BSAC'88 DECOMPRESSION TABLES [Archive] - BSAC Scuba Diving Forums

MattDuke

09-11-2005, 12:30

BACKGROUND:

BSAC'88 DECOMPRESSION TABLES
In 1985 BSAC commissioned Dr Tom Hennessy, a world renowned researcher in the field, to jointly produce the first decompression tables specifically designed for sports diving

EXAMPLE DIVE FOR REVIEW OF REQUIRED DECO:

A dive to 50m, for 21minutes having been out of the water for 24 hours: Acsent rates and definitions of "Bottom Time" differ, so this should be considered not 100% the same dive, but close enough for discussion.

50M, 21 Minutes :- BSAC88:

(15M a minute acent rate up to first deco stop)

Deco: 1 min @ 9m
Deco: 6 min @6M

1min form 6M to surface

50M, 21 Minutes :- VPM-B +2

Dec to 50m (3) Air 15m/min descent.
Level 50m 17:40 (21) Air 1,25 ppO2, 50m ead
Asc to 27m (23) Air -9m/min ascent.
Stop at 27m 0:27 (24) Air 0,77 ppO2, 27m ead
Stop at 24m 2:00 (26) Air 0,71 ppO2, 24m ead
Stop at 21m 2:00 (28) Air 0,65 ppO2, 21m ead
Stop at 18m 3:00 (31) Air 0,59 ppO2, 18m ead
Stop at 15m 3:00 (34) Air 0,52 ppO2, 15m ead
Stop at 12m 6:00 (40) Air 0,46 ppO2, 12m ead
Stop at 9m 8:00 (48) Air 0,40 ppO2, 9m ead
Stop at 6m 44:00 (92) Air 0,34 ppO2, 6m ead
Surface (92) Air -9m/min ascent.

50M, 21 Minutes :- Proplanner

Depth1= 50msw: 21mins 21%O2 Run Time= 21mins
DIVE 1 DECOMPRESSION REQUIREMENTS
Stop= 31msw: 1mins 21%O2 Run Time= 24mins
Stop= 21msw: 1mins 21%O2 Run Time= 26mins
Stop= 12msw: 3mins 21%O2 Run Time= 30mins
Stop= 9msw: 7mins 21%O2 Run Time= 37mins
Stop= 6msw: 61mins 21%O2 Run Time= 98mins

I raised the question on the BSAC 88 tables a few years ago, but I raise it again. Is it time to remove the BSAC 88 tables? (Or at least advise against using them for decompression dives OFFICIALLY?)

Or have the other deco algorithms got it wrong, and we should stick with the 88's?

Today in the BSAC we teach divers to decompress & dive on AIR, and give them the BSAC 88 tables for their deco. (Say a Dive Leader without any Nitrox/ERD qualification)

My proposal for discussion:

"I do not think the BSAC 88 tables give decompression times that are in line with current research & available programes for decompression planning. As they give significantly less decompression, I propose the BSAC 88 tables should be urgently reviewed as for what role they play in the BSAC system. Today we have divers qualified to dive to 50M on air using air as a decompression gas, and the 88 tables as a promoted tool to do this."

ADDENDUM:

Clearly there has been a strong focus on getting divers to use Nitrox for decompression (Advanced Nitrox & ERD) and soon a BSAC Trimix course will be released that will further promote a better breathing gas for a 50M dive in UK waters, but despite this good work, we still teach and sell the BSAC 88 tables.

Is it time to drop the tables? Are they being used for decompression dives anyway...?

/Matt Duke

Adrian Kelland

09-11-2005, 12:46

Matt,

This ties in well with this http://www.bsacforum.co.uk/forums/genforum/posts/8644.html post on the old forum. Perhaps it is worth you posting there to continue that aspect here?

Adrian

Nigel Hewitt

09-11-2005, 14:43

BSAC'88 DECOMPRESSION TABLES
In 1985 BSAC commissioned Dr Tom Hennessy, a world renowned researcher in the field, to jointly produce the first decompression tables specifically designed for sports divingIt's a secret algorithm so who can criticise? It may be the only one that works, we don't have all that many bent divers, but who knows who uses it? Bühlmann has suffered years of pier review and people tweak it for more conservatism but nobody has ever published a rebuttal.

Does anybody use BSAC tables after OD classes? Or do we just look at them then dive the computer on our wrist? I learn to use them for my DL exam but I don't know anybody who takes them seriously now for deco.

MattDuke

09-11-2005, 15:09

It's a secret algorithm so who can criticise? It may be
Does anybody use BSAC tables after OD classes? Or do we just look at them then dive the computer on our wrist? I learn to use them for my DL exam but I don't know anybody who takes them seriously now for deco.

No-one I know. That's why I posted, I can't see this being more than a no-brainer from the diving side. But then like I said too, maybe the 88 tables are right and everyone else is wrong.

Once divers start doing any sort of planned deco diving, (Like Sports Diver level and beyond) they might want to have a look at other tables to see why their gas plan based on 88 tables has left you at 6M with 45minutes of stops on your Uwatec/Aladin with 30bar in your tank.

Try telling the dive marshall "It's OK, I planned on the BSAC 88 tables" these extra 45 minutes were just mircobubble ****. I'm up for the next dive, if only I could get this silly "five 'o five" message to go away....

I know I know, they would have used the dive computer to plan, and seen the stops rack up, but I'm just showing that it's just doesn't seem right to have such large differences of opinion on the deco! I tried to show the VPM/Proplanner slates as way of example of two differing algorithms STILL giving shed loads more stops, and both of them having the first stop way deeper than 9m!

Matt

Tony Dwyer

09-11-2005, 15:27

<<Does anybody use BSAC tables after OD classes? Or do we just look at them then dive the computer on our wrist? I learn to use them for my DL exam but I don't know anybody who takes them seriously now for deco.>>

I only use them in training.

I dive on the computers (I take two on every dive - the backup is by way of being a timer!). Since most of my dives are on Nitrox anywhere between 21% & 40% the 88 tables aren't much use anyway.

Our students buy computers pretty quickly after qualifying. Our branch owns several computers and we lend them to trainees, prior to them buying their own. They still have to demonstrate understanding of the 88s and plan their dives using them.

MattS

09-11-2005, 15:40

other Matt

I have thought long and hard on this subject. The big problem with comparing tables is that you can't. You can get an indication from comparing a single initial dive, but you can't draw conclusions from such a limited comparison. The 88s use a different maximum altitude to Buhlmann, on which Proplanner is based. I haven't run these particular numbers, but I have looked at others in the past and altitude could explain the sort of difference we see between 88s and Buhlmann et al.

When I look back at how my diving progressed to where it currently is I feel like I have been my own decompression guinea pig. I started on 88s, was deco diving with them until the headaches and fatigue became unbearable. I slowed down the ascent rates, took hydration more seriously, started doing deep-stops, went to IANTD for Nitrox, started doing last stops shallower than 6m and felt much better for it all. I returned to IANTD for Technical after getting all the symptoms again when trying to accelerate on 50%. I am now routinely doing dives with depths and bottom times my initial instructors would view as suicidal, but getting out the water feeling better than ever.

So ultimately I don't think that a table or algorithm holds all the answers. Decompression diving requires a holistic approach to DCI avoidance.

I completely agree with your premise that the 88s are no longer the best choice of table for decompression diving. Each year we see more evidence from the bubble modellers and projects like DANs dive explore, supporting what we all know. The 88s are dated and allow for practises that no educated decompression diver would subject themselves to these days. We also now know that the headaches and drowsiness I associate with decompression dives on 88 tables are DCI symptoms, albeit not so obvious as to persuade one to seek treatment.

The 88s are very easy to use and to teach and I would not like to see them completely abandoned for that reason. They are perfectly safe for no-deco diving and in some ways safer than the alternatives. Having been sat with a group of instructors scratching our heads trying to remember how Buhlmann's RNT works I would not underestimate the ease of use. So I can see some reasons to retain them for no-decompression air and Nitrox diving. The same sort of diving new divers do until they get around to buying a computer.

Once decompression enters the equation the 88s become just too difficult to defend. They do not include deep stops. They allow an overly fast ascent rate. There is no stop above 6m. Didn't BSAC invite JP Imbert to DOC last year to tell us all how essential these things are? There was even a follow up in Dive.

There are added complications introduced by the 88s when considered alongside dive computers. If I plan a dive on Buhlmann tables it will be pretty close to my Buhlmann based VR3 computer and fairly close to my buddies Buhlmann + RGBM Vyper. Now should my BSAC 88 trained buddy attempt to plan a deco dive on 88s and then carry it out using a Vyper they could potentially run low on gas before being able to complete the significantly longer stops the Vyper will call for. I will do my deco at 4.5m using less gas and off-gassing about 50% faster than my 88s buddy stuck at 6m. If my computer breaks I can pull out my in-water Buhlmann tables, apply EAD and have a safe bail out plan. My buddy however can do little but wait out their gas supply in shallow water and hope.

So whether or not the 88s stop schedule is better or worse than Buhlmann's, there are a host of other factors which make the 88s inappropriate for modern decompression diving.

After all that I would like to see some upto date thinking applied to the 88s. Slow the max ascent to 10m/min, make a safety stop at 5m or shallower mandatory (on the understanding that breaking it is not a reason to rush to the pot) and perhaps even introduce deep stops on ascents of 20m or more (basically a stop of 2minutes at 10m). Due to the elegance of the Dive Time concept, this does not have to be expensive. It does not require the tables to be re-written, re-validated or re-printed. It just needs an A5 sticker on the inside front cover saying that in view of advances in decompression theory since 1988 these measures are HIGHLY recommended.

At Sport Diver we could teach Buhlmann on the grounds that the majority probably are using Buhlmann based computers and probably would benefit from knowing how to use the table their computers are based on. It also makes it easier for BSAC members to seek specialist technical training from other agencies most of whom teach Buhlmann tables. If the subject is too wide for the SD course stick it in an SDC. I would love the opportunity to teach Buhlmann regularly and perhaps the next time I will not be sat embarrassingly scratching my head when I need to work an RNT.

Sorry I am not sufficiently up to speed on VPM to offer an opinion of it. I just hope BSAC don't end up adopting Buhlmann at the same time the rest of the World switches to VPM.

garethwebber

09-11-2005, 21:13

Are there any web pages that go into a discussion of the two other planning methods and their benefits?

Thanks

Gareth

MattS

10-11-2005, 08:50

You can download a copy of the Buhlmann tables here
http://www.diving.co.za/buhlmann_dive_tables.htm

There is a bunch of information on VPM here
http://www.decompression.org/maiken/VPM/VPM_Program_Site_Map.htm

purdy

11-11-2005, 18:01

|Good arguments on both sides here.

We have two BSAC Advanced divers, who still dive on tables with there trusty dive timers, and they use tables.
Must admit they do not do much deco diving, but they do some, and more importantly are capable of doing it.
As is anyone with the grade of Sports Diver or above, surely we are wrong to tech someone to use something that we believe is "not right" even if we do try and justify it to ourselves that they are never going to use it anyway!

Anyone got any idea what it would cost to revamp and re calcualte the 88's??

Taff Griffiths

11-11-2005, 23:27

Hi
This subject is of great interest to me not only as a diver but until recently I was a Hyper & Hypo Baric chamber operator in the research field.

To cut a very long story short IMHO tables BSAC tables like other tables are a system of lowering the odds of a DCI. If I personnaly follow tables and or my dive computer I will get out of the water after multiple dives a tired man. After completing some research I now plan my dives to include a multilevel dive approach where at certain depths I will linger to help off gassing in conjuction with recommended stops, hence now I get out of the water feeling a lot better.

I must stress this works for me and I use my informed judgement to plan a dive to include these multilevel lingerings.

I am therefore am of the opinion that tables/computers regardless who has devised them are only there for risk management. As more information and research is completed on the subject they must be reviewed to acertain if they are still current or need amending.

COST of doing this should not be an issue as the people who produce these tables have a duty of care to update them :o

Adrian Kelland

11-11-2005, 23:38

as the people who produce these tables have a duty of care to update them :o

Now there's a point.

So could BSAC be failing with this duty of care with a set of tables that would appear to be out of step with 2 other major models?

Adrian

Andy MacLeod

11-11-2005, 23:53

The BSAC tables are relatively old. Conservative (that helps) but need to account for new research & theory. We should be open minded (Like NAUI) and be prepared to accept a range of solutions. The venerable age of the tables may be making the Governing body seem stuck in the mud and inflexible.

Andy Wade

12-11-2005, 01:28

Now there's a point.

So could BSAC be failing with this duty of care with a set of tables that would appear to be out of step with 2 other major models?

Adrian

Well IIRC the previous BSAC/RNPL tables were printed in 1972
So they lasted until the BSAC '88 tables came out in 1988 - which is 16 years.
It's now 2005 (last time I looked) so the BSAC '88 tables are now 17 years old.
Maybe it is time for a new set. Maybe we should adopt someone else's tables?
Can BSAC afford to have new tables produced?
:(

Alwassia

12-11-2005, 11:15

Both Matt's, Nigel and others,
I do decompression diving on a weekly bases, having read the post on this and YD forum on the latest toughts on the DECo theory i got concerned and wanted to add some type of safety net. Following normally the Aladin Air and the BSAC 88 tables just semed out of time, even i included Pyle stops on every Deco dive. Reading now all your comments does not help my confidence either.
So i enrolled and finished the DSAT TecRec Deep Diver course, padding profiles and accelerated Deco will make things safer i guess.

During the course the instructor (the only instructor from any agency able to do technical course here in Jeddah, so no bashing please) introduced me to the Z-planner and the V planner in addition to the Sunto Viper (i think), my Aladin Air gave an error message during the preparatiuon for the first dives and had to be send in.

Having done 50m deco dives before i was very surprised that the dive schedules created with Z & V planner where so much longer on the deep stops and as well the shallow once.

Now 3 days after completing the course i do not know what to follow, the proven BSAC table or some computer software created by someone i do not know or recognise by name. The most worrying part is that my instructor said that some of the persons / companies which published deco software where sucessfully sued and are out of business now. You can immagine what that did for my confidence in any software.

So here i am siting reading your discussion not knowing whom to listen to, and not knowing which tables, sofware, practice or phisolophe to follow.

As the local tec community consists of three, including me, i have to depend on you to give me some guidance.

Many thanks for any replys.

Khaled

MattS

12-11-2005, 11:37

garethwebber

12-11-2005, 11:49

I think that would be great - again provided the NDC is happy.

Alwassia

12-11-2005, 12:01

Matt,
that was one of the articles which made me wonder about deep stops, reading then the webpage from others Pyle.... i started doing deep stops.
But then i do not know a lot about the subject, what dive planing software is being used in the UK, or an other way around which once are safe to use?

Khaled

Gareth

12-11-2005, 20:09

Khaled

I can understand your confusion over which table to use, how safe they are etc.

Remember the first & most importent point is that no table (algorithm) is 100% safe. If you dive you risk getting bent!

About which table to use or trust, this is more difficult.

What I would normally recommend is that if you have been using a table todate that you are happy with - use this as your bench mark (reference).
Run a series of exercises comparing a series of known dives on your existing table with the new table you are looking at. If, under 'like for like' profiles the new table will get you out of the water quicker - you can ask yourself why & if you are happy with this.

The other question to ask is - are you suffering any symptoms of sub-clinical DCI on your existing table (or new table). If so you may wish to be more conservative in future.

Please also note that if your existing diving has been 30m max depth, no stop dives, your existing 'good' table may not be as safe when you apply it to 50m 40minute bottom time dives with extensive decompression!

I am not a great fan of 'just adding a bit extra if I don't feel so good on the way up'. I prefer diving an established table which I trust with consistent safety factor settings.
Granted, if I have done something to 'increase the risk of a bend' during the dive, like working harder than usual, being scared etc, then I will 'pad' the table - or move to the next longest plan to increase the decompression.
I believe it is better to dive on a table that I am happy with, if for any reason I feel that I am experiencing subclinical DCI I will increase the conservativity setting until I am happy with the table again.
Under this method I can review my diving over a series of dives & am certain I am applying a consistent series of rules on which to make any future adjustments. - I prefer 20minutes longer in the water than a treatment in a chamber! I would also be more conservative if access to a chamber is difficult or impossible - at least in the UK we know that a chamber should not be very far way!

Most importent - make changes slowly - increase your depth in slow measured steps, increase your bottom time & decompression requirements in small amounts. Hopefully you will get some warning if you are being too aggressive with your decompression, fatigue after dives etc.

Safe diving

Gareth

Alwassia

13-11-2005, 14:22

Gareth

13-11-2005, 18:17

Alwassia

I still use Proplanner - unfortunately it is no longer available. (I add the pyle stops manually).

I will be experimenting with V-planner & a number of other versions of software over the winter. This is mainly because my Proplanner version does not include CCR - which I am currently moving onto. So next season I will also be making small steps!

On proplanner I use a safety factor of 25% - this equates to 5% more inert gas in the mix than stated i.e. on Air with the safety factor at 25% the Nitrogen is calculated as if it is 84% of the breathing mix.

I think Matt is using V-Planner, I do know one of his old friends thinks he cuts the table a bit agressively - but he seems to be OK on it, & he dives in cold water!

Matt - which software are you using, & what are your settings?

Gareth

MattDuke

13-11-2005, 19:58

Alwassia

I still use Proplanner - unfortunately it is no longer available.

I will be experimenting with V-planner & a number of other versions of software over the winter. This is mainly because my Proplanner version does not include CCR - which I am currently moving onto. So next season I will also be making small steps!

On proplanner I use a safety factor of 25% - this equates to 5% more inert gas in the mix than stated i.e. on Air with the safety factor at 25% the Nitrogen is calculated as if it is 84% of the breathing mix.

I think Matt is using V-Planner, I do know one of his old friends thinks he cuts the table a bit agressively - but he seems to be OK on it, & he dives in cold water!

Matt - which software are you using, & what are your settings?

Gareth

Hmmm.

I think it's a bit harsh to say that I am cutting the tables mate. But OK things can be lost a bit on email. Lets hope this post clears that up. The last thing I want is a reputation as a risk taker!

I use V-Planner on +3, and I add minimum 5 minutes at 6M, plus a further 6 minutes from 6M to the surface. Generally I run a square profile, and never have an average depth as deep as the max either. Deco has been 2 gases every time, either EAN40 &EAN80 or EAN50 & OXYGEN, (The later I am using more and more, due to the kind nature of fjord diving conditions, and my increasing belief that 100% is a good gas to be on, if you are in non tidal protected areas, doing just 1 mix dive a day with a bottom PPo2 of around 1.2)

The past few months I've been "Trying my body out" with VPM-B, (As you rightly say, it's the only way to know!) Since September when I fully switced to V-Planner, I've done 7 Trimix dives, in the 55-60M range and had varying results of well being. This is a very small amount of dives compared to others out there, and I would say I'm at the early part of this process. So I wouldn't base **** on my experience, as it's so limited.

I basically run my dives through V-Planner, and then Proplanner, and try and make sure I do no more than 10 minutes less deco at 6M when comparing. This is very conservative and would not be possible on big dives. But 60M-70M & 25minutes BT is all I am interested in. I am limited by gas (2x12 2x Alu80) and my comfort level of 75-80minutes.

In conclusion, the run times aren't that different, but yes the 6M stop has been somewhat shorter than for similar dives we have done together back in the UK. But as I never dive a square profile at the absolute max depth planned, and also leave the bottom about 1 minute early, I think I am the same careful guy you dived with in the UK! Having become a father in 2003, technical diving took a back seat 2003-2004. I now have my life back in control and have been somewhat "Active" getting the team well back into the swing of things. Every time we dive we take a chance, and using trimix and multiple deco gases pretty much is the highest risk we can do. But don't mistake being my now being "Active" mix diving, as being "Agressive" I have no desire to end up in a wheelchair, or worse. I respect the risks & my young family too much for that, and havng nothing to prove with cutting tables. If you lived 45minutes from world class wrecks & deep walls, you'd probably want to dive at least once a week too. :p

You want aggressive? Take a look at what the BSAC 88 tables would give you for an air dive of 21minutes at 50M, doing the deco on AIR!

Does that clear things up?

/Matt

Janos

13-11-2005, 22:33

I think that for no-stop diving, BSAC 88s are just as aggressive / conservative as the PADI RDP. I posted a comparison on YD a little while back but can't find it now. However the BSAC tables come in for far more of a kicking. For deco diving, then yes, I think they are aggressive.

I think 88s have a place as a minimum deco schedule - just like Bulhman 100/100 - and I can pad this as necessary.

Anyone got any idea what it would cost to revamp and re calcualte the 88's??

First dive would be easy to do, it's just a question of plugging numbers into Decoplan or something. Repetetive dives would require a little more work but I'm sure I could come up with something.

Janos

Adrian Kelland

13-11-2005, 22:57

First dive would be easy to do, it's just a question of plugging numbers into Decoplan or something. Repetetive dives would require a little more work but I'm sure I could come up with something.
A surface interval table could still be used. I think that works well and is easy to explain.

It's the altitude tranfers that I find fun to explain. No one else wants to teach that bit :D

David Walker

13-11-2005, 23:19

It's the altitude tranfers that I find fun to explain. No one else wants to teach that bit :D

I taught that bit fairly recently. Nice and easy to understand (although some of the logic of "you don't have a surface interval as you're driving over a mountain, but you do on a plane" took a bit of explaining...

The main problem I have teaching altitude transfers is explaining the point of them. The "driving over a hill after diving" things are fine, you can see where they'd come in. Its the "dive in the sea, drive over a hill, then dive in a mountain lake" ones I have trouble explaining - why would anyone want to do that? I usually come back to "well just in case you ever want to, now you know how"! :D

David

Adrian Kelland

14-11-2005, 09:11

(although some of the logic of "you don't have a surface interval as you're driving over a mountain, but you do on a plane" took a bit of explaining...
David

Oh yes :( benefit/no benefit at lower pressure depending on time...

Gareth

14-11-2005, 09:18

Hmmm.

I think it's a bit harsh to say that I am cutting the tables mate. But OK things can be lost a bit on email. Lets hope this post clears that up. The last thing I want is a reputation as a risk taker!

/Matt

Sorry Matt didn't mean to sound so negative, you're the only one I know/trust using V-planner & i know how safety conscious you are!

As I said to you a couple of weeks ago I have only one experience of V-planner, & that was diving with someone who had cut their own table using V-planner. They wanted to get out of the water WAY to early for me.

I would be interested to know if V-planner has a conservativity setting, & what you have it set to, + any other settings you've manually set up.
At least i would have reference point from a source I trust! - & you're doing the Beta testing instead of me!

Safe diving Matt, talk to you soon cheers.

Gareth

MattDuke

14-11-2005, 09:33

Sorry Matt didn't mean to sound so negative, you're the only one I know/trust using V-planner & i know how safety conscious you are!

As I said to you a couple of weeks ago I have only one experience of V-planner, & that was diving with someone who had cut their own table using V-planner. They wanted to get out of the water WAY to early for me.

I would be interested to know if V-planner has a conservativity setting, & what you have it set to, + any other settings you've manually set up.
At least i would have reference point from a source I trust! - & you're doing the Beta testing instead of me!

Safe diving Matt, talk to you soon cheers.

Gareth

Well that is somewhat OT from my original post: Which is about how suitable the BSAC'88 Tables are for Deco diving. So we can V-Planner in another thread. :)

/Matt

purdy

14-11-2005, 09:38

MattDuke

14-11-2005, 10:05

Sorry Guys
You have completely lost me know, gone all technical and the like

Most divers are still surely on Air or maybe Nitorx
but then again maybe most divers do not do much Deco and as such the 88's are Ok for majority?

Purdy

Back on topic, my original proposal was:

My proposal for discussion:

"I do not think the BSAC 88 tables give decompression times that are in line with current research & available programes for decompression planning. As they give significantly less decompression, I propose the BSAC 88 tables should be urgently reviewed as for what role they play in the BSAC system. Today we have divers qualified to dive to 50M on air using air as a decompression gas, and the 88 tables as a promoted tool to do this."

I do not think we should abandon the tables, but we should review what their role is. For non deco dives they are quite in line with everything else, not counting the ascent rate. Even the ascent rate could be worse. (eg 18m a minute for the Nordic Navy..)

But as the stops indicated for deco diving, on air, are somewhat aggressive, I have suggested that they are retired as being promoted for that type of diving. Not just informally on ERD courses, but an official line from the BS-AC. Something like, "We do not recomend using the 88 tables for planned decompression diving" or "We fully support the use of the 88 tables for planned decompression diving"

If council feels that the BSAC 88 tables do not need review, and that they offer a safe tool for planning such dives, then fair enough, make such a statement. Just seems to me that it's time for an update. (Deep stops, slow ascent rate, reduced bubble theory etc etc) The diving computer manufactures haven't stayed still since 1988, nor should the BS-AC.

But as you rightly say, I suspect that very few advanced deco dives are done soley on the 88's. But does that mean we should ignore the issue?

/Matt

MattS

14-11-2005, 10:39

other Matt
First the OT stuff.

Gareth was a little inaccurate and that needs clearing up.
Proplanner is still available. The latest version includes deep stops, which are pretty much Pyle. Trimix and CCR versions are available.
http://www.vr3.co.uk/vr3/main.php?content=proplanner

The main problem with Proplanner is that it is not Windows XP friendly.

Personally I still use Proplanner, although I mainly use the version embedded in my VR3. The Dive Plan function is a little clunky but works well enough.

DeltaP is due to release a VPM-B version of the VR3 on 14th November. It is an expensive upgrade but we should see an increase in the numbers running VPM without the conservatism of square profile tables.

Khaled.
Proplanner was/is used for the BSAC ERD course. It was the first desktop deco package. It is squarely Buhlmann based but includes several modifications as a result of chamber tests and real World profiles carried out by Kevin Gurr and the various dive teams he has lead. Kevin's pedigree is that he was one of the three that brought technical diving and Trimix to the UK and arguably the most prolific of UK technical divers.

purdy

14-11-2005, 12:54

MattDuke

14-11-2005, 14:21

Matt (duke) just to prevent confusion!
As the BSAC issued them in the first place and on the whole have been used safetly for quite a while, asking them to again say they are safe, is a bit much.
As for the need to re work or whatever the phrase is, I would agree.
We have learnt so much more on physiology and computer modeling has got so much better, that yes they can be made safer I am sure
No quite so sure that it is an "urgent" problem, or even that the issue can be addressed urgently.
The BSAC are a club that promote the fact that they allow Deco diving on air, when propeerly trained and also that we have a depth limit of 50mtrs.
So they must provide something to allow us to do this safetly.
However surely it is just as important that they are easy to use and reliable, which you must admit the 88's are a piece of cake to use, especially on the fly!

We have two instructors from other dive organisation and they both struggle to expalind the wheel to me and I am not that dim (just apear so some times)

So maybe the question we should be advising the BSAC what we want of our tables and stuff like that.

Is that making any sense at all??

Hi Purdy,

I raised the issue as a member, as I feel we should look at the tables, and what I'm hoping to hear to that a working group is setup to do just that.

All tables have a % risk associated, and I don't expect our club to come out with a "Our tables are bad and will bend you" statement. Just like when we moved to the 88 tables in the first place. Change is a normal part of life, and it's quite a while since the 88 air tables were reviewed isn't it?

A team of divers/doctors with the correct skills, could do such a review for our club, and if they feel we should change some of our advice on deco, then great. If not, then great too as it means folk with far more knowledge than me have made the call for the best interests of the club.

It's important that these sorts of things are not seen as "Anti-BSAC" quite the oppositt, these topics should be seen as membership debate on what we think some resources could be spent on, to improve safety & the reputation in our club. A club I might add, I have spent almost half my living life, and all my adult life being a proud member of. I still get to push some of our excellent SDC's on the Norwegians too. :cool:

So far, I have found this an excellent debate with some reasoned and interesting arguments. A great start to our new club forum! Thanks for the replies.

Matt Duke.
sat in the airport.

Chris Cherrington

15-11-2005, 14:22

So far, I have found this an excellent debate with some reasoned and interesting arguments. A great start to our new club forum! Thanks for the replies.

Not even a few days and back on this old chesnut!! :)

There are a couple of points here to add. Firstly that the Buhlmann tables are public domain so they should not cost anything in theory. However, should anyone get bent on the "new" tables what is the liability to BSAC? Surely if you "endorse" the model you must take liability for it? Maybe this is part of the reluctance to move to other deco models (newer ones) for dives requiring stops.

Second, AFAIK the BSAC are looking at another deco model to teach in conjunction with the Trimix course? EAD is being introduced :eek: to deal with the accelerated deco?

My view? If the Buhlmann tables can be adopted without liability issues they should be and then taught for stop diving and the 88s either relegated to no-stop diving for new divers or scrapped altogether.

Chris

Andy Wade

15-11-2005, 15:06

Not even a few days and back on this old chesnut!! :)

There are a couple of points here to add. Firstly that the Buhlmann tables are public domain so they should not cost anything in theory. However, should anyone get bent on the "new" tables what is the liability to BSAC? Surely if you "endorse" the model you must take liability for it? Maybe this is part of the reluctance to move to other deco models (newer ones) for dives requiring stops.

Second, AFAIK the BSAC are looking at another deco model to teach in conjunction with the Trimix course? EAD is being introduced :eek: to deal with the accelerated deco?

My view? If the Buhlmann tables can be adopted without liability issues they should be and then taught for stop diving and the 88s either relegated to no-stop diving for new divers or scrapped altogether.

Chris

Do any tables come without the disclaimer that 'diving and using decompression tables are not risk free' etc etc?

Mike Rowley

15-11-2005, 16:47

Gareth,
thanks for the reply, do you have any recomendations which deco program / deco planner to use? I have the V-planner (paid & register version) but when reading on forums i see other mentioned. Any recomendations from any one which once are, at least in their local dive group, tried and tested?

If someone from Dive or Diver reads this they could take it as an idea for an article.

Alwassia

One fact that you might like to consider is that BSAC has adopted the use of Pro-Dive Planner for the dive planning in the BSAC Inspiration CCR course, the BSAC Drager Dolphin SCR course and is likely to use it for the BSAC open circuit and CCR trimix courses that are currently under development.

The reason we decided to opt for Pro-Dive Planner is that the BSAC 88 tables do not allow the use of EAD concept and do not facilitate constant PO2 use. Pro-Dive Planner has a proven track record for dives within normal recreational range to mixed gas 80 - 90 metres. The programme is versatile, facilitates open circuit and rebreathers for air, nitrox and mixed gas. The algorithm is currently available in computers such as the VR2 and VR3 and as a computer programme for PCs and laptops. Kevin Gurr was happy to allow BSAC to use it on our courses at no charge for which we are gratefull.

So you see BSAC does have an open mind on this issue, does use a decompression tool other than the 88 tables and does allow the use of many decompression tools in branch diving.

VPM-B is a good tool as is GAP which incorporates RGBM but these do not have quite the proven track record across the depth range and are more likely to be used by technical divers at the deeper end of diving. However, that said I believe RGBM is now incorporated into some Saunto models. VPM-B is about to be made available in VR3 computers.

Mike Rowley
BSAC NDC Technical Development Leader

Mike Rowley

15-11-2005, 16:57

[QUOTE=purdy]Matt (duke) The BSAC are a club that promote the fact that they allow Deco diving on air, when propeerly trained and also that we have a depth limit of 50mtrs.

Actually Purdy BSAC allows diving to 70m when using trimix and for appropriately certified divers.

Cheers

Mike

richard scarsbrook

15-11-2005, 17:07

Its the "dive in the sea, drive over a hill, then dive in a mountain lake" ones I have trouble explaining - why would anyone want to do that? I usually come back to "well just in case you ever want to, now you know how"! :D

David
A shore dive in the sea followed by a dive in a quarry or lake is quite a popular day out in North Wales or Northwest England. It also saves washing your gear out when you get home :) . Trefor or Menai for the sea; and Dorothea (altitude c100m), Vivian (c100m) or Fron (c250m) are common combinations. I've also used the altitude table a few times to check how soon I can climb a hill after a dive.

I've always felt that the BSAC88 deco times are suspiciously short. Matt Duke's example 21 minutes at 50m dive is 'off the table' for BSAC88's predecessor BSAC/RNPL, while the full RNPL1972 Air Table requires 5mins at 15m, 10 at 10, and 60 at 5 - much closer to what the latest models demand (not that I'm suggesting we go back to the old tables). I suspect that the reasons there wasn't much protest at the time the 88s were introduced were that decompression theory was not much in the public domain; and that most of the divers doing long deco dives had already switched to computers so the tables were irrelevant to them.

I'm in favour of a review of the use of 88 tables for decompression diving, including their nitrox derivatives. However I don't think BSAC should spend a lot of money on a replacement, should one be necessary, given that there are several alternatives already available.

Chris Horan

15-11-2005, 17:19

Khaled

I am not a decompression scientist and can only suggest that you read the article in Dive which was published after JP Imbert presented to DOC last year
http://www.divemagazine.co.uk/news/article.asp?SP=&v=1&UAN=1830

I would be willing to attempt an understandable article but only with the NDCs approval.

Jean-Pierre Imbert's paper can be read in full at http://www.bsac.org/techserv/ndc/doc2004/doc2004jpimbert.htm

Mike Busuttili (posting on Chris Horan's computer)

Alwassia

16-11-2005, 08:20

Mike,
I appologize for not attending a BSAC Extended Range Course, i would have loved to but we got no instructors here so i opted for the next best thing.
As DSAT does not recommend any deco program / planner as such and the tech comunity here is in its infancy i wanted to find out what others (BSAC members) use or recommend.
Your statement regrading the Pro-planner was pritty much what i was looking for. :0)

Thanks for the help.

Khaled

purdy

16-11-2005, 09:30

Mike

Yes but we are refering to the 88's table
Surely they only have a relavance to those on AIR

You know the one sometimes refered to as a 21% mix!

Purdy

Mike Rowley

16-11-2005, 12:49

Mike

Yes but we are refering to the 88's table
Surely they only have a relavance to those on AIR

You know the one sometimes refered to as a 21% mix!

Purdy

Air and selected nitrox mixes. Not very flexible, don't cover mixes other than those selected by the tables and don't recognise EAD concept so can't be used with custom mixes. In addition, cannot be used for accelerated decompression. This makes the depth range where they can be used quite narrow. I'm not sure that anyone would wish to do a 40m - 50m dive these days as a no-stop dive and without using accelerated decompression gas. That is why we now use Pro-Dive Planner on all BSAC Technical courses. However, I would point out that we do discuss the use of other equally valid tools such as VPM-B and RGBM etc.

My general point is that BSAC no longer teaches the 88 tables only. Currently they are used for entry level training and diving but there rapidly becomes a point as a diver progresses when they become innapropriate and other tools have to be taught.

Cheers

Mike:)

Janos

17-11-2005, 17:15

Ok,

Out of sheer curiousity, I've had a go at doing some dive tables in a BSAC stylee. The attached are for first dive only. Seocnd dive (ie after a surface interval) are a little more complicated but shouldn't be too bad. It will be more interesting though!

Hopefully it's all self-explanatory. Dark blue is stop depths. Turquoise is dive depth, and yellow is time.

This took me about an hour.

Janos

Paul Watts

21-11-2005, 00:47

Dave

21-11-2005, 10:28

Is it time to drop the tables? Are they being used for decompression dives anyway...?

/Matt Duke

Whilst slagging these tables off, has anyone actually got evidence that says they are bad rather than just that other tables different to them

Dave

MattS

21-11-2005, 19:09

Whilst slagging these tables off, has anyone actually got evidence that says they are bad rather than just that other tables different to themIf you have not already done so, please read my other posts on this thread Dave.

MattD and myself can both provide personal evidence that using the 88s for extensive decompression diving lead to sub-clinical symptoms that we have since learnt to avoid. I don't know MattD particulary. It turns out we were up to similar things at similar times and we share a name.

Read the report given to DOC last year by JP Imbert and the follow up article by Dr Philip Larkin in Dive magazine. JP presented a great deal of evidence supporting deep stops for dives of intermediate bottom time in the 30m to 50m range. He also provided evidence to support a last stop shallower than 6m, safety stops on no decompression dives and a max 10m/min ascent rate for all dives.

Note, in the context of decompression science a short bottom time is a bounce of a few minutes. A long bottom time is one of many hours which results in saturation. It should be clear that sport divers are concerned with intermediate bottom times. Decompression dives and bottom times in the 30 to 50m range have increased enourmously since 1988.

For additional evidence refer to DANs Project Dive Explore and of course the work of the bubble modellers such as Yont, Maiken, Baker.

Personally I disagree with MattD's assertation that BSAC stop times are too short. I see this as a need for BSAC to promote current good practise. Many of the changes in understanding decompression over the last 17 years has resulted from Dopler monitoring techniques which were not available in 1988.

If you ask the question can BSAC do it better? In the case of decompression practise I have to say I believe the answer to be a resounding YES.

Chris Cherrington

21-11-2005, 22:37

I agree. Its not that the 88s are "bad" as such, just that things have moved on. (I'm sure Tom would agree himself with that). Deep stops, for example, are now pretty much accepted as a good DCI prevention (again Hennessey talks of "defizzing" stops even back in the 70s..)

Some people see it as BSAC-bashing. Its not. The PADI RDP is as old and as old-fashioned (17mps ascent...) Deco is a dynamic science and diving today a far cry from what we did in the late 1980s....

The bit that mystifies me (and many others) is the near religious devotion to the 88s by some divers. Like anything new is bad and somehow or another all diving knowledge stoped 20 years ago.....(stab jacket bad - ABLJ good..)

BSAC is a club, not a commercial organisation so maybe there is a cost issue. Me, I don't think so. I think this smells of politics. I don't like politics in a sport that can kill you.

The adoption of rebreathers and now the start of the trimix training suggests that the 88s are under review. Lets all drop the politics and look at the facts. BSAC was once at the cutting edge of diving is that where we want it or just a sideshow pulled apart by backward looking arguments?

Chris

IainC

22-11-2005, 13:52

The bit that mystifies me (and many others) is the near religious devotion to the 88s by some divers.

Never met any of those types myself.
But if they are devoted to them and still diving and still not bent, that is a pretty good argument that the '88s are OK, maybe not the latest and greatest, but not unsafe ;)

Chris Cherrington

22-11-2005, 15:00

I've never heard of them (88s) being called "unsafe", as in they bend you. Just outdated and liable to generate sub-clinical DCI. I don't think sub-clinical DCI was much discussed in the late 80s?

But you raise a valid point as to whether or not anyone dives them very much .

Chris

Tristan Green

23-11-2005, 00:36

Hi,

If you are wondering where all the ERD discussions have gone - they are in a new thread here:

http://www.bsacforum.co.uk/forums/showthread.php?t=345

The topic seemed to warrant it's own thread.

Cheers,
Tristan

Tom Hennessy

24-11-2005, 01:59

Do I detect an error in Matt Duke's sample dive, viz. confusion between dive time and bottom time in using the BSAC'88 Tables?

After 17 years, we (and me) have all moved on to deal with longer and/or deeper dives, but we do need to keep in mind that dive time, as per the BSAC'88tables, is not equal to bottom time.

In 1988, I designed my tables for short and/ or shallow dives, and, at the request of the BSAC, I restricted the bottom depth-time curve to one that what was then perceived as the sports diving depth-time envelope. Outside this region decompression penalties very rapidly escalate to the point where they would have been deemed to be impractical and dangerous to perform with the dive kit and procedures then available.

In the subsequent 17 years decometer software developers have reacted by gradually increasing the decompression requirements on very ordinary dives. Why? not because they had better data (DCS risk incidence is very low and dive data is notoriously subjective and Doppler microbubble echosound is inconsistent with subsequent DCS, particularly on relatively low risk dives, viz. sports dives),

The software designers have moved gradually towards longer decompression times because they needed to offer a decompression profile, not just for the sports diver, but one that could be justified to their financial backers. The grounds were (and still are) (a) their perception of the risk of litigation (b) that the diver market was becoming accustomed to increased decompression times, deeper stops, slower ascent rates etc from (albeit very limited) technical diving experience, and (c) that this shift fitted in very nicely with the proliferation of decompression experts who had become embroiled in the developemt of commercial decosoftware and technical diving, using their (it seems to me) rather limited offshore trials experience.

No sour grapes here, please. I had my reasons for not publishing my model and ideas at that time. I knew Buhlmann very well and I also knew that he was astounded that his published algorithim later became so biblical. He always cited the case of Hannes Keller, who, given a computer and his (Buhlman's) earlier deco data and initial mathematical model, went on to develop a decompresson model that eventually attracted the mighty USN. In 1962, a 1000ft USN dive was planned off Catalina Island, California, using Keller's IBM-based decomodel with Peter Small (founder member of the BSAC) as support diver. Check out the outcome on:

http://www.divinghistory.com/kellerdive.htm

So there you have it, mathematical models, diving trials.DCS/Doppler as end points, and/or numerical overconfidence are a minefield for the unwary. I have said all this before in an earlier paper.

Rivalry between USN and RN experts on air decompression tables has cooled a bit over the years, but what should they now do about decometer software models for navy operatives (who require ultra short in-water deco-stops)? Who knows? Decompresson prediction is the equivalent of alternative medicine. As in all adventurous activities, like NHS comsultants despite their learned pedigrees, someone will always emerge to megabaffle the hapless patient (diver?).

Safe diving!

Tom Hennessy

Alwassia

24-11-2005, 08:04

Tom,
Welcome back & thanks for your comments.
May i ask you a question? Is the strategy of the deco software developers as discribed a positive necessary step for the point of pure dive safety / deco requierments?

Thanks

Khaled

MattDuke

24-11-2005, 09:19

Do I detect an error in Matt Duke's sample dive, viz. confusion between dive time and bottom time in using the BSAC'88 Tables?

Safe diving!

Tom Hennessy

Thanks for your comments Tom. I am not sure you read this bit of my original post: (Just to clear up the error you mentioned, I did state the fact that Dive Time & Bottom time definitions differ)

A dive to 50m, for 21minutes having been out of the water for 24 hours: Acsent rates and definitions of "Bottom Time" differ, so this should be considered not 100% the same dive, but close enough for discussion.

Anyway, it was very interesting to hear your views, and it was good of you to give us some feedback.

Matt

Tom Hennessy

03-12-2005, 01:29

Matt

I hope that your transoesophageal echocardiograph proves that you don’t have a patent foramen ovale (pfo). If you don't have one then you might like to look again at the BSAC'88 Tables. These are very similar to the USN Standard Air Tables (revised 1999) which, apart from the 30 fpm ascent rate, have been in use for decades (1937, I think). I have used them myself (from 1966 – 1976, down to 60m/10 min) when I was an active BSAC diver (never did like the BSAC/RNPL Tables, being a sport diver, even though I worked at RNPL for about 9 years).

You might also like to revise your deco calc for a 50m/ 21 min depth/bottom time dive on the BSAC’88 Tables. You should have entered the tables for a 51m 24 min Dive Time giving a total of 39 min total in-water time. This USN Standard Air Decompression Table (1999) does not give an exact equivalent (167 ft/ 21 min) but a 160ft/ 20 min dive requires a total of 37 min in-water time and a 170 ft/ 20 min bottom time requires a total of 42 m in water time. For a 1 min increase in bottom time, a total of 39 min on the BSAC’88 Tables is quite comparable. You will notice that there is a very rapid increase in decompression for the next longer bottom time entry of 25 min on the USN tables: a 160 ft/25 min dive and a 170ft / 25 min dive of 45 and 60 min total in-water times. You should also note that both of these dives are disallowed on the BSAC’88 Tables! So you happened to choose a dive which was very close to the depth/bottom time cut-off envelope of the BSAC’88 Tables. Your VPM-B+2 and Proplanner results are OTT. Why don’t you compare some popular and well-established no-stop dives that were in use for 40 years with these deco programs?

You might wonder why it is that the decometer decompression requirements have escalated in the past few years, even for short dives which are known to be ultra-safe on the USN tables. Contrast this with the lack of progress in useful DCS- risk data – pretty thin data over the last 2-3 decades, I believe.

The most obvious reason is that divers and dive doctors have all become mightily aware of decompression-induced microbubble phenomena (Doppler studies, etc), an awareness that was virtually non-existent among dive doctors and certainly among divers 2-3 decades ago. It would seem that divers and dive doctors may have gradually become aware of deco microbubbles, and decometer software designers have reacted to these influences, and often inconclusive results by introducing well-meaning but unproven software amendments to their models (e.g. RBGM, Pyle stops, ‘Uwatec understands microbubbles’ etc).

But this awareness has not been accompanied by a concomitant understanding of the true aetiology of ‘microcavitation’ (my word, by the way, like my word ‘poxtox’ and 'noxtox', for historians to chew over) during diving and subsequent DCS onset. Instead, this lack understanding on the site of origin, onset and effects of decompression-induced microcavitation has provided an opportunity for various gurus step in and to pontificate on decompression procedures.

Had DCS data progressed at the same rate as people's awareness of the effects of microcavitation on decompression, and that it was proven that the ascent rate needed to be slowed, and depth of first stop and subsequent stop times needed to be increased, then, of course I would have immediately revised the BSAC'88 Tables.

I knew more than 20 years ago that a massive increase in in-water deco was needed for longer and deeper dives. When I produced the BSAC'88 tables, I clipped the depths and bottom times to limit dives to the then perceived 'sports dive' envelope, so avoiding the exponential increase in in-water decompression.

I am, however, now beginning to wonder whether the BSAC should issue extended air tables for longer and deeper dives to cover the extended air range. If I am asked to produce these, do not assume that I would just jesus-factor in a fix-up so as to equate with current decometer predictions. Indeed, divers might be rather shocked by the very long decompression penalties required on my extended air tables The decompression time curve changes very rapidly after dives that exceed 50m depth and/ or 25 min ascent time.

Of course, if my extended air tables were ever to appear, then I would anticipate that divers would just carry on using current decometer predictions for some of the deeper/longer of the extended dives on the new Tables, because they would give shorter in-water deco times, an about turn for current negative comments about the in-water dives on the BSC’88 Tables!

Tom Hennessy

MattDuke

03-12-2005, 09:57

Matt

Of course, if my extended air tables were ever to appear, then I would anticipate that divers would just carry on using current decometer predictions for some of the deeper/longer of the extended dives on the new Tables, because they would give shorter in-water deco times, an about turn for current negative comments about the in-water dives on the BSC’88 Tables!

Tom Hennessy

Thanks Tom! This was an excellent reply! Much to chew on! :) I can imagine you have a busy schedule, and must say it is excellent for us BSAC members to get such feedback.

Quite a heavy post, so I'm going to need to go through it a few times! Might have a question or two afterwards. :rolleyes:

Matt

Chris Cherrington

03-12-2005, 23:31

.....
Of course, if my extended air tables were ever to appear,.....

Tom, I am sure you are aware many of us would be much happier with a trimix breathing mixture for what is generally termed "extended range".

There is a lot of noise about the differences between He and N2 on and off gassing. I and many others would love to know your views about this - particularly in respect of the forthcoming BSAC Tx course.

This development in the club's history seems to many (well me at least :D) to be logical point at which to review the role of the 88s and tables in general in UK diving as it is today.

Chris

Rory O'Conor

05-01-2006, 02:18

I am a novice diver, only done my Open Water Course and reached 10m.
So treat these comments accordingly...

Following my course, I have bought one piece of diving kit, because I can hire the rest. I bought a dive computer - after much deliberation, I bought an Aladin Tec - because it had additional level stop conservatism over the Prime, and the PC download enables review of completed dive profiles.

I have also done quite a bit of study of decompression models in order to understand "aggressive" v "conservative" etc in order to avoid DCI.

I have not looked at the BSAC 88 tables, but I assume that these use a neo-Haldane model, as do Buhlmann, DCAP, PADI DSAT, COMEX etc. Of course, there are also the newer "bubble models" such as Vplanner, RGBM, HKBM etc.

Whilst it is comparatively easy to create a neo-Haldane model in an Excel spreadsheet, I am still having difficulty coming to terms with the equations and their implications in the "bubble models". There may be a qualitative difference between the philosophy of the bubble models and just adding some compartments with fast halftimes to a neo-Haldane model. But I cannot yet understand this and have not seen a clear explanation.

There are also some linear-exponential models out there with a Haldanian approach to on-gassing, but a linear approach to off-gassing. I have not yet seen relevant equations.

My point is: If you are going to update an old table, you need something to replace it with. It is not simply just a matter of changing a few halftimes and ascent rates and re-computing. You need to consider if the model is qualitatively correct and what its operational scope is.

Having said that, if the scope of BSAC 88 tables are "repetitive, multiday recreational nitrox (including air) dives at varying altitudes with decompression (single gas) to a maximum depth of 60m, ppO2=1.6", my expectation is that bubble models will need to be explored. If the words "repetitive and multiday" are excluded and "reasonnable profile (not too many yo-yos)" included, then an adjusted Haldane model would probably be fine.

ps: Anyone diving to these depths on nitrox mixtures is probably at some risk of oxygen toxicity, so they wont be worrying too much about the DCI risk !!

I see little point in replacing one Haldane model with another one, without a clear understanding of whether this is the appropriate underlying model. I dont think this is a simple task.

So consider carefully the required scope of the tables before making changes.

Regards

Rory
(ps: probably rubbish from a novice)

Chris Cherrington

05-01-2006, 11:47

I have not looked at the BSAC 88 tables, but I assume that these use a neo-Haldane model,

The 88s are different in the way they work to the Buhlmann/Haldane algorithm at the heart of just about every other set of tables.

Rory, mate in the best and nicest way please remember an old saying:-
when you ASSUME you make an ASS of U and ME........ :D

The algorithm for the 88s has never been published - that's why they alway draw so much traffic when discussed on sites like this. We are all therefore, in effect, discussing our personal guess of what the 88s measure.

My point is: If you are going to update an old table, you need something to replace it with. It is not simply just a matter of changing a few halftimes and ascent rates and re-computing. You need to consider if the model is qualitatively correct and what its operational scope is.

Agreed.

Chris

Vic

05-01-2006, 12:42

I have not looked at the BSAC 88 tables, but I assume that these use a neo-Haldane model

Well, you're not off to the best of starts, because the BSAC '88 model uses gas-phase modelling; it's not a Haldanean model...

Whilst it is comparatively easy to create a neo-Haldane model in an Excel spreadsheet, I am still having difficulty coming to terms with the equations and their implications in the "bubble models". There may be a qualitative difference between the philosophy of the bubble models and just adding some compartments with fast halftimes to a neo-Haldane model. But I cannot yet understand this and have not seen a clear explanation.

You're never going to get an accurate 10-minute precis of this, but here goes...

All tissues are subject to Henry's Law, and so the amounts of dissolved gas in a tissue can be calculated if we know the applied partial pressure of inert gas. There is some discussion as to how accurately we can determine that (see the ubiquitous diffusion/perfusion arguments), but the end result of all that is just a tweaking of parameters, so from the point of view of implementation, we can consider the tissue saturation level to be proportional to the ambient pressure, with the ongas/offgas rates being determined by the model in use (again, these are just parameters to a common model).

So far so good. Now let's add some bubbles.

Bubble models declare that bubbles are generated. This may be as a result of decompression, or as a result of body processes not related to diving. Or both. There are a few ideas floating around.

If we were not diving, these bubbles would be irrelevant - they are initially very small, so they cause no problems. But as we decompress, they will grow.

This has several effects; firstly, and most importantly, a growing bubble is a DCI risk. There comes a radius at which we should consider the subject bent; in short, we should attempt to keep the bubbles as small as practical.

Now modelling the bubble lifecycle is non-trivial. It is simple enough to calculate the internal pressure of a bubble (which is higher than the ambient pressure because of surface tension - see Laplace's Theorem). However, the rate of diffusion of gas from the bubble to the surrounding tissue is dependent both on the pressure differential and also on the permeability of the interface - this is where things like VPM come in. There will also be a range of tissues where the bubble might develop/be transported to, and there will be a spectrum of probable bubble radii in each.

The main upshot of all this bubble theory is that deep stops minimise bubble problems - both because they prevent the bubbles from growing to the point where they become difficult, and because they maintain the increased internal pressure on small bubbles. Unfortunately, deeper stops means that some tissues will almost certainly be on-gassing whilst the bubbles are being eliminated - thereby creating decompression issues in the faster tissues that must be resolved later.

There are also some linear-exponential models out there with a Haldanian approach to on-gassing, but a linear approach to off-gassing. I have not yet seen relevant equations.

It remains to be seen whether this sort of tweak really makes much difference. Some people like this sort of thing.

My point is: If you are going to update an old table, you need something to replace it with. It is not simply just a matter of changing a few halftimes and ascent rates and re-computing.

Isn't it? How do we know that?

You need to consider if the model is qualitatively correct and what its operational scope is.

And now you're onto a very sticky wicket indeed.

The thing is, you see, none of the models are "qualitatively correct". Not one. They're all mathematical approximations of a biological process that no-one understands. Indeed, the more research there is on decompression, the more we discover that our approximations are decidedly dodgy.

The best you're going to get is a model with a low incidence rate of DCI. Buhlmann fills this niche rather well - probably because there have been so many people diving it and feeding data back in to refine the model. The initial version - determined by supposedly cutting-edge science - was appalling, with an unacceptably high incidence.

So - are the BSAC '88s any worse in terms of incidence? There is no data to suggest that they are. But is that purely because there is insufficient data? Tolerance bars get very large with the sample sizes we've got...

Having said that, if the scope of BSAC 88 tables are "repetitive, multiday recreational nitrox (including air) dives at varying altitudes with decompression (single gas) to a maximum depth of 60m, ppO2=1.6", my expectation is that bubble models will need to be explored.

Why? How do we know a bubble model will do any better? Take a look at VPM-A if you believe such a model is inherently safer :-(

ps: Anyone diving to these depths on nitrox mixtures is probably at some risk of oxygen toxicity, so they wont be worrying too much about the DCI risk !!

Now that really wasn't the sharpest thing you could have said. The DCI risk is mitigated by using Nitrox (which is why we do it), but unless you're diving in an enormously irresponsible fashion, the DCI risk still outguns the Tox risk by some significant margin.

I see little point in replacing one Haldane model with another one, without a clear understanding of whether this is the appropriate underlying model.

That leaves you in the realms of religious argument, then - the real, biological processes underpinning decompression are largely unknown. Any model that arises is merely one group's attempt to codify their belief structure about a representation of those unknown processes. That makes the definition of "appropriate" somewhat problematical.

HTH

Vic.

Gordon

05-01-2006, 15:26

Vic - sounds like you know more about this than me, but dont all models come from how many fit and able seamen can survive at that depth for that time?

Its all statistics, you can have much shorter deco times with more bent people or long deco times and few bent people.

All tables and software are trade offs to various degrees.

Chris Cherrington

05-01-2006, 15:41

Vic - sounds like you know more about this than me, but dont all models come from how many fit and able seamen can survive at that depth for that time?

No.

The authors of many tables (inc 88s) had access to empiric data on dive times and DCI incidence. Haldane used data from caisson workers - diving as such was not invented...

The USN used its data as did the RN to hypothetically test their "new" algorithms. Robert Workman invented a thing called M values that gave a line to which the data could be plotted. However this data is old and limited in both its depth (fsw) and application since military and commercial divers rarely do multi-dive days.

An urban myth was borne about the unsuitability of some rec. tables as they were "Navy tables with a fudge factor" and this was "unsafe 'cos they were all strapping young blokes with above average levels of fitness" etc. In truth DSAT were the leading light in chamber & doppler testing the M Values for a new table in 1987 for their client - an agency called PADI... In addition work was undertaken by the commercial operators to test their models.

In recent years the testing has dropped off and many new models have been generated by people with little DCI experience but with computer programming skills or some (like Weinke for example) with bubble mechanics knowledge.

Weinke's RGBM is therefore an example of a model that has been generated by knowledge rather than empiric data from drowned squaddies.

Modern deco thinking is being hampered by the lack of empiric data in fact. Health and safety considerations mean that commercial companies no longer "push the envelope" of deco theory. In fact most "new" stuff comes from the WKPP - sadly the noise generated by the spin-off from this group (DIR) and its former gob-on-a-stick GI3 serve to cover up a lot of useful info.

Chris

Nigel Hewitt

05-01-2006, 16:38

The authors of many tables (inc 88s) had access to empiric data on dive times and DCI incidence. Haldane used data from caisson workers - diving as such was not invented...Sorry wrong.
Haldane was working on a Royal Navy budget to produce a solution to the problem that their hard hard divers were getting broken. He had a compression chamber but his 'volunteers' were mostly goats. He ran the solutions on many people including himself.

Chris Cherrington

05-01-2006, 17:31

Sorry wrong.
Haldane was working on a Royal Navy budget to produce a solution to the problem that their hard hard divers were getting broken. He had a compression chamber but his 'volunteers' were mostly goats. He ran the solutions on many people including himself.

I thought he had some caisson stuff. But yes, I am on a go-slow today - he was of course working for the RN. Brain elsewhere (MS Access to be exact..)

Chris:p

Adrian Kelland

05-01-2006, 19:23

There was a good presentation on his work at this year's DOC.

http://www.bsac.org/techserv/ndc/doc2005/doc2005pjames.htm

Adrian

Gordon

05-01-2006, 19:37

I know, the presentation at the DOC looked very good, but had a band concert in the evening so had to leave early :(

Vic

06-01-2006, 13:18

An urban myth was borne about the unsuitability of some rec. tables as they were "Navy tables with a fudge factor" and this was "unsafe 'cos they were all strapping young blokes with above average levels of fitness" etc.

It's not entirely unreasonable to describe certain tables in that way - considering that the USN tables were the root for much of the diving technology from about the 1950s onwards.

In truth DSAT were the leading light in chamber & doppler testing the M Values for a new table in 1987 for their client - an agency called PADI...

I think that's laying it on a bit strong...

The compartment half-times in the RDP are (according to the original PADI/DSAT documentation) exactly the same as the USN compartments, but with added 30 and 60 minute compartments. The M-values are reduced from the USN tables, but the same Surface Interval Credit Table is used.

Now the tables were tested (both in dry-chanber and in water), and good Doppler scores were obtained, but nevertheless, all this is really just to tune the USN model. The RDP is very much a derivative of its Naval forbear...

and many new models have been generated by people with little DCI experience but with computer programming skills or some (like Weinke for example) with bubble mechanics knowledge.

No, I don't like Bruce either :-)

In fact most "new" stuff comes from the WKPP

I'm none too sure about that, either. The WKPP have, allegedly, made significant advances in decompression research - but how much of that is actually available? There are claims, of course, but I'm unconvinced as to whether or not some of these can be verified (especially in light of the "slobitis" threads).

sadly the noise generated by the spin-off from this group (DIR) and its former gob-on-a-stick GI3 serve to cover up a lot of useful info.

Indeed. We'll probably never know whether the WKPP know everything, nothing, or something in-between...

Vic.

Chris Cherrington

06-01-2006, 13:40

I'm none too sure about that, either. The WKPP have, allegedly, made significant advances in decompression research - but how much of that is actually available? There are claims, of course, but I'm unconvinced as to whether or not some of these can be verified (especially in light of the "slobitis" threads).

As you say mate, unless they publish its just noise. No doubt if they did there will be all sorts of "spoilers" just because of who it is..

All the debates about "deco on the fly" prove that.

I'll stick to me V Planner..;)

Chris

Gareth

06-01-2006, 13:49

Chris Cherrington

06-01-2006, 16:42

I do remember reading (I think it was in John Lippman's Deeper into Diving). That when they researched the USN tables they found that the USN's own dive supervisors didn't use the USN tables as published.

They added there own fudge factor - along the lines of next deeper next longer. This was not 'offical policy' but the policy of the dive supervisors, the fudge factors where passed from supervisor to supervisor.

Gareth

I think its important to put things in context. Not having read Lippman I can't comment on the above, but my understanding is that the USN table ascent rate came about as a result of the ability of the winchmen to raise hard-hat divers. Let's also not forget also there are several itterations of the USN tables...

We all use "fudge factors" to some extent. Adding a "conservative" stop to your Suunto is a fudge.....

It all makes for good forum discussions though. Someone will start on about Isobaric Counter Diffusion any minute now:)

Chris

Gareth

06-01-2006, 18:58

Chris

The ascent rates weren't my point. It was the base table they applied that was modified.

The main reason I mentioned it was that prior to (,as I say I think it was Lippman,) the official USN statements & decompression incident analysis was done on the wrong premis.

The USN believed that their divers where diving to the official USN tables.
So they printed statistics saying X dives where done on tably Y resulting in Z number of bends of which W where Type 1 & V where Type 2.

When in actual fact the official USN tables where not being used.

I have just had a quickscan of Lippmans Book. I cann't find the note I referred to above but this may be either poor speed reading, or I 'm muddling up another author. However Lippman does state the following:-

The USN report of 1980 refering to the scheduled dives between 1971-1978 reported incidents of 1.4%. Doubt about the accuracy of the report was highlighted when it was stated that the actual dives where not logged but the table schedules where logged (i.e. the table used on the dive, rather than the actual dive time & depth).
Controlled experiments where later carried out, the incidents of bends was four times or greater than those stated by the Navy. Further experiments (Merrill Spencer & Bruce Bassett) revealled bends in 5% of cases although doppler tests detected venous bubbles in 30% of divers.

This is not a direct qoute, I've abreviated his notes. It also only refers to No Stop Dives - NDL. It should also be noted that the USN where not doing repeat dives.

Continuing on the USN table data.
DAN stated that in 1987 58% of recreational cases of DCI where using USN tables. In 1988 this had increased to 72% of DCI cases.

The conclusions where that the USN tables where not suited to recreational NDL diving with out adding a significant safety factor - no guideance on this is available!

Gareth

Chris Cherrington

07-01-2006, 11:09

The conclusions where that the USN tables where not suited to recreational NDL diving .....
Gareth

I don't think anyone would argue otherwise!!

Thanks for the background stuff BTW - very interesting whatever the source. I've forgotten where we were in the discussion, but someone made the point about fit navy divers and I think your observation suggests that the USN no-deco times were too aggressive for all humans. I perhaps didn't make the point very well (or at all IIRC) that there is nothing about known deco physiology that suggest young active males are better able to withstand nitrogen loading than other groups of humans. That said being physically fit is generally agreed to be a benefit in terms of cardio-vascular efficiency and deco theory suggests this lowers DCI incidence.

The question that comes from this is whether/when/why the USN tables were revised? The DAN quote is '87, the year the PADI RDP came out. Things sure have changed since then!!

I have a copy of the latest USN and the no-deco times are not far from other tables. Nevertheless the ascent rate IS part of the equation. For example the French MN90 tables (used by FFESSM) have a very slow ascent in the last 6 meters. Changing the ascent rate changes the no-deco bottom time, since the ascent is the deco - there being, in reality, no such thing as a no-deco dive.

In conclusion I would say once again that the methodology of the USN is not therefore wrong simply that the chosen M values and level of conservatism are such that the resultant tables are not suited to sports divers. However you look at it the modeling technique of half times, M values and controlling tissue groups has stood the test of time. If you rubbish the USN you rubbish just about every other tables in the world along with just about every dive computer ever made. The disagreement seems to start when this technique is extrapolated to mixed gas and deep diving. Buhlmann never finished his work with Helium mixtures so there is no solid base of data.

Dive safe everyone.

Chris

Gareth

07-01-2006, 14:21

Hi Chris

I think we are in broad agreement.

I wasn't rubbishing the methodology of the way the USN tables where created. It just worries me when they are qouted as the base sports diver table, with thousands of hours of safe diving & minimal incidents. I just wanted to make the point that the way the USN applied them was not as it first looked. In addition the USN tried to avoided repeatative diving, which is not how sports divers apply them!

I think the most importent points that have been made by yourself & others is that.

1. Since the 80's very little research has been done on decompression theory using test dives. This is since due to two primary factors, economic necessity (COMEX etc now rely more on ROV's), it is also no longer ethically acceptable!

2. Most of the testing is now done on rerunning old dives through new decompression models & seeing if the model accurately predicts what happened. (There some running test on themselves but this is statisticaly limiting.)

3. A lot of the new tables are created by mathmaticians or computer specialists rather than physiologists or decompression medicine specialists.
(especially 'freely available downloads').

4. You do need a new 'gimic' to encourage people to buy another dive computer.

5. The available capacity of new electronics allows the space/capacity to add in additional safety factors that have been known, but were previously left to the individual to apply rather than the computer. i.e. saw tooth profiles, short surface intervals, low temperature, high work loads etc.

I guess we all have our own preferred tables/theories & practices. I personnaly subscribe to the deep stops are great group!

Any way we are way off the 88's, thread that was originally started.

Gareth

Gordon

07-01-2006, 21:23

It comes back a bit to what I was saying earlier - you can have slower ascent rates and bend fewer people, or faster ascent rates and bend more...

Another possible comment is that I perceive the Navy as having a chamber nearby, so aggressive tables can be countered by sticking them in the pot for a bit, just in case. Correct me on that if Im wrong, I have no Navy experience.

MattS

08-01-2006, 16:19

It comes back a bit to what I was saying earlier - you can have slower ascent rates and bend fewer people, or faster ascent rates and bend more...What Vic was saying earlier is that it really is not that simple. The notion of gas being dissolved in the body encourages a fast ascent rate in order that a gradient is produced between dissolved and inspired inert. The notion of bubbles being part of the decompression process indicates that too fast an ascent is provocative. The notion of a continual spectrum of tissue half times means that the only way to decompress all tissues is to surface.

David_de_Marneffe

25-02-2006, 14:10

Hi guys,

New to this thread, but just before I read it I sent the following email to HQ (technical issues). If I get a reply from them, I will post it. The whole point, I think, is that experienced divers are aware of all this and can make informed decisions. The average BSAC Sport Diver does not have a clue (unless the local club instructors decided by themselves to teach them proper deco theory), and does not question the '88 tables or his pure Buhlmann computer before he does an ERD or tekkie course. Here is the email:

Hi,

I am a member of BSAC 406 (Sharjah, United Arab Emirates). I am (almost) a Dive Leader, I am assistant instructor.

The following summarizes my concerns about the way decompression diving is currently taught within BSAC. If a debate on this is already open at a higher level, please pardon my impertinence. If not, I think it should be opened.

Since when I started diving, I have been interested in decompression theory and have followed the development of the VPM/RGBM algorithms vs the "classic" neo-Haldane models (Buhlmann, etc). Also, I have recently followed the IANTD Advanced Recreational Trimix course with the excellent team at Tech Asia in the Philippines.

I have come to think that the way deco theory is taught within BSAC, as well as the BSAC tables are now out of date. Indeed, there is strong evidence (both theoretical and experimental) that the neo-Haldane models overlook important parts of the physical and physiological phenomenons that occur during a dive. The most obvious experimental evidence is the number of divers affected with pains, aches and tiredness for many hours after diving, despite the fact that they dive "perfectly" according to the tables or their computers, including safety stops. This is not even mentioning the few who develop DCI despite their "flawless" dive profiles. I speak of personal experience when I say that diving VPM/RGBM profiles or even including Pyle stops resolves the problem.

Of course, I realise that this is an issue for recreational diving because of the much larger deco requirements of VPM/RGBM. Indeed, at the Sport Diver level, it is difficult to ask divers to carry deco gas and start doing gas switches and explain that deco should be almost as long as the dive itself.

However, consider the following example: a 30 minutes dive (surface to first deco stop) at 30m, according to the BSAC tables, calls for a 3 minutes deco stop at 6m. According to VPM-B, if the last stop is at 6m, deco on air, conservatism
+2 (average diver) we get:
1min at 12m
4mins at 9m
22mins(!!) at 6m (total deco: 27mins)

If the last stop is at 3m:
1min at 12m
3mins at 9m
6mins at 6m
10mins at 3m (total deco: 20mins)

Looking at this and knowing that VPM-B is the latest and most likely most accurate decompression model, don't you think that the BSAC 3mins at 6m are grossly underestimated?

In view of the above, may I kindly suggest the following:

- I think the very least we owe to our members is to make them aware of these facts so that they can make an informed decision about what they want to do. This would involve:
1) At the basic level, modify the contents of the Sport Diver / Dive Leader trainings regarding decompression theory
2) Offer an "Advanced Decompression Theory" course as a skill development course, available well before Extended Range Diving, and induce divers who want to practice deco diving to follow it. It could even be made mandatory

- Reevaluate within which limits decompression diving should be made available at the Sport Diver level.

- Induce divers who practice decompression diving to follow a nitrox course and suggest that they carry deco gas in order to reduce deco requirements.

- Review the safety stops policy.

- Review the BSAC tables.

There you go. I hope I wasn't out of place by suggesting this. But personally, knowing what I know, I definitely won't dive anymore the way I did when I just qualified as a Sport Diver.

Best regards,
David de Marneffe

David Walker

25-02-2006, 15:13

The most obvious experimental evidence is the number of divers affected with pains, aches and tiredness for many hours after diving, despite the fact that they dive "perfectly" according to the tables or their computers, including safety stops. This is not even mentioning the few who develop DCI despite their "flawless" dive profiles. I speak of personal experience when I say that diving VPM/RGBM profiles or even including Pyle
stops resolves the problem.

What? So you're saying, on BSAC tables, that some people get bent on "perfect" profiles, yet from your personal experience, you've never been bent using Pyle etc? I'd suggest that isn't a massive theoretical basis for changing the tables. What if these people getting bent had a PFO, were overweight/unfit/smokers/drunk/etc? Deco theory is exactly that - a theory! It is not an exact science, and as a result you will ALWAYS get some people getting bends on what the tables say is a safe dive - that is a fact of ANY table ever created. Its merely the incidence of it that changes.

However, consider the following example: a 30 minutes dive (surface to first deco stop), according to the BSAC tables, calls for a 3 minutes deco stop at 6m.

At what depth? I'm also thinking that most people would NOT use conservatism +2 when using VPM-B, and so a bad comparison. +0 would be the base theory, +2 just adds in padding - you could do that with BSAC tables too.

Looking at this and knowing that VPM-B is the latest and most likely most accurate decompression model, don't you think that the BSAC 3mins at 6m are grossly underestimated?

Hmmmm - "latest" does not translate into "most accurate"! Thats a fundamental flaw in your argument I think. As already mentioned, deco is a theory - there is not such thing as a "most accurate" model because you aren't modelling a uniform entity, ie the divers you're attempting to build a deco model for are all different. If you make it accurate for the most unfit diver in the world, then for everyone else it will be extremely conservative. If you make it accurate for the world's most fit diver, for many it will be extremely aggressive. All the tables / algorithms are based on assumptions about the divers, none of which will ever be right for you. The question is how conservative they make it. Often, they will simply suggest padding for unfit divers.

The problem with your proposals for change is that a lot of divers don't want to know about deco theory. They don't want to be told that there are 10 different algorithms / tables, and that they have to evaluate them all to decide which is safest. At much higher levels that may be appropriate, but the average SD or even DL does not want to have to do a PhD in Deco Theory to work out what is safe. They want someone to tell them which model is known to work well, and wherever necessary we can (and do) highlight the fact that not every model is perfect, we don't encourage people to push the limits, and encourage people to add in extra stops etc if they are having a bad day / unfit / whatever. The BSAC tables as they are get people out safely on the huge majority of dives - the odd bend is considered an acceptable risk, I believe. Even if it were possible to make a table so conservative that no one ever had any chance of getting a bend ever, chances are people would ignore it and use something else because they don't want to spend their entire dive at 6m decompressing just because the tables have been designed for the lowest common denominator, ie the world's oldest, most unfit, most drunk chain-smoking and overweight person!

In short, there is no easy answer to deco theory - which is why there are so many different algorthms around. Your way seems to be based on the assumption that newest = best - that is a seriously flawed assumption in my opinion. I could be wrong though, which is part of my point - I don't really know, neither do you, neither does anyone else to any great degree. Empirical evidence suggests that BSAC tables are fine for the vast majority of dives - making them more conservative would just mean people use something else, and is for most people unnecessary. Everyone is different, and for most people BSAC tables are suitably conservative already.

They're not perfect, but then neither is any other table / algorithm. I'm sure with every algorithm we could sit and pick holes in it saying "well table X says that this dive should need deco, but algorithm Y says I don't, and when I did that dive last week with table Z which said I needed 15mins of stops I got a bend!"

There probably is a good argument to change - but yours really has no scientific basis other than "I got a bend once when I dived BSAC tables, but not when I tried Pyle stops".

David

David_de_Marneffe

26-02-2006, 01:37

What? So you're saying, on BSAC tables, that some people get bent on "perfect" profiles, yet from your personal experience, you've never been bent using Pyle etc? I'd suggest that isn't a massive theoretical basis for changing the tables. What if these people getting bent had a PFO, were overweight/unfit/smokers/drunk/etc? Deco theory is exactly that - a theory! It is not an exact science, and as a result you will ALWAYS get some people getting bends on what the tables say is a safe dive - that is a fact of ANY table ever created. Its merely the incidence of it that changes.

I never got bent. Before I knew about deep stops, I would feel tired, and achy after lots of my dives, and so do lots of people, just ask around you. I also know a couple who did get bent, without any apparent reason (not overweight, non smoker, and I don't know about you but around here we usually don't dive drunk), and they certainly aren't the only ones. Deep stops fixed my problem, and again, I am far from being the only one (ask Pyle!). It suffices to read around to realise that. It's also not enough to ask that people don't end up in a recompression chamber after diving. If people feel bad after diving, it's not only an annoyance, it's their body complaining that something is not exactly right.

At what depth? I'm also thinking that most people would NOT use conservatism +2 when using VPM-B, and so a bad comparison. +0 would be the base theory, +2 just adds in padding - you could do that with BSAC tables too.

The dive was at 30m. Sorry for the typo. If you read V-planner's manual, conservatism +2 (which is the default) and +3 are for average people. "Nominal is the base value, and is for the super fit and divers with a proven track record of fast deco. Plus 2 or 3 is the normal setting for most divers. Set to plus 3 or 4 with strenuous, cold, a series of multi day dives, extra safety, or a prior history of DCS or symptoms.
This setting increases the Critical Radii of N2/He inside the VPM algorithms, from these base values, by this increase: 1 = 5%, 2 = 12%, 3 = 22%, 4 = 35%, 5 = 50%. The Critical Volume setting is on by default."

there is not such thing as a "most accurate" model because you aren't modelling a uniform entity, ie the divers you're attempting to build a deco model for are all different.

Last time I checked, on- and off-gassing happen in every diver. Also in goats, guinea pigs, any living creature and even inert stuff like rubber and all solid and liquid material. That's called physics. So is the case for bubble physics: it applies to every diver. In this sense, the bubble models are more accurate because they take bubbles into considerations, AS WELL AS on- and off- gassing. The Haldanian models don't do that. What differs is the consequences of these phenomenons for a given diver. That's why modern algorithms and modern computers allow to set a conservatism factor: to suit as many people as possible. Not the case for older computers. Not the case for the '88 tables.

The bubble models ARE better (read: more complete, closer from reality, I never said perfect: no model is perfect), in the sense that they account for phenomenons not considered by previous models. Same goes for Einstein's relativity vs Newton's classic gravity theory. That's not to say that Newton or Buhlmann models were crap. That's simply the fate of ALL models to be replaced by more realistic ones as science and knowledge progress. Anybody with a scientific background will tell you that. But I don't blame you: people also had a hard time to believe Einstein when he came up with his new / better theory. Some even burnt Galileo when he said the Earth was round. But it is not enough for a model to account for more than its predecessor. It still has to be verified experimentally, and in the case of the bubble models, it appears to be the case.

The problem with your proposals for change is that a lot of divers don't want to know about deco theory. They don't want to be told that there are 10 different algorithms / tables, and that they have to evaluate them all to decide which is safest. At much higher levels that may be appropriate, but the average SD or even DL does not want to have to do a PhD in Deco Theory to work out what is safe.

Well, when I teach deco theory, people are usually very interested in what I have to say. Because it's their a*** down there and they want (and have a right!) to choose what is good for them and to know what are their alternatives, and because they often recognize themselves in the sleepy - achy diver full of skin rashes after his dives. I fully realise that not everybody has the background to understand the full blown theories, and I try to present the mainstream theories and the resulting deco profiles in a simple manner. Once properly educated, they can make an INFORMED DECISION about what they want to do, they are FREE TO CHOOSE what is good for them, which is the whole point of what I am saying, and is sufficient by itself as an argument for change: currently, until you've done an ERD or tekkie course, within BSAC's official teaching, there is NOT A WORD about deep stops. And it doesn't take a PhD to understand them.

Also, I'd much appreciate if you stopped rephrasing me to come up with stuff like "newest = best" or "I got a bend once when I dived BSAC tables, but not when I tried Pyle stops", which I never wrote.

Cheers,
David

David Walker

26-02-2006, 04:13

It suffices to read around to realise that.

But is that a good basis for changing the tables? Reading around a few diver's comments is not a scientific basis, much of it may be placebo effect and the like. I'm not saying that anything you say is wrong, i'm just questioning the scientific basis behind it... and if we're going to endorse any kind of tables, I think most people would like to know that there is some kind of evidence behind it other than speculation and "well it seems better" type comments from divers who've used it.

If you read V-planner's manual, conservatism +2 (which is the default) and +3 are for average people. 0 and +1 are for the ultra-fit. +4 and +5 are for people with a history of DCI or other problems like PFO, overweight, etc

Fair enough, never used V-planner.

Last time I checked, on- and off-gassing happen in every diver. Also in goats, guinea pigs, any living creature and even inert stuff like rubber and all solid and liquid material. That's called physics.

It may be physics that it happens.... the rate at which it happens (and therefore the potential damage it can cause) varies massively between these things you list though. The deco schedules for different people, never mind different animals, would, if you could measure them, vary enormously. Remember how different tissues take up gases at different rates, fast/slow tissues? Well what is fast in one person may not be quite as fast in another - thats why there is no single model which can possibly be described as accurate for all divers.

The bubble models ARE better (read: more complete, closer from reality, I never said perfect: no model is perfect), in the sense that they account for phenomenons not considered by previous models.

Not strictly true - even "more complete" doesn't necessarily make them better / more accurate. Its a fundamental principle of almost any kind of modelling: the simpler the model the better (to a point). I don't know though whether that point of diminishing returns happens to be before or after bubbles have been included. However, the fact that they are included does not automatically make the model better. In many cases it will make it worse. I think the problem is that currently a lot of people doing Pyle stops for example add these as additional to their normal deco, which means they're naturally doing slower ascents, more stops, more time in-water, and so will likely feel better after the dive. That doesn't however mean that Pyle is more accurate. In fact, I could come up with the new David's Deco Theory (DDT) which is basically the same as the BSAC tables but for any given dive, you use the BSAC tables to your depth + 10m. So if you do a 20m dive, read the tables as if you did a 30m dive and thats the outcome of DDT. I'm sure I could do a lot of research to prove that mine is safer, but is it better? On what basis have I created my theory?

I should make clear that i'm NOT saying that Pyle or any bubble model is wrong - just that your justifications for changing the BSAC tables are not really very strong. If we're going to do it, there needs to be some good evidence and research into it, long-term tests, scientifically sound and based on a solid theoretical basis. Simply "feeling good after a dive" or whatever isn't that good a reason to change the tables. After all, if BSAC tables are getting people out of the water safely most of the time then there is something to be said for that. Its not recommended to push the tables to their limit, but rather than removing them and putting extra stops and things into them, why not keep what we've got but tell people "its good to add extra stops to this if you've got the gas / time to do it". Then you're giving divers the tools to know what is likely to get them out of the water safely, ie the current BSAC tables, but they know that if they want to feel good at the end of a dive, and they aren't decoing in dodgy conditions and have plenty of gas left that they can add in extra padding themself. Comes back to what you're saying about giving people the options, the understanding to choose themself rather than being told what to do.

currently, until you've done an ERD or tekkie course, within BSAC's official teaching, there is NOT A WORD about deep stops. And it doesn't take a PhD to understand them.

Definitely worth mentioning to divers at SD level and above, but we can't just pick random theories to throw at people - not officially anyway. What is there to say that Pyle is better than anything else out there? Why not some of the modified models used in computers? Why not the tables GUE or other agencies use? There's a lot of different models out there, the feel-good-factor you get from one particular model isn't a solid basis for changing what we teach. For BSAC to do that without a very good basis could be considered negligent, in the end.

There was some recent discussion about teaching things like deep stops. As I remember the consensus was that it was worth mentioning to divers who are getting into deeper dives, but not as part of BSAC teaching officically. It was also pointed out that most of what we know about deep stops etc has come from other sources, often unverified, claims on the internet or whatever. Is that really the best basis for teaching people about something which could kill them if they get it wrong? How do we know how much of the real research has been "lost in translation" so to speak, or omitted from people's understanding? If we don't know that, how can we teach it to people? To do it properly needs proper materials and support behind it, with a verified amass of evidence and testing of the theories - i'm guessing thats probably what you want, and I agree that a review of BSAC deco teachings is due (and maybe already in progress) given that they've been around since 1988, its just the reasons and logic you gave that I was wondering about.

"newest = best"

You actually said "latest and most likely most accurate", I was paraphrasing... I don't think my summary is significantly different from what you said?

David

Gareth

26-02-2006, 08:12

Just want to pointout that the BSAC88's do have a conservatism factor.

There are several ways.

1. Move up a level i.e. use A/2 instead of A/1.
(very like early suunto's)
2. Move up a table - use a tissue code table mov
re conservative. i.e use B in place of A. C inplace of B etc.
3. Always plan the dive as if it was the next deepest & /or longest.

The 'user' can select which factor he/she wants to apply!

Gareth

David_de_Marneffe

26-02-2006, 22:03

But is that a good basis for changing the tables? Reading around a few diver's comments is not a scientific basis, much of it may be placebo effect and the like. I'm not saying that anything you say is wrong, i'm just questioning the scientific basis behind it...

I was not just speaking of diver's comments. Want scientific evidence? Read this: http://www.decompression.org/maiken/VPM/VPM_Program_Site_Map.htm and this: http://www.hhssoftware.com/v-planner-cde/vpmdetail.html More than 30 years of research... That may not be sufficient to you as a basis for change. It is to me and to reputable agencies like IANTD, NAUI, DAN research, the Danish Maritime Authorities and the major dive computer manufacturers. I think it should be to BSAC.

if we're going to endorse any kind of tables, I think most people would like to know that there is some kind of evidence behind it other than speculation and "well it seems better" type comments from divers who've used it

a lot of divers don't want to know about deco theory. They don't want to be told that there are 10 different algorithms / tables, and that they have to evaluate them all to decide which is safest. At much higher levels that may be appropriate, but the average SD or even DL does not want to have to do a PhD in Deco Theory to work out what is safe. They want someone to tell them which model is known to work well

Contradicting yourself, mate? And what more evidence is there behind the '88 tables than behind VPM? All I see is people feeling bad or getting bent when they dive Buhlmann type of profiles (so badly that they sometimes end up in a wheelchair, like Pyle) and divers hearing the bubbles inside themselves when tested with Doppler. So far, there is no report of such things with VPM. And the theoretical basis sustains the experience: neo-Haldane models simply DO NOT consider free phase gas, and that is WRONG because it is present. There is no denying that fact.

It may be physics that it happens.... the rate at which it happens (and therefore the potential damage it can cause) varies massively between these things you list though. The deco schedules for different people, never mind different animals, would, if you could measure them, vary enormously. Remember how different tissues take up gases at different rates, fast/slow tissues? Well what is fast in one person may not be quite as fast in another - thats why there is no single model which can possibly be described as accurate for all divers.

Correct. Hence the various conservatism settings of modern algorithms / computers... and the absence of these in tools like the '88 tables.

Not strictly true - even "more complete" doesn't necessarily make them better / more accurate. Its a fundamental principle of almost any kind of modelling: the simpler the model the better (to a point). I don't know though whether that point of diminishing returns happens to be before or after bubbles have been included. However, the fact that they are included does not automatically make the model better. In many cases it will make it worse. I think the problem is that currently a lot of people doing Pyle stops for example add these as additional to their normal deco, which means they're naturally doing slower ascents, more stops, more time in-water, and so will likely feel better after the dive. That doesn't however mean that Pyle is more accurate. In fact, I could come up with the new David's Deco Theory (DDT) which is basically the same as the BSAC tables but for any given dive, you use the BSAC tables to your depth + 10m. So if you do a 20m dive, read the tables as if you did a 30m dive and thats the outcome of DDT. I'm sure I could do a lot of research to prove that mine is safer, but is it better? On what basis have I created my theory?

Models that completely overlook the free gas phase are definitely not a better description of reality than those which do. Concerning the benefits of diving the resulting profiles, read previous posts and follow links above. "In Many cases, it will make it worse". Examples, please? Your DDT is based on nothing. Bubble theories are based on multi-phase fluids mechanics. Note that Pyle, as he says himself, is not a decompression theorist. He is simply a diver who was getting all the symptoms described previously when diving "classic" profiles (including getting bent so badly that he temporarely ended up in a wheelchair). He pioneered the inclusion of deep stops into his dive profiles. The story of how he discovered their benefit is amazing. Read his famous article here: http://www.bishopmuseum.org/research/treks/palautz97/deepstops.html

Definitely worth mentioning to divers at SD level and above, but we can't just pick random theories to throw at people - not officially anyway. What is there to say that Pyle is better than anything else out there? Why not some of the modified models used in computers? Why not the tables GUE or other agencies use? There's a lot of different models out there, the feel-good-factor you get from one particular model isn't a solid basis for changing what we teach. For BSAC to do that without a very good basis could be considered negligent, in the end.

The bubble models are not just one of ten completely different models. As I said before, there are basically two types of models:
a) neo-Haldanian models (Buhlmann, etc) which (wrongly) do not consider free gas phase but only dissolved gas
b) bubble models which consider both.
Last time I knew, there were only two bubble models: VPM and RGBM and they give virtually identical results: all that differs is the way they run their calculations. RGBM is actually a commercial algorithm, mostly "stolen" from VPM, which is public domain. VPM-B is the latest version of VPM, improved for very deep dives. Follow links above for more info.

if BSAC tables are getting people out of the water safely most of the time then there is something to be said for that
But what is safely? I say again: if you feel bad after diving, if you sleep like a dead, it's your body complaning that something is wrong. There is very strong suspicion of long term damage caused to the body by bubbles.

its good to add extra stops to this if you've got the gas / time to do it
Why not: "you should plan for the gas / time to do extra stops, which are good for you"?

As I remember the consensus was that it was worth mentioning to divers who are getting into deeper dives, but not as part of BSAC teaching officically
50m IS deep, and I think that going straight from 50m to 9m for 1min and 6m for 6min (20 min dive) all on air is no good for anybody.

It was also pointed out that most of what we know about deep stops etc has come from other sources, often unverified, claims on the internet or whatever. Is that really the best basis for teaching people about something which could kill them if they get it wrong? How do we know how much of the real research has been "lost in translation" so to speak, or omitted from people's understanding? If we don't know that, how can we teach it to people? To do it properly needs proper materials and support behind it
Follow links above, with references to all the scientific publications on the subject. Why would it be irresponsible for BSAC to do that if it's not for all the agencies mentioned above?

I agree that a review of BSAC deco teachings is due
Well, at last, we agree on the essential!

Nigel Hewitt

27-02-2006, 08:58

BSAC 88s *@#=$£&!! *bangs head*

Today in the email I get a question from an established club member who has been diving years longer than I have but has only recently moved up from single tank short stop diving.

"Why do all these software programs give me 15 to 20 minutes of stops for 20 mins at 39 meters when '88s only ask for a minute?"

Now I want to be reasonably loyal to the BSAC brand but he's a friend. I have to tell him that 88s for deco are so close to the limit I wouldn't touch them. They make US Navy look conservative. The reason you get out feeling trashed after an '88 style deco is because you are bent.

88s date from the days when bent meant writhing on the floor and real men didn't complain about a little niggle in their shoulder. BSAC divers don't do denial about the symptoms of being bent - they have been told that's what to expect from deco diving.

Sorry. I know the thread had gone elsewhere but this is a sore point.

MattS

27-02-2006, 11:18

"Why do all these software programs give me 15 to 20 minutes of stops for 20 mins at 39 meters when '88s only ask for a minute?"

Now I want to be reasonably loyal to the BSAC brand but he's a friend. I have to tell him that 88s for deco are so close to the limit I wouldn't touch them.Nigel, what happens to the Buhlmann stop schedules when you lower the altitude range to that of 88 L1? Buhlmann's sea level altitude range is about twice that of the 88 L1 table - he was Swiss, they have a lot of mountain lakes. Reduced surface pressure has a startling affect on deco times although it is often overlooked.

They make US Navy look conservative. The reason you get out feeling trashed after an '88 style deco is because you are bent.

88s date from the days when bent meant writhing on the floor and real men didn't complain about a little niggle in their shoulder.Perhaps I am a little unusual these days in that on my path to technical diving I was completeing dives to the limits of the 88s decoing on air in the 30 to 40m range. Certainly it left me feeling very crap and my buddy almost gave up diving because of sub-clinical symptoms.

Personally, slowing the ascent rate to 10m/min helped a great deal - for a while. Pyle stops improved post dive symptoms further. Hi-FO2 deco in conjunction with the slowed ascent rate and Pyle stops saw the end of post dive symptoms completely. Not scientific but good enough for me.

It took a lot of scratching around to find the information at the time. I do find it rather frustrating that our new entrants are still reaching for the Red Bull post dive. It does not have to be that way as most of the current generation of instructors appear to acknowledge.

Considering just how rough decompression science is I don't think we need to wait around for the physiologists to catch up. As far as I can tell decompression hypothesis has always followed diver observation all the way back to JS.Haldane. There are enough plausible theories to explain how slow ascents, deep stops and shallower final stops help to reduce subclinical DCI. Personally I think BSAC has a duty to start passing the information on in a structured manner. At the moment it is left to informed instructors to try to fill in the gaps.

Perhaps it would be interesting to see what a poll throws up. Do the membership think a change is overdue?

David_de_Marneffe

27-02-2006, 11:31

Matt, Nigel,

Happy to see I am not the only one to think this way. Been battling with David Walker for three days now, so a little support is most welcome!

Cheers,
David

Vic

27-02-2006, 12:10

I have come to think that the way deco theory is taught within BSAC, as well as the BSAC tables are now out of date. Indeed, there is strong evidence (both theoretical and experimental) that the neo-Haldane models overlook important parts of the physical and physiological phenomenons that occur during a dive. The most obvious experimental evidence is the number of divers affected with pains, aches and tiredness for many hours after diving, despite the fact that they dive "perfectly" according to the tables or their computers, including safety stops.

And what number would that be?

AFAIK, this sort of thing isn't recorded; although you'll get plenty of apocryphal stories, I generally consider it a bad idea to base my decompression procedures based on what I heard from a bloke in a pub[1].

Looking at this and knowing that VPM-B is the latest and most likely most accurate decompression model, don't you think that the BSAC 3mins at 6m are grossly underestimated?

Oh, I can't believe you said that.

If you believe that "new == better", just take a look at VPM-A. VPM-B only occurred because VPM-A was bending so many people.

If you really want to find out about decompression theory, get yourself on the decompression.org mailing list (I think it's still going, although I've not had time to read it for some time). You'll talk to loads of extremely knowledgeable people - the people who develop the algorithms that we all talk about. One message comes through clearly from this - there is no "right way" to do decompression. It is all approximation.

1) At the basic level, modify the contents of the Sport Diver / Dive Leader trainings regarding decompression theory

To what? Although there are passionate proponents of any particular scheme you could come up with, the people developing those schemes will tell you that they are all experimental. "Nobody knows" is a mantra you'll often hear repeated in such circles.

2) Offer an "Advanced Decompression Theory" course as a skill development course, available well before Extended Range Diving, and induce divers who want to practice deco diving to follow it. It could even be made mandatory

It would make a very interesting talking shop (well, it interests me, anyway). But there is little, if any, fact to be worked on; deco procedures are belief systems, and no more.

There you go. I hope I wasn't out of place by suggesting this. But personally, knowing what I know, I definitely won't dive anymore the way I did when I just qualified as a Sport Diver.

There is a great statement - "a wise man knows what he does not know". I'd heartily suggest a bit more research before making recommendations about deco theory - there is a *vast* amount to learn, but once you've trawled through it all, the only thing you'll come out with is that there is no such thing as certainty when it comes to decompression...

Vic.

[1] Hactually, I've learnt loads about deco theory from certain blokes in certain pubs. The trick is to be very selective, both in terms of the pubs you drink in, and the people to whom you speak...

Vic

27-02-2006, 12:18

Last time I checked, on- and off-gassing happen in every diver. Also in goats, guinea pigs, any living creature and even inert stuff like rubber and all solid and liquid material. That's called physics. So is the case for bubble physics: it applies to every diver.

That's true. The physics applies to every diver.

Do you know how to do those physics? Because nobody else does. Everyone else is making their best stab at it - but no-one claims to know what's going on; these are models, not simulations.

In this sense, the bubble models are more accurate because they take bubbles into considerations, AS WELL AS on- and off- gassing.

So a model is more accurate because it takes more things into consideration? Rubbish.

I've mentioned VPM-A in another post, but I'll do it again. VPM-A took bubbles into consideration. It proved significantly worse at modelling decompression than, say ZHL-16B. Your argument is fatally flawed.

The Haldanian models don't do that. What differs is the consequences of these phenomenons for a given diver. That's why modern algorithms and modern computers allow to set a conservatism factor: to suit as many people as possible. Not the case for older computers. Not the case for the '88 tables.

Wrong, I'm afraid. There are a number of "extra conservatism" procedures for using the '88s. That you don't know them is not an indictment of the tables...

The bubble models ARE better

You have absolutely no evidence to support that statement.

closer from reality

You have absolutely no evidence to support that statement.

Once properly educated, they can make an INFORMED DECISION about what they want to do

That would be an admirable goal. But it's never going to happen - some of us devote large amounts of time to learning everything we can about deco theory. The thing we come back to, time after time, is that Nobody Knows. Expressions of support for any particular algorithm are more about faith than science

Vic.

David Walker

27-02-2006, 12:21

Been battling with David for three days now, so a little support is most welcome!

Have you? Sorry I should have paid more attention to this battle i've been in then!

As I said many times in my posts, I am NOT saying that BSAC tables are good, i'm NOT saying that there are not better tables / algorithms out there, but what I am saying is that if you want to persuade BSAC to change their system you're going to need some better evidence for that than what you put in your letter. To anyone reading your letter it comes across as someone who's been reading about Pyle stops on the internet, tried them, and found they seem to work - that won't get a great deal of attention, they've heard this all before i'm sure. They can read the same websites and forums that you do... I'd just hope that the information BSAC use to make their decisions comes from a reputable source - no matter how much information the internet has, very little of it has any kind of credibility behind it.

David

David Walker

27-02-2006, 12:24

Vic

Happy to see I am not the only one to think this way. Been battling with David_de_Marneffe for three days now, so a little support is most welcome!

... hehe, sorry couldn't resist ;) :D:D:D lol

Cheers,
David :rolleyes:

Vic

27-02-2006, 12:42

Want scientific evidence? Read this: http://www.decompression.org/maiken/VPM/VPM_Program_Site_Map.htm

I've spent quite some time talking to Erik Maiken (and he is actually one of the most approachable deco theorists I know). If you asked him if VPM is right, he'd tell you that he believes so, not that it definitely is.

Dave Yount would have said something similar (you can't ask him because he died some years back - a great loss to us all).

I'm sure Tom Kunckle (sp?) would say something along the same lines, but he's not someone I've had dealings with personally.

And what more evidence is there behind the '88 tables than behind VPM?

Lots! Henessey's work goes back a very long way, and there has been quite a bit of testing done on the '88s in one way or another.

I'm not sure about the '88s place in modern diving - but that's more because people don't understand them than because they might be erroneous. They are by far and away the easiest dive-planning system I've ever used (zero calculations!). I'd like to see them remodelled with a slower ascent rate, and I'd like to see deeper stops coming out of them, but that's just because of my own belief system.

All I see is people feeling bad or getting bent when they dive Buhlmann type of profiles (so badly that they sometimes end up in a wheelchair, like Pyle)

Quoting Pyle is a very bad way to win an argument - he readily admits than he frequently did things that were beyond stupidity and verging on suicidal. He has used his experiences to help others, but that doesn't for one moment mean that he has known what he was doing (nor even that he knows now!).

and divers hearing the bubbles inside themselves when tested with Doppler. So far, there is no report of such things with VPM.

No reports of Doppler scores for VPM? You're reading the wrong literature...

And the theoretical basis sustains the experience: neo-Haldane models simply DO NOT consider free phase gas, and that is WRONG because it is present. There is no denying that fact.

That may be so, but do you understand the significance of free-phase gas? Because I don't. And nor does anyone. Nobody Knows.

Models that completely overlook the free gas phase are definitely not a better description of reality than those which do.

How do you know?

None of these models accurately simulate what really goes on in our bodies - nor do they attempt to. They are there to model decompression and get us out of the water as safely as is possible. So to compare models, you need statistical data comparing the bends incidence - do you have that? You also need an unambiguous and testable definition of what constitutes a bend - do you have that?

Bubble theories are based on multi-phase fluids mechanics.

What they are not based on is knowledge of what goes on in the human body.

As I said before, there are basically two types of models:
a) neo-Haldanian models (Buhlmann, etc) which (wrongly) do not consider free gas phase but only dissolved gas
b) bubble models which consider both.
Last time I knew, there were only two bubble models: VPM and RGBM and they give virtually identical results

They actually give very different results. VPM, for example, gives you great credit for removing surfactants - e.g. by a bounce to >100m. RGBM does not, and will hit you heavily for such behaviour.

But you appear to have omitted one very popular bubble model - the BSAC '88 Tables.

Follow links above, with references to all the scientific publications on the subject. Why would it be irresponsible for BSAC to do that if it's not for all the agencies mentioned above?

It would be irresponsible for anyone to portray as fact that which is actually just belief. We have to recognise what we do know, and what we don't.

Vic.

MattS

27-02-2006, 13:53

As I said many times in my posts, I am NOT saying that BSAC tables are good, i'm NOT saying that there are not better tables / algorithms out there,Does it not depend on what you are using the table for? As Vic says, the 88s are one of the best tables for ease of use and teaching purposes. The risk of DCI (including sub-clinical) within no-decompression limits (anyones) are sufficiently small as to make any percieved inadequacy of the table of little practical concern. Ignoring any inadequacy that there may or may not be with the 88s, they are not particularly suitable or simple for the mixed gas, extended bottom time and rebreathing facets that are now so common in the decompression diving seascape.

but what I am saying is that if you want to persuade BSAC to change their system you're going to need some better evidence for that than what you put in your letter. To anyone reading your letter it comes across as someone who's been reading about Pyle stops on the internet, tried them, and found they seem to work - that won't get a great deal of attention, they've heard this all before i'm sure.So how many divers does it take before BSAC will pay attention? Unfortunately we can not expect all divers to make their arguments accurately or even logically. They are divers not scientists. However it is divers making the dives and we should not ignore their first hand experience.

You remember BSAC - Leading by experience. Experienced divers, particularly those operating beyond the traditional limits of 1988, are throwing out the 88s as soon as they find an alternative. BSAC are spending time teaching an increasingly irrelevant table and the membership are not thanking them for it. The only people that seem to be happy with this situation are those that have not looked further than BSAC for their decompression guidance.

They can read the same websites and forums that you do... I'd just hope that the information BSAC use to make their decisions comes from a reputable source - no matter how much information the internet has, very little of it has any kind of credibility behind it.Many of the active divers on Council and NDG sport VR3s and/or use Pro-Planner. The rest are carrying Suuntos, Aladins or whatever. The coaching teams appear happy to admit that they do not dive 88s on anything more adventurous than training dives. Personally the current "do as I say not what I do" policy lacks credibility.

It must have been Council/NDG that invited JP.Imbert along to DOC a few years ago. I think he is pretty credible and his oppinion was quite clear; to paraphrase the explanation which I suspect went over the heads of most of the audience - 10m/min ascent rate, deep stops, last stop shallower than 6m are beneficial additions to traditional decompression practise. Even chamber staff with their rather traditional and conservative views are now supporting these modifications. We accept the importance of teaching good hydration and many other factors not encompasssed by 88s, so why not these things as well? My main concern is that we are failing to teach accepted good practise as a matter of course.

You will not hear me criticising the work of Hennessey, he is a clever man in this area. I may criticise his beligerance in accepting that diving has moved on. As an organisation I believe we are letting that beligerance cloud our judgement. There is a better way, most of us know there is a better way. We should be teaching that better way IMVHO.

David_de_Marneffe

27-02-2006, 14:00

OK. I won't start again replying in detail to everything because I have a life out of this forum. Basically, I said what I had to say. The point is: if people are not presented with all possible "belief systems", how can they make up their mind about what they want to believe? Even if I gave up on the idea of promoting deep stops as being the right thing to do (which is my belief, as you call it, and the one of more and more people), I can't possibly admit that it is the right thing to do to not even mention them during training. And unfortunately, that is what BSAC does at the moment (at least in the "official" training).

MattDuke

27-02-2006, 15:04

The dive was at 30m. Sorry for the typo. If you read V-planner's manual, conservatism +2 (which is the default) and +3 are for average people. 0 and +1 are for the ultra-fit. +4 and +5 are for people with a history of DCI or other problems like PFO, overweight, etc
Cheers,
David

Hello David,

On the Vplanner settings you mention, I'm not sure I would agree with your conclusions on the use of +4 conservatism. From the manual:

Set to plus 3 or 4 with strenuous, cold, a series of multi day dives, extra safety, or a prior history of DCS or symptoms.
This setting increases the Critical Radii of N2/He inside the VPM algorithms, by this increase: 1 = 5%, 2 = 12%, 3 = 22%, 4 = 35%. The Critical Volume setting is on by default.

Which I would think means most divers using VPM-B in cold temperate water, in high current, would want to go at least with +4, not just if they are overweight and/or bent previously.

Just a little note, as you made it sound like +4 was for divers with a specific phyisilogical issue, rather than making plans to suit the dive environment itself. I believe you misquoted the manual, and as the bits you missed out are of high importance to divers in cold water. (Most BSAC members, but not you!) this could be misleading to divers reading the boards. Therefore I thought I would post.

A also think you are a mis-guided if you feel VPM-B is "Best" as it is newest. It is early days to conclude on that. Just ask the divers who dive on VPM-A. Ouch!

But I would agree that one can find what is "Best" by actually diving tables and feeing the results for themselves. But at the same time, we must not lose touch with the fact that we are sports divers, often with no life insurance for trimix diivng.

Therefore, what factors influence your choice of deco?

- Gas volume?
- temperature?
- RISK?

I liked Gareth's post about adding conservitism to the BSAC 88 tables. (Altitude or CTC adjustment) I would also point out that most dives done on VPM-B have been runtime dives. That means we dive a depth and a BT. Many dives are not done on a square profile. Take a look at the attached. The plan was 60M 20minutes Run Time.

DEPTH:
Apart from the end, when I was freeing the shot, I didn't actually stay at 60M.
RUN TIME:
I also left the wreck 1 minute early at 19minutes. (As always, I leave 1 minute for the unexpected)
DECO:
I did an additional 8 minutes at 6m on EAN80 (As I was warm) and then used a full 6 minutes from 6M to the surface.

When looking at my VPM-B dives, pretty much all of them are the same. Conservative Depth adjustment, Conservative Run Time, Conservative Deco. I am NOT alone! So therefore, I find it hard to advise on VPM-B, as I am diving well inside it, just like I did Proplanner, Just like I did on BSAC 88 tables.

Now that the VR3 is offering VPM-B support, and as divers invariably will dive "Up to a computer limits" I will be interested to see how people get on. Maybe we will read up on VPM-C in 2007?

So whatever program you choose, don't forget tool #1. The lump inbetween your ears. :D

The best quote I've heard on Deco, is: "It's all bollox." I try and keep that in mind, as I hang at 6M.

Safe Diving.

Matt

Chris Cherrington

28-02-2006, 09:17

Anyone got any idea what it would cost to revamp and re calcualte the 88's??

The Buhlmann tables are widely available and would cost nothing so replacing the 88s with them is the best option.

The "cost" is in changing all the training and re-training all the instructors.

Cutting new tables is not an option.

Chris

Nigel Hewitt

28-02-2006, 11:53

The Buhlmann tables are widely available and would cost nothing so replacing the 88s with them is the best option.
The "cost" is in changing all the training and re-training all the instructors.Do we then have the question of what and how much is the 'official' conservatism or do we just print 'raw' Bühlmann and teach that this is the limit and you add some conservatism yourself on every dive?

I do like the idea that a beginner looks at a set of tables as a 'must do' time and knows that longer is better. A safety stop being a 'longer is better' on a table 'no stop' dive. Provided you have the gas I rate slow ascents and dragging out the shallowest stop as a good idea.

David_de_Marneffe

28-02-2006, 15:21

On the Vplanner settings you mention, I'm not sure I would agree with your conclusions on the use of +4 conservatism. From the manual:

Set to plus 3 or 4 with strenuous, cold, a series of multi day dives, extra safety, or a prior history of DCS or symptoms.
This setting increases the Critical Radii of N2/He inside the VPM algorithms, by this increase: 1 = 5%, 2 = 12%, 3 = 22%, 4 = 35%. The Critical Volume setting is on by default.

Which I would think means most divers using VPM-B in cold temperate water, in high current, would want to go at least with +4, not just if they are overweight and/or bent previously.

Just a little note, as you made it sound like +4 was for divers with a specific phyisilogical issue, rather than making plans to suit the dive environment itself. I believe you misquoted the manual, and as the bits you missed out are of high importance to divers in cold water. (Most BSAC members, but not you!) this could be misleading to divers reading the boards. Therefore I thought I would post.
Fair enough Matt, thanks for correcting me on this.
A also think you are a mis-guided if you feel VPM-B is "Best" as it is newest. It is early days to conclude on that. Just ask the divers who dive on VPM-A. Ouch!

But I would agree that one can find what is "Best" by actually diving tables and feeing the results for themselves. But at the same time, we must not lose touch with the fact that we are sports divers, often with no life insurance for trimix diivng.

Therefore, what factors influence your choice of deco?

- Gas volume?
- temperature?
- RISK?

I liked Gareth's post about adding conservitism to the BSAC 88 tables. (Altitude or CTC adjustment) I would also point out that most dives done on VPM-B have been runtime dives. That means we dive a depth and a BT. Many dives are not done on a square profile. Take a look at the attached. The plan was 60M 20minutes Run Time.

DEPTH:
Apart from the end, when I was freeing the shot, I didn't actually stay at 60M.
RUN TIME:
I also left the wreck 1 minute early at 19minutes. (As always, I leave 1 minute for the unexpected)
DECO:
I did an additional 8 minutes at 6m on EAN80 (As I was warm) and then used a full 6 minutes from 6M to the surface.

When looking at my VPM-B dives, pretty much all of them are the same. Conservative Depth adjustment, Conservative Run Time, Conservative Deco. I am NOT alone! So therefore, I find it hard to advise on VPM-B, as I am diving well inside it, just like I did Proplanner, Just like I did on BSAC 88 tables.

Now that the VR3 is offering VPM-B support, and as divers invariably will dive "Up to a computer limits" I will be interested to see how people get on. Maybe we will read up on VPM-C in 2007?

So whatever program you choose, don't forget tool #1. The lump inbetween your ears.
Fair enough as well. It's true that people tend to dive "within" the limits when they "plan the dive and dive the plan", and they tend to dive the limits of the models if they have a computer. I would be interested to see experimental / statistical data about VPM A/B, or even single (bad) dive reports. Do you have any link? Further developments of VPM: yes, I would even hope so. All models that are a mix of theory and experimental data need continuous adjustment as more data becomes available... I also just found this: http://www.inspired-training.com/decochek.htm this software seems to troubleshoot the plans generated by popular algorithms based on experimental data (but it is unclear to me how exactly they do that, based on what data)
The best quote I've heard on Deco, is: "It's all bollox." I try and keep that in mind, as I hang at 6M.
Well, one may think that. But if the theory is all crap. then we need sufficient statistical data. This means: for every single possible dive profile, we need thousands of dives, and be able to fully analyze the harm done by the profile in question to the diver (both long term and short term). This is simply impossible to obtain. That's where the theory has to come in support. Basically: have a theory as good as can be (even if that's not very good), and adjust it as experimental data becomes available...it's simply the only way. Quote from Eric Maiken: "In the laboratory, a current focus in decompression modeling involves formulating ascent schedules that report the statistical confidence in a certain probability of decompression illness (DCI). For example, a schedule might state that you can be 95% sure that the profile will have a bends incidence of 2%. Statistical models form reliable predictions within the "known." However, without a fundamental model, statistics cannot be used to extrapolate to untested regions (new combinations of depths and bottom times, gas mixtures and switches, etc)."

Cheers,
David

David Walker

28-02-2006, 15:43

Out of interest, am I right in thinking that there was never any detailed published information about how the BSAC tables were put together?

David

Vic

28-02-2006, 16:07

Out of interest, am I right in thinking that there was never any detailed published information about how the BSAC tables were put together?

Depends how you define "detailed"... :-)

The algorithm was indeed published; however, it relies on a number of parameters which AFAIK were not. One day, I'll get around to reverse-engineering them.. One day...

Vic.

David Walker

28-02-2006, 16:12

The algorithm was indeed published; however, it relies on a number of parameters which AFAIK were not. One day, I'll get around to reverse-engineering them.. One day...

Oh... kind of a "the algorithm used is a+b-c/d x log e^-f, where a,b,c,d,e,f = ?"

Sounds good.

In general, I just meant do we know what kind of factors it includes, that kind of stuff. But i'm guessing we do if we got an algorithm (even if its incomplete)

David

Vic

28-02-2006, 16:18

Oh... kind of a "the algorithm used is a+b-c/d x log e^-f, where a,b,c,d,e,f = ?"

Far more than that.

It's a multi-compartment model with both dissolved and free-phase inert gas in each compartment.

There is a matrix of parameters to define the migration of gas between compartments and phases - that's the bit I don't have. It would be possible to generate those parameters - we have a fairly large set of output data, after all. But I haven't...

In general, I just meant do we know what kind of factors it includes, that kind of stuff.

Yes, we do.

But i'm guessing we do if we got an algorithm (even if its incomplete)

Indeed.

Vic.

Nigel Hewitt

28-02-2006, 17:09

It's a multi-compartment model with both dissolved and free-phase inert gas in each compartment.

There is a matrix of parameters to define the migration of gas between compartments and phases - that's the bit I don't have. It would be possible to generate those parameters - we have a fairly large set of output data, after all. But I haven't...Interesting.
But I would have thought that any gas flow model would still equate to a simple compartment model. Can a gas transfer be modeled in an non-linear way? Flow is proportional to tension difference and hence transfer through a set of compartments should resolve to a single equivalent compartment with a changed time constant.

Either way... do you know how it models the equivalent to Bühlmann's a/b factors? That always seems key to me to what stops are generated. Compartments are just a matter of having enough and close enough together.

Go simply as the only model I understand in any detail is the one from Tauchmedizin.

Philip Smith

28-02-2006, 23:51

The algorithm was indeed published;

Hi Vic, where was the algorithm published?

Phil S

David_de_Marneffe

01-03-2006, 12:06

Vic

01-03-2006, 13:26

where was the algorithm published?

A quick Google (I don't have any info with me...) leads to :-

Hennessy TR 1988 Modelling human exposure to altered pressure environments. In: Environmental Ergonomics (eds Mekjavik, IB, Banister, EW, Morrison, JB,). London: Taylor & Francis.

AFAICT, this is not available on-line :-(

Vic.

Vic

01-03-2006, 13:29

Nigel Hewitt

01-03-2006, 13:52

Interesting.
But I would have thought that any gas flow model would still equate to a simple compartment model.I initially thought something similar, but TBH, I got rather side-tracked before I'd thrashed out the maths, and the whole project got a little bit forgotten...My first guess would be that the flows are linear (flow proportionate to tension difference) so it's a simple integral so any linked list of compartments the last compartment could be modelled with one compartment.Either way... do you know how it models the equivalent to Bühlmann's a/b factors?
That always seems key to me to what stops are generated.
Compartments are just a matter of having enough and close enough together.Not off the top of my head! It's been a couple of years since I even looked at it.

This is more your bag than mine - are you seriously interested in this? IOW, is it worth the couple of hours it'll take me to dig out my copy of the paper?Sadly I don't have any free time at the moment but if the paper does appear put it somewhere memorable. This was big stuff in it's day and I think the main thing that has moved is our perception of what is bent. 88s delivered what they promised but we want to be a bit further from the grey area today.

For example with my medical history I can't afford to take risks. Not that I believe I'm in danger but I'm quite sure that if I go to renew my UKSDM and say "Oh and I got chambered after a 60m dive" I could easily push the doctor over the line and pull my diving or put restrictions on me that would effectively do so.

David Walker

01-03-2006, 14:01

Hennessy TR 1988 Modelling human exposure to altered pressure environments. In: Environmental Ergonomics (eds Mekjavik, IB, Banister, EW, Morrison, JB,). London: Taylor & Francis.

AFAICT, this is not available on-line :-(

An abstract is available at http://www.lboro.ac.uk/departments/hu/groups/EEC/ICEE/textsearch/1986/Hennessy-1986.pdf

Other than that, doesn't seem to be available even in online journals through our uni subscriptions / collections... :(

David

Bob Elliott

01-03-2006, 14:15

Hi

An interesting point that some of you who were not around in 1988 as divers may not know is that little or no research was carried out on th 88 tables before launch. They were infact trialed on the membership and with certain dives found to be lacking, an NDC report was issue to amend certain details on the tables before a reprint was issued.

Bob Elliott

Paul Morris

01-03-2006, 17:14

I wan't around (well, diving wise) in 88, but were the NDC bulletins related to corrections due to printing errors or actual changes to the model.

I suspect the former, as a couple of years ago my set of Nitrox tables came with a paper insert advising me to make certain corrections to the tables, which clearly were misprints. The later print runs are now correct.

Paul.

David_de_Marneffe

02-03-2006, 14:19

Hi again,

To follow up on my previous posts and the claims of some about the lack of experimental data related to bubble models, here is a link to an article by Wienke about the field testing of his RGBM model. Start on page 3 and read through page 6:
http://www.rgbmdiving.com/site/rgbmhis3.htm

Cheers,
David

Philip Smith

02-03-2006, 20:45

A quick Google (I don't have any info with me...) leads to :-

Hennessy TR 1988 Modelling human exposure to altered pressure environments. In: Environmental Ergonomics (eds Mekjavik, IB, Banister, EW, Morrison, JB,). London: Taylor & Francis.

Thanks Vic. I had a previous note of this article, but I haven't seen it and it's not clear from the abstract David found whether it relates specifically to the BSAC88 model.

Phil S

Sue Mitchell

09-03-2006, 20:31

MattDuke

10-03-2006, 09:05

Hi Matt

just looking at your run times there - you are comparing a 21minute dive time on BSAC88 tables where the 21mins includes the ascent to 9m, against 21minute bottom times on two other programs where 21mins is the time you leave 50m.
so, just to compare like-for-like (as far as one ever can....)

the run time to leave the bottom at 21 mins on BSAC 88 would mean you get to 9m at 24mins IF you do the maximum ascent rate and then 1min at 9m at 12minutes at 6m and 1 minute to the surface (total of 38mins)

If you choose to ascend at less than 15m per minute, you would move along to reach 9m by either 25min or 26min (no later than this, its the end of the table!:eek: ) when you could do up to 3min at 9m, 18min at 6m and a minute to the surface. (total of 48mins)

the ascent rate is a MAXIMUM of 15m per min, and you can choose to ascend slower than this (as the dive time ends when you reach the first stop depth) unlike tables where the ascent rate is critical to the deco time at each stop. (like the previous RNPL tables, for example and many of the computer programs). This choice (to use a slower ascent rate, so long as you reach your check depth within the dive time) is a point we emphasise when teaching how to use the BSAC88 tables in my branch.
we also emphasise not to dive the tables to the limit. The dive you suggest is pushing the tables near the limit, and we would recommend an alternative plan (could be a different dive or use a different planning tool and/or deco gas mix)- afterall, you can't even plan for a worst-case just exceed your max depth scenario, which even ocean divers are taught is good dive practise! :)

As I said, this is to just trying to make the comparision more like for like ... Obviously its still a much shorter run time than the 92mins or 98mins on your two programs, but not quite as short as the dive time vs bottom time comparison initially posted. :)
I'm not contending your point that decompression theory has moved on considerably in the last 15-20 years, that more modern algorithms take into account. I suspect the differences are more pronounced for deeper dives with longer times beyond the no-stop limits (since the no-stop times are not all that different to my dive-computer planner option, though deco stops can be quite different!) and that these days, most divers who are doing much deep diving with long stops tend to use other planning tools anyway - inlcuding computers and nitrox deco mixes.

At the end of the day, deco theory is just that - theory. In practise, it never works ideally as there are so many other stressors that affect the system. It would be interesting to see if there were any stats on numbers of bends without any obvious cause where divers were diving on 88 tables and within the decompression boundaries set for long deep dives compared to the total numbers of dives using the 88 tables and the same profiles ... it would go a long way towards knowing whether it is worth developing a new set.

cheers
Sue

Hello Sue. Earlier on in the thread we discussed the differences between "88" BT and a runtime "BT", it of course does effect things somewhat, & so I agree with your sentiments.

The BS-AC drops the 88 tables at the ERD level, as they are not designed for extended decompression diving. At this stage divers are asked to "Re-learn" other tools, like Proplanner & V-planner. You can get a free test of V-planner at www.v-planner.com (http://www.v-planner.com), and have a look for yourself. Well worth a quick look. V-planner is based on VPM, which is also an option now on VR3 (Proplanner Computer from Kevin Gurr et al) Then we can also look to companies like Suunto who are implementing an RGBM based solution on their D9.

So things are changing, and new tools are available.

My feeling was that we should look at these other options and evaluate them for the club. It was not my intention to shine too negative a light on the 88 tables. Tom Henneys's work kept me safe on many a dive in the early ninties. :)

I'm just a simple member. I've done a bit of diving, and I'm in no position to give any advice on decompression theory. Just as a "Spectator" I notice things seem to be changing.
As way of example, lets look at a 15minute Vplanner BT (Leave bottom & 15m)

Dec to 50m (3) Air 15m/min descent.
Level 50m 12:40 (16) Air 1,25 ppO2, 50m ead
Asc to 24m (18) Air -9m/min ascent.
Stop at 24m 1:07 (20) Air 0,71 ppO2, 24m ead
Stop at 21m 2:00 (22) Air 0,65 ppO2, 21m ead
Stop at 18m 2:00 (24) Air 0,59 ppO2, 18m ead
Stop at 15m 3:00 (27) Air 0,52 ppO2, 15m ead
Stop at 12m 4:00 (31) Air 0,46 ppO2, 12m ead
Stop at 9m 6:00 (37) Air 0,40 ppO2, 9m ead
Stop at 6m 34:00 (71) Air 0,34 ppO2, 6m ead
Surface (71) Air -9m/min ascent.

And that program is considered as "too agressive" by some divers.

Happy & safe diving, whichever way you plan your dives.

Thanks for some good comments on the thread.

Matt

Sue Mitchell

17-03-2006, 20:43

hi Matt
thanks for replying... yes things are changing, both in terms of deco theory and more people doing longer and deeper diving ..... here's to all that new technology that makes it possible! :)
happy diving
Sue

adcmay1976

23-04-2007, 12:03

Your 21mins in this case is not the time you leave the bottom, it is the time you reach your ascent check depth which in this case is 9m. So using the 15m/min ascent rate you need to leave the bottom on 18mins.

How does a run time of 18mins compare with the others/

Andy

cancerchris

03-11-2008, 17:15

To cut a very long story short IMHO tables BSAC tables like other tables are a system of lowering the odds of a DCI

The BSAC 88 tables have always been criticised for being unsafe - in general if you dive an aggressive dive using BSAC 88 tables you should call the chopper before you touch the water!

As demonstrated earlier in the thread the deco calculations by '88's are terrible. They are incredibly aggressive and only practical for the fittest of divers, I wouldn't even entertain the thought of following the 88's. I use 88's as a loose planner, I then use the IANTD tables - I then pad it out with Pyle Stops to make a deco curve I'm happy with.

I also use my noodle and think - is a 50m dive for 21mins on air responsible? And is 20 mins of deco sensible? Definately not! To Both! You want to be on 18/45, or even something a bit richer on the helium to get the END down to about 25m.

If you insist on using tables you should use the Buhlmann ZHL-16 tables - get them from IANTD.

Or even better - do the DIR courses and learn about decompression theory and how to do it properly :p It should be said that the WKPP has developed an excellent decompression theory that is rumoured as much, if not more, scientific basis than VPM/RGBM/Buhlmann etc

Richard Whitcombe

03-11-2008, 17:47

It should be said that the WKPP has developed an excellent decompression theory that is rumoured as much, if not more, scientific basis than VPM/RGBM/Buhlmann etc

That unfortunately is complete bollocks. You clearly have no idea of what ever your own agency teaches.
It may have completely escaped your attention but they use Buhlmann or VPM for planning purposes. And even SELL decoplanner.
Suggest you go back and read up on things you clearly have no understanding of.

Vic

03-11-2008, 18:01

Well done. You've just posted misinformation to a 3-year old thread.

The BSAC 88 tables have always been criticised for being unsafe

No they haven't. They might not be appropriate for all situations, but most of the criticisms of the '88s are from people who simly haven't read the instructions.

If you've got any sort of evidence to show the '88s as less safe than anything else, I'd love to see it...

- in general if you dive an aggressive dive using BSAC 88 tables you should call the chopper before you touch the water!

Ah. Hard Facts(tm). Reference?

And is 20 mins of deco sensible? Definately not!

Ah. you'll be one of those experienced divers I keep hearing about, then.

Why, prezackerly, is 20 mins of deco not sensible?

Or even better - do the DIR courses and learn about decompression theory and how to do it properly

Now you're lost.

Read the slobitis threads and tell me again how wonderful WKPP deco is. I wouldn't tolerate the symptoms they consider normal.

It should be said that the WKPP has developed an excellent decompression theory that is rumoured as much, if not more, scientific basis than VPM/RGBM/Buhlmann etc

No, no, and no.

The WKPP put great stock in Wienke's RGBM. Wienke originally claimed that this was derived from Yount's VPM work - although he later claimed it isn't. So the WKPP's work *is* VPM/RGBM; it can hardly be claimed to be of a "more scientific basis".

Buhlmann? That's the dissolved-gas model. The integrals it uses dictate the tissue tensions that are used in VPM et al. So on that basis, there is more theory in the bubble models than in straight Buhlmann. Does that make in "more scientific"? Only if theory is science - i.e. if it is right. The jury is definitely out on that one - there isn't nearly enough statistical information to be able to judge for certain, even if we all have our own pet theories.

So - 0/10 for decompression knowledge. I'd suggest you learn a bit before trying to teach anyone else...

Vic.

p.s. Reputation's looking good, isn't it?