View Full Version : Latest Statement on O2 and deco
iainmsmith
23-03-2004, 10:27
Oh well...it looks like BSAC have no intention of reconsidering their position on deco gases. Nice of them to tell us formally.
Or maybe they were just trying to avoid the firestorm.
Hopefully someone has time to take the linked article apart as someone has clearly missed the fact that logical conclusions can only derive from the correct, logical premises.
Iain
Hopefully someone has time to take the linked article apart as someone has clearly missed the fact that logical conclusions can only derive from the correct, logical premises.
Well lets's just start with the first paragraph ....
"When the BSAC created its own Nitrox Training programme in 1995 the authors of these manuals and courses recommended a maximum PPO2 (Partial Pressure of Oxygen) of 1.4 bar. This was decided upon from information known about the safe levels of oxygen to ensure the diver is kept well away from oxygen toxicity. Now nearly tens years on it can be seen that this recommendation was a good one as there are no reports of Oxygen toxicity problems in BSAC recreational or technical diving.":=
Please correct me if I'm wrong, but evidence of no BSAC
incidents on 80% is NOT evidence that other agencies 100% is
in any way dangerous or harmful. To be truly objective you need
to make a comparison between agencies and there max
percentage. Looking at BSAC in isolation tends to lean towards
the conclusion not that they are right, but that the barrier is
set too low.
TerryH
Dominic Humphries
24-03-2004, 09:05
Well lets's just start with the first paragraph ....
:="When the BSAC created its own Nitrox Training programme in 1995 the authors of these manuals and courses recommended a maximum PPO2 (Partial Pressure of Oxygen) of 1.4 bar. This was decided upon from information known about the safe levels of oxygen to ensure the diver is kept well away from oxygen toxicity. Now nearly tens years on it can be seen that this recommendation was a good one as there are no reports of Oxygen toxicity problems in BSAC recreational or technical diving.":=
Please correct me if I'm wrong, but evidence of no BSAC
incidents on 80% is NOT evidence that other agencies 100% is
in any way dangerous or harmful. To be truly objective you need
to make a comparison between agencies and there max
percentage. Looking at BSAC in isolation tends to lean towards
the conclusion not that they are right, but that the barrier is
set too low.
According to my TDI instructor, nobody has ever toxed whilst diving within the TDI limits either. And they allow 1.6ppO2 and the use of 100%
So even if the concern is keeping the limits to where no incidents happen, there's no justification for the current limitations..
Hopefully someone has time to take the linked article apart as someone has clearly missed the fact that logical conclusions can only derive from the correct, logical premises.
Personally I think that taking it apart is a waste of time. The flaws are obvious to anyone that has recieved ERD training outside of BSAC. To me it looks like a sensible set of guidelines have been subjected to various special interests hence what we are left with is a 'camel'. We know it don't make sense. I expect the people responsible for it, know it don't make sense.
There is one spark of hope left. The statement makes it clear that this is a recommendation. If the NDC remove the wordings in the SDPs, BOH and Nx Equivalency statements which indicate it could be a rule, we can get back to running branches and going diving.
andy botten
24-03-2004, 13:06
According to my TDI instructor, nobody has ever toxed whilst diving within the TDI limits either. And they allow 1.6ppO2 and the use of 100%
At the DOC: Professor David Elliott said he had a case at 1.52
His view less is best.
Dominic Humphries
24-03-2004, 13:10
At the DOC: Professor David Elliott said he had a case at 1.52
His view less is best.
But was that within the 100% CNS value?
Even 1.4 will give you a hit if you stay on it long enough..
Mike Rowley
24-03-2004, 21:41
As I read it it does not say you cannot use 100% O2. It merely recommends a maximum PO2 of 1.4 bar.
Oh well...it looks like BSAC have no intention of reconsidering their position on deco gases. Nice of them to tell us formally.
Or maybe they were just trying to avoid the firestorm.
Hopefully someone has time to take the linked article apart as someone has clearly missed the fact that logical conclusions can only derive from the correct, logical premises.
Iain
Anke Otto
25-03-2004, 14:42
What gets me about this statement is this sentence:
'This weaker mix of Nitrox 80 can be used safely at deeper depths, which allows for a faster and therefore shorter decompression schedule.'
If 'at deeper depth' means deeper than 6m then why is 80% at e.g. 10m safer than 100% at 6m?? ppO2 for both would be 1.6, would it not?
I would also argue that 80% used to max ppO2 of 1.4 could ever give you a 'faster and therefore shorter deco schedule' than using 100% at 6m with max ppO2 of 1.6.
Regards
Anke
andycarroll
25-03-2004, 16:03
What gets me about this statement is this sentence:
'This weaker mix of Nitrox 80 can be used safely at deeper depths, which allows for a faster and therefore shorter decompression schedule.'
If 'at deeper depth' means deeper than 6m then why is 80% at e.g. 10m safer than 100% at 6m?? ppO2 for both would be 1.6, would it not?
I would also argue that 80% used to max ppO2 of 1.4 could ever give you a 'faster and therefore shorter deco schedule' than using 100% at 6m with max ppO2 of 1.6.
Regards
Anke
Hi Anke
What I think this statement means is that 80% is a better deco gas because if you restricted yourself to 1.4 po2 then 100% would have to be deco'd at 4 mtrs as opposed to 80% at 6 mtrs (actually 7.5 mtrs at 1.4po2).
When these restrictions are in place then 80% probably does result in a shorter decompression when used with decompression software as the majority of the stops are calculated at 6 mtrs, below the 1.4 threshold of 100%.
I find the statement kind of one sided and very defensive, but it is no surprise really. We should be looking forward, not justifying decisions and policies made in the past, rightly or wrongly.
Kindest Regards
Andy
When these restrictions are in place then 80% probably does result in a shorter decompression when used with decompression software as the majority of the stops are calculated at 6 mtrs, below the 1.4 threshold of 100%.
It does depend on the profile, but in most cases the fastest way out the water is to switch to 80% for the 9m stop. This will generally provide a shorter decompression than using 100% at 6m. By moving the final stop to between 5m and 3m decompression is speeded up further.
I find the statement kind of one sided and very defensive, but it is no surprise really. We should be looking forward, not justifying decisions and policies made in the past, rightly or wrongly.
By far my 'favourite' bit of this statement is the line about BSAC 88s. When we talk about mixes >70% you can pretty much assume we are talking about accelerated decompression. You can't accelerate on 88s...Now what were you saying about looking forward!
By far my 'favourite' bit of this statement is the line about BSAC 88s. When we talk about mixes >70% you can pretty much assume we are talking about accelerated decompression. You can't accelerate on 88s...Now what were you saying about looking forward!
Actually... They do teach accelerated deco based on the 88s on the ERD SDC. Whether you would want to follow those practises is another matter entirely
:=You can't accelerate on 88s.
Actually... They do teach accelerated deco based on the 88s on the ERD SDC. Whether you would want to follow those practises is another matter entirely
Hi Pete
I thought they just taught a deco package. I have only met one person that has done BSAC ERD though.
On a purely a academic level I am intrigued. You can't use EAD on 88s and I have never seen or heard of a set of accelerated tables. So is there a set of accelerated 88s, or is there some other method? Feel free to mail me off the forum - remove noham.
nick kay
26-03-2004, 13:56
Actually... They do teach accelerated deco based on the 88s on the ERD SDC. Whether you would want to follow those practises is another matter entirely
Err, NO they don't...
The BSAC ERD course teaches accelerated deco, using Nitrox mixes upto 80% in conjunction with 3rd party software such as (normally) ProPlanner, though its the principle thats taught - and as such you can choose your own software, e.g. GAP, DecoWeenie, etc.
:=Actually... They do teach accelerated deco based on the 88s on the ERD SDC. Whether you would want to follow those practises is another matter entirely
Err, NO they don't...
They did when my mate did it - it was pretty crude you took the deco from the 88's and reduced it by a set percentage
:=:=Actually... They do teach accelerated deco based on the 88s on the ERD SDC. Whether you would want to follow those practises is another matter entirely
:=
:=Err, NO they don't...
They did when my mate did it - it was pretty crude you took the deco from the 88's and reduced it by a set percentage
Your % chance of getting into the SBS possibly ;-)
The idea of reducing 88 stop times sends a shiver down my spine. The tox risk of a 1.6PPO gas switch doesn't worry me but I'm not brave enough for that!
David Walker
26-03-2004, 20:20
The idea of reducing 88 stop times sends a shiver down my spine. The tox risk of a 1.6PPO gas switch doesn't worry me but I'm not brave enough for that!
I'm surprised reducing stop times on tables worries anyone - do some dives on a computer, just 2 or 3 per day for a couple of days, and then after you've done the dives work out what deco you should have done if you were diving on tables! If you work out the averages then you'll probably find the tables give you several minutes of extra deco after a couple of days, where you could still be 30 or 40 minutes away from deco on your computer. Even on relatively 'square' dive profiles my computer always gives me much more than the tables (and i'm using a Vyper, which is supposed to be rather conservative anyway).
That said, if I didn't have a computer then i'd be a bit wary of going over the tables, but in reality if you look at the numbers then the tables will always be massively more conservative than a computer would be, so don't think i'd ever be all that worried
.... I think! :O\
David
nick kay
26-03-2004, 21:48
So, why is it, that deco theory seems to be moving in the direction of inserting deep stops & increasing the length of stops???
nick kay
26-03-2004, 21:49
:=:=Actually... They do teach accelerated deco based on the 88s on the ERD SDC. Whether you would want to follow those practises is another matter entirely
:=
:=Err, NO they don't...
They did when my mate did it - it was pretty crude you took the deco from the 88's and reduced it by a set percentage
Wouldn't like to be the ERD Instructor if Jack Ingle ever discovered who it was...
David Walker
27-03-2004, 14:57
So, why is it, that deco theory seems to be moving in the direction of inserting deep stops & increasing the length of stops???
My understanding was that inserting deep stops on the whole reduced the overall length of stops - a few short stops at depth could reduce shallow stop times significantly! I don't do anything like that myself, but from what i've read thats how the deep stops work.
In any case what I said in my last post does stand, unless you say the algorithms in computers are dangerous and wrong - which I don't believe anyone thinks they are (or not significantly anyway).
David
> My understanding was that inserting deep stops on the whole
> reduced the overall length of stops - a few short stops at depth
> could reduce shallow stop times significantly!
Not usually.
Adding deep stops to a "conventional" (neo-Haldanian) model increases the length of the shallow stops.
Some people (not me!) use GFhi > 100 to reduce the length of short stops. Others use a bubble-based model (e.g. VPM). But IME these people are very much in the minority.
Vic.
> I'm surprised reducing stop times on tables worries anyone -
> do some dives on a computer, just 2 or 3 per day for a couple
> of days, and then after you've done the dives work out what
> deco you should have done if you were diving on tables!
> If you work out the averages then you'll probably find the
> tables give you several minutes of extra deco after a couple
> of days, where you could still be 30 or 40 minutes away from
> deco on your computer.
That depends very heavily on the sort of diving you do.
If you do square profiles, then a table and a computer offering the same model will give exactly the same decompression obligation.
The difference you mention above comes from spending time during the dive at other than maximum depth; this leads to the table penalising you for being at max depth and the computer not doing so.
> Even on relatively 'square' dive profiles my computer always
> gives me much more than the tables (and i'm using a Vyper,
> which is supposed to be rather conservative anyway).
But it depends on the model you're running in each; I can give you tables that will make the computer look either ridiculously conservative, or suicidally permissive.
> That said, if I didn't have a computer then i'd be a bit wary
> of going over the tables, but in reality if you look at the
> numbers then the tables will always be massively more
> conservative than a computer would be
Completely disagree. Computers are invariably more conservative for a given (square) profile; that's why the ZHL-16C model was devised, for example.
> so don't think i'd ever be all that worried
I'd strongly recommend a refresher on the relevant deco systems before making any rash assumptions...
Vic.
David Walker
27-03-2004, 17:27
> My understanding was that inserting deep stops on the whole
> reduced the overall length of stops - a few short stops at depth
> could reduce shallow stop times significantly!
Not usually.
Adding deep stops to a "conventional" (neo-Haldanian) model increases the length of the shallow stops.
Some people (not me!) use GFhi > 100 to reduce the length of short stops. Others use a bubble-based model (e.g. VPM). But IME these people are very much in the minority.
Hmmm - doesn't quite make sense in my head yet. If you add deep stops, and then do more in shallow water too, then (if you're assuming the standard tables / algorithms are OK) thats just like having a longer dive, and the deep stops aren't doing anything to help decompress.
Can't really think of a concise example:
eg Standard tables - 30mins @ 50m => 5mins @ 6m stop
- 32mins @ 50m => 8mins @ 6m stop
Deep stop tables - 30mins @ 50m => 2mins @15m, 8mins @ 6m
So if you do a 30min dive to 50m, and stop at 6m for 5mins you're fine. If you do the same though, but add a 2min stop at 15m, and then also extend your 6m stop, you're just following the tables anyway, and the deep stop doesn't seem to add any benefit.
No idea if that makes any sort of sense, or if i've just completely misunderstood the concept.
Basically, what i'm saying is that if deep stops require you to decompress for longer later, then the deep stops don't seem to provide any benefit unless you assume the standard tables are wrong!
I'll have to work out where I read about deep stops before, because i'm sure there is one model that gives deep stops credit and thus reduces length of shallow stops...
David
Nigel Hewitt
27-03-2004, 18:24
>Basically, what i'm saying is that if deep stops require you to decompress for longer later, then the deep stops don't seem to provide any benefit unless you assume the standard tables are wrong!
Look for the stuff Pyle wrote, it's on the web. He was a fish research man and noticed that on dives where he had to stop and puncture swim bladders to keep the specimens alive he came out of the water feeling much better. He did an awful lot of diving and kept good logs.
His original stuff was largely empirical but it is reasoned that the stop plan that bears his name pauses you and lets your lungs flush the micro-bubbles that are building up. DAN have published some stuff recently that looks just like it based on Doppler bubble measurements.
However you are spending more time at depth so some of your tissues are still on-gassing so you need to off-gas them later streching your shallow stops.
The bubble models promise to get you out of the water faster by basing their calculations on bubble formation and gas gradients over bubble walls but be sure you are using the current versions as the early ones were really quick and had a higher incident rate. I'm not brave enough to use a bubble model routinely especially on multi-day dive trips.
nigelH
:=The idea of reducing 88 stop times sends a shiver down my spine. The tox risk of a 1.6PPO gas switch doesn't worry me but I'm not brave enough for that!
I'm surprised reducing stop times on tables worries anyone
David, with the greatest of respect you may be a little out of your depth here. It sounds like you do not do much mandatory stop diving.
There is a World of difference between stop dives and no stop dives.
If you compare no stop times, for a first dive, between differing tables, they are all roughly the same. We have a pretty good understandig of how gas disolves into the body and how long it takes before it reaches a dangerous level.
If you compare stop times on different tables you see significantly different numbers and significantly different decompression strategies. What you are looking at is differences in opinion between individual deco scientists and modelers. We simply do not understand off-gasing as well as we understand on-gasing.
The upshot of this is that stop dives are inherrently riskier than no-stop dives. The statistics fully support this, you are far more likely to get bent as a result of a stop dive than a no-stop dive, even if you follow the schedule (unearnt bend).
Take a simple 30m 30min air profile. BSAC 88s give a schedule of 1min at 6m. On the IANTD Buhlmann tables the same dive requires 1 min at 6m + 12 mins at 4.5m. In fact if I used 80% for the decompression stops on the IANTD table I still need to do more stops than the 88 air profile!
I am sure that Dr Hennessey (BSAC 88s) is a clever man and believes his schedules are in no way dangerous. But he is not diving them. I have dived 88 deco schedules in the past and actually got to the point where I thought fatigue and a banging headache were normal post dive symptoms. So yes the idea of further accelerating an 88 deco schedule frankly scares the willies out of me.
That said, if I didn't have a computer then i'd be a bit wary of going over the tables, but in reality if you look at the numbers then the tables will always be massively more conservative
Your statement is just downright wrong.
Behind every schedule is a mathematical algorithm. The algorithm is a rigid set of rules, which will for the same inputs provide the same outputs. If you plan a dive on a Buhlmann table and dive that exact plan using a Buhlmann computer you will get pretty much the same results. There is usually some padding around the algorithms, but it can be ignored for the sake of the discussion. A table shows the output of the algorithm for an assumed time/depth/gas profile. Your computer shows the realtime output of the algorithm based on the profile you have actually dived. T
The first difference between a computer and table schedule, is the obvious one. Any time spent shallower than the maximum assumed depth will be 'credited' by the computer. Put another way, the computer shaves off the inherent safety factor in a multi-level profile that was planned as a U-profile on the tables. There are some other factors which will influence the difference in what the computer displays compared to the table. For instance, tables assume an instantaneous descent to the maximum depth. The majority of computers will also display decompression assuming an instant ascent whilst the tables schedule is based on having made the ascent.
For what is generally termed recreational or traditional sport diving you do not need a particularly detailed understanding of decompression science. To remain relatively safe, it is sufficient to stay within tables or a modern computer. An appreciation of good practice will reduce risks further.
Extended Range Diving requires balancing often conflicting risks in order to venture past traditional recreational limits. For instance this thread is fundamentally concerned with balancing oxtox risk against DCI risk. You really need to know as much as possible about decompression and it's effect on physiology in order to make the risk assessments. ERD diving places it's proponents into the areas of diving science which are least understood. Hence ERD divers will attempt to use everything we do know, and some things still being debated, to mitigate the risks to an acceptable level. It starts with such things as positive hydration and can encompass significant diet and lifestyle changes.
Regards
Matt
Hmmm - doesn't quite make sense in my head yet. If you add deep stops, and then do more in shallow water too, then (if you're assuming the standard tables / algorithms are OK) thats just like having a longer dive, and the deep stops aren't doing anything to help decompress.
Well we absolutely positively know that the standard algorithms are not completely OK. People get bent inside table schedules every year. People have also been known to walk away from profiles which the algorithms predict will put them in hospital. The algorithms are a rough guide to decompression, they are not an accurate and complete representation of it.
The traditional algorithms (Haldane/USN/PADI, Hempleman/Hennessey, Buhlmann/Workman etc) only model one aspect of decompression - dissolved gas. The algorithms are no more than simple math applied to a conveniently arranged set of compartments. This is a long way from the chaotic assembly of tissue and the fluids being pumped around our bodies. If our bodies behaved the same way the models do, the best decompression strategy would involve rushing as fast as possible to the shallowest safe depth, which has been tried and we now know it is a very provocative method.
Since the introduction of Doppler monitoring we know that not all gas is dissolved. On every dive free gas, or bubbles, also exists in the blood stream. Usually these bubbles are not big enough to cause a problem, they pass through the heart to the lungs and the lower pressure in the lungs causes the bubbles to 'burst' and the gas they contain is exhaled.
There are two scenarios which are thought to turn these symptomless bubbles into DCI causing bubbles. The first comes down to the ascent rate. If the ascent is too rapid a small bubble can grow and lodge before it reaches the lung bed. The second concerns deep stops. If enough bubbles are allowed to form, the density of small bubbles in the blood will increase and some of these small bubbles will combine to create a single bigger bubble. Both of these scenarios can potentially cause a bend inside what a dissolved algorithm forcasts is a safe profile.
Deep stops are intended to reduce the density of bubbles in the blood. By pausing you both stop existing bubbles growing larger and discourage new bubbles from forming, giving the existing bubbles time to reach the lungs and be eliminated. The downside of this is that you are still disolving additional Nitrogen into the tissues. This additional disolved Nitrogen still needs to be eliminated by a longer traditional shallow stop.
So if you do a 30min dive to 50m, and stop at 6m for 5mins you're fine. If you do the same though, but add a 2min stop at 15m, and then also extend your 6m stop, you're just following the tables anyway, and the deep stop doesn't seem to add any benefit.
It makes sense if you believe that there is more to decompression than dissolved gas.
I'll have to work out where I read about deep stops before, because i'm sure there is one model that gives deep stops credit and thus reduces length of shallow stops...
The way the dissolved gas models were developed was largely based on blowing up goats and pigs in recompression chambers. By observing what did and did not cause a bend, Haldane hypothesised what was going on and then compiled the math to model his observations. Since then the original model has been refined as knowledge has increased. But ultimately our understanding has been based on how the majority of people get out of the water unbent.
The argument for deep stops and shorter shallow stops is based on the belief that bubbles are more significant than dissolved gas. It is thought by some, that the traditional shallow stop not only eliminates dissolved gas, but also large numbers of bubbles that have formed during the direct ascent to the stop depth. Therefore, if the number of bubbles is controlled sufficiently during the ascent by making deep stops, then the shallow stop can be reduced.
This model is known as the Variable Permiability Model (VPM) and was originally developed in Hawaii during the 70s using sophisticated computers to attempt to model bubble behaviour. The intial chamber trials indicated that this new strategy could shave significant time off the total stop time. However the proof of the pudding is in the eating. Once VPM was released to the diving public, the schedules caused more bends than was initially predicted. The VPM schedules have been gradually made more conservative, and are now fairly similar to traditional schedules but include deep stops.
Now before anyone rushes to tell me that Hennessey and Buhlmann both predicted the existance of bubbles, I have simplified the explanations a little for the sake fo clarity. After all I have tried to encompass all the highlights of 100 years of deco research into a couple hundred words.
David Walker
27-03-2004, 21:10
Well we absolutely positively know that the standard algorithms are not completely OK. People get bent inside table schedules every year. People have also been known to walk away from profiles which the algorithms predict will put them in hospital. The algorithms are a rough guide to decompression, they are not an accurate and complete representation of it.
Ahh yeah, OK then - obviously people are different, but then you woulnd't want tables designed for the most DCI prone person in the entire world ever to have existed, otherwise we'd never get underwater! If someone knows they are more prone to DCI than normal then they will know not to push the limits (or set their computer to a more conservative level).
I don't imagine there is a single model which works perfectly, but if there are two different models which both give say 1% of people diving to those limits DCI, and one of those has half of the amount of deco needed then both would be considered as safe as each other.
... I have tried to encompass all the highlights of 100 years of deco research into a couple hundred words.
And a brilliant job you did of it too!
I realise why the deep stops are beneficial but don't reduce stop times now - I think i'll just stick with whatever my Vyper tells me for now... although I do always make much slower ascents than it allows, I never liked the idea of rushing up to a stop.
David
David Walker
27-03-2004, 21:17
David, with the greatest of respect you may be a little out of your depth here. It sounds like you do not do much mandatory stop diving.
Hehe - I agree! Made sense at the time! And from multi-day no-stop diving, a computer does always allow far more than following (BSAC) tables would. I assumed the same sort of pattern would follow into deco too, but I do only do the odd deco dive, and its only ever a few minutes of deco, nothing long.
I'll shut up now, let people who know what they're talking about talk! :O)
David
David Walker
27-03-2004, 21:21
If you do square profiles, then a table and a computer offering the same model will give exactly the same decompression obligation.
The difference you mention above comes from spending time during the dive at other than maximum depth; this leads to the table penalising you for being at max depth and the computer not doing so.
Thats what I was referring to - all the dives I do are not square, and inevitably aren't spent all at the max depth, and I assume that applies to everyone to at least some extent - even a flat wreck you might drop into a hole or over the side. From that you will get penalised from tables for depths and times you've never dived, which is why I said I wouldn't be overly worried.
That said, I wouldn't dive based on guessing what the tables might be for a multi-level dive, and as i've been told once you're into deco small changes can make a huge difference.
Made sense at the time - now I know better! :O)
David
nick kay
28-03-2004, 14:09
Not sure whether the observation is "real" or "perceived", but I feel better (or don't feel as tired) when doing a dive and using deep stops.
Is it just pyschological or real? No idea, but works for me...
iainmsmith
28-03-2004, 19:09
:=:=Actually... They do teach accelerated deco based on the 88s on the ERD SDC. Whether you would want to follow those practises is another matter entirely
:=
:=Err, NO they don't...
They did when my mate did it - it was pretty crude you took the deco from the 88's and reduced it by a set percentage
IMO, that's pretty terrifying. A friend of mine ran some numbers and did a picture showing the different deco profiles for a nitrox dive resulting from:
1)& 2) what we were diving at the time, ie modified Buehlmann with gradient factors of GFlo=20% and GFhi=80% with and without accelerated deco (50%) from 21m
3) and 4) "raw" Buehlmann (100/100) with and without accelerated deco
5) BSAC Nitrox tables
The only model that got the diver out of the water faster than the BSAC '88 table was the "raw" Buehlmann with accelerated deco.
Iain
iainmsmith
28-03-2004, 21:09
Oh well...it looks like BSAC have no intention of reconsidering their position on deco gases. Nice of them to tell us formally.
Or maybe they were just trying to avoid the firestorm.
Hopefully someone has time to take the linked article apart as someone has clearly missed the fact that logical conclusions can only derive from the correct, logical premises.
Having a little more time on my hands this evening, I thought I would look at the BSAC statement in more detail.
As Terry has pointed out, the statement that "Now nearly tens years on it can be seen that this recommendation was a good one as there are no reports of Oxygen toxicity problems in BSAC recreational or technical diving" is flawed as no data are presented to allow a comparison with the frequency of oxygen toxicity problems in recreational or technical diving in which a max ppO2=1.6 bar is used.
A review of the BSAC Incident Reports from 1997-2003 (using "toxicity" as a search term) lists only one case from that period:
"June 2001, 01/186
An instructor was teaching on a rebreather course. He dived to a depth of 35m and is reported to have suffered an oxygen toxicity convulsion as he made his ascent. He died as a result of this event."
However, no details of oxygen exposure are provided, thus no conclusions can be drawn from this.
Regarding the "Partial Pressure of Oxygen", the statement fails to differentiate between recommendations for maximum ppO2s for working and decompression phases of the dive. It does note that the BSAC recommendations for the working phase of the dive are, in fact, more aggressive than some other agencies (and conveniently ignores the fact that BSAC recommended ppO2max for no-decompression dives exceeds that of (to the best of my knowledge) all the mainstream technical agencies.). However, the limits for the working phase of the dive are not those that result in ongoing controversy.
The BSAC statement fails to acknowledge that the diver at rest (ie decompressing) is at less risk of an oxygen toxicity event and can therefore make use of a higher ppO2. In contrast, both of the technical agencies from which I hold qualifications (one of which the BSAC claims to "recognise") allow ppO2=1.6 bar for decompression on this basis. These limits are commonly used throughout the world, whether in the warms waters of Florida or the "harsher conditions of northern Europe" - including Scandinavia, whose conditions can be considerably less favourable than are our own.
Moving on to the "Use of Nitrox 80 for decompression", we are told that the maximum of nitrox 80 was to allow the ERD course to tie in with the '88 tables. As has been discussed elsewhere in this thread, the ERD course does not use '88 tables as a basis for decompression planning, hence there should be no need to use the '88 tables any sort of basis for choice of deco gas.
This illogicality became even more pronounced when BSAC permitted the use of trimix. Surely we should not be trying to plan trimix dives around parameters derived from air tables, but should rather be starting with a clean piece of paper.
The statement continues by referring to the increased safety advantages of a weaker nitrox mix as a decompression gas. This goes back to the old question of whether BSAC trusts its divers doing advanced dives to be able to hold stops or not. If a diver cannot hold a stop well enough to be safe to use pure oxygen, then IMO, that diver has no business doing any sort of diving where 80% might be an alternative.
"This weaker mix of Nitrox 80 can be used safely at deeper depths, which allows for a faster and therefore shorter decompression schedule."
There are two "interesting" points about this sentence.
Firstly, given that the maximum ppO2 allowed by the BSAC is 1.4 bar, that would limit the use of 80% to 7.5m.
However, I'm not sure that I've ever heard of anyone doing 7.5m stops. Unless I'm misinformed, the ERD course also looks at 9m and 6m stops...
I conclude that there are three possible explanations:
1) there are non-standard stops being introduced
2) the 80% is not, in fact, being used at deeper depths
3) the 80% is not being used in accordance with BSAC Safe Diving Practices.
To examine the statement that 80% "at deeper depths" results in faster and shorter deco schedules, I ran a number of profiles through my software.
Using my preferred gradient factors (20/90), I looked at profiles from 30-50m for Bottom Times of 20-50 minutes, with a maximum deco of 40 minutes.
Using air as a bottom gas:
Obviously 80% from 6m came out worst. However, the difference between 80% from 9m and <a href="mailto:100%@6m">100%@6m</a> was in all but a few cases, 1 minute. In those few cases, there was a maximum time saving of 2 minutes. Big deal.
I then looked at 21/35 trimix for the same range of dives, and found even less difference between using 80% from 9m and <a href="mailto:O2@6m">O2@6m</a>.
Introducing <a href="mailto:50%@21m">50%@21m</a> showed the 50%/100% combination being faster than 50%/80%. I then looked at <a href="mailto:40%@21m">40%@21m</a>/<a href="mailto:80%@9m">80%@9m</a> and <a href="mailto:50%@18m">50%@18m</a>/<a href="mailto:80@9m">80@9m</a>. Both of these were slower than 50%/100%.
I then compared dives using 18/45 to 66m, using GF20/90 and 50%from 21m and either 80% from 9m or O2 from 6m, with a max deco time of 80 minutes. Again, 80%, even used outside BSAC SDPs, was slower to get the diver out of the water.
Looking at compartmental analysis of various dives from the above fails to show the "clearly identifiable enhancement of the offgassing phase."
All of the above, of course, ignores the difference in effects on bubbles of breathing a gas with no inert gas vs a gas with an FN2 of 20%. That might be something better modelled by VPM or RGBM, but I'm sure that we are all aware of why O2 is a better gas for dealing with bubbles than is 80%.
It would be interesting see a comparison of 80% vs 100% for other people's typical mixes/conservatism settings.
In terms of my diving and preferences, 80% used within BSAC SDPs is less effective in all circumstances. Used at the normally accepted max ppO2 for deco (1.6 bar) it is at best minimally faster at getting the diver out of the water and when used for a multiple-deco-gas dive is slower than commonly used decompression gases.
It may well be that someone can produce a set of figures which would produce minimal benefits over comparable depth ranges using 80%. However, I think that it is clear that the conclusions of the BSAC statement regarding the use of 80% are, at best, hugely overstating the minimal benefits of 80% and in the cases outlined above, are incorrect. Further, to get those "benefits", the diver has to violate both BSAC Safe Diving Practices and the ppO2 limits laid out in the earlier section of the statement.
I would therefore ask the BSAC to withdraw this published statement and to reconsider the issues of pure oxygen and maximum ppO2s for decompression from a fresh perspective, rather than simply restating ten-year old thoughts.
Iain Smith
Philip Smith
29-03-2004, 13:54
"This weaker mix of Nitrox 80 can be used safely at deeper depths, which allows for a faster and therefore shorter decompression schedule."
There are two "interesting" points about this sentence.
Firstly, given that the maximum ppO2 allowed by the BSAC is 1.4 bar, that would limit the use of 80% to 7.5m.
Presumbaly, nitrox80 was considered the nearest "standard" or easily-remembered mix with P02<1.4 bar at 6m, and/or a safety margin of -7.5% F02 was added.
In terms of my diving and preferences, 80% used within BSAC SDPs is less effective in all circumstances.
Did you compare using 80% from 6m with using 100% from 3m, which I think is the comparison the author of the statement had in mind?
Philip Smith
David Martin
29-03-2004, 19:37
:=In terms of my diving and preferences, 80% used within BSAC SDPs is less effective in all circumstances.
Did you compare using 80% from 6m with using 100% from 3m, which I think is the comparison the author of the statement had in mind?
I've run through a couple of profiles, similar to those that Iain described:
Model - ZHL16B
Immediate descent - off
GF Low - 20
GF High - 90
Bottom gas: Air
Gas switch depths (ppO2 1.4)
Nx50 18m
Nx80 6m
O2 3m
Listed below are total run times for 3 sample profiles
(max deco approx 40min, max bottom time 50min)
The first run time is with the last stop at 3m, the second with the last stop at 6m
Depth = 50min
Bottom Time = 28min
Total time (deco Nx50) 66min, 68min
Total time (deco Nx80) 70min, 70min
Total time (deco O2) 77min
Total time (deco Nx50 & Nx80)61min, 61min
Total time (deco Nx50 & O2) 61min
Depth = 40min
Bottom Time = 40min
Total time (deco Nx50) 78min, 80min
Total time (deco Nx80) 82min, 82min
Total time (deco O2) 88min
Total time (deco Nx50 & Nx80)72min, 72min
Total time (deco Nx50 & O2) 73min
Depth = 30min
Bottom Time = 50min
Total time (deco Nx50) 75min, 76min
Total time (deco Nx80) 76min, 76min
Total time (deco O2) 81min
Total time (deco Nx50 & Nx80) 71min, 71min
Total time (deco Nx50 & O2) 71min
To summarise:
Deco at max ppO2=1.4 for dives in the above range:
Nx80 is faster than pure O2 alone
But Nx50 is faster than either.
Nx50+Nx80 is no faster than Nx50+O2 for two of the profiles, and one minute faster for the third.
These figures cannot be reconciled with BSACs statement: "If comparative schedules are run on any decompression software the enhancement of the off gassing phase will be clearly identified"
I therefore, fully endorse Iain's comment:
"I would therefore ask the BSAC to withdraw this published statement and to reconsider the issues of pure oxygen and maximum ppO2s for decompression from a fresh perspective, rather than simply restating ten-year old thoughts."
David Martin
iainmsmith
30-03-2004, 00:57
:=:=In terms of my diving and preferences, 80% used within BSAC SDPs is less effective in all circumstances.
:=
:=Did you compare using 80% from 6m with using 100% from 3m, which I think is the comparison the author of the statement had in mind?
:=
I've run through a couple of profiles, similar to those that Iain described:
[snip]
To summarise: Deco at max ppO2=1.4 for dives in the above range: Nx80 is faster than pure O2 alone But Nx50 is faster than either.
David,
Thanks for looking into that.
The other observations that I would make are:
Firstly, given that BSAC are claiming that the choice of 80% was to allow compatability with BSAC '88 style decompression, making any sort of comparison with 3m stops is irrelevant as such profiles are not typical of '88-style diving. (As I said earlier, I fail to see why it is thought to be necessary to use recreational air tables as a basis for extended range/trimix dive planning, but that's a whole other topic).
Secondly, if the author had in mind any such 6m vs 3m comparison, it would have been useful if they'd actually stated as much, rather than expecting that the reader would infer this. Postition Statements are supposed to be unambiguous and designed to avoid possible misinterpretation, rather than relying on the reader to correctly guess at what happened to be going on in the author's head at the time of writing!
Iain
Philip Smith
30-03-2004, 02:28
To summarise:
Deco at max ppO2=1.4 for dives in the above range:
Nx80 is faster than pure O2 alone
I think that is the point the article was trying to make, but it was not very clear and, as you point out, subject to the constraints of a max P02 of 1.4 bar during decompression and a single deco gas.
Philip Smith
David Martin
30-03-2004, 12:08
:=To summarise:
:=Deco at max ppO2=1.4 for dives in the above range:
:=Nx80 is faster than pure O2 alone
I think that is the point the article was trying to make, but it was not very clear and, as you point out, subject to the constraints of a max P02 of 1.4 bar during decompression and a single deco gas.
But, Nx50 is a better gas than Nx80 under the same conditions.
The author's simple two-way comparison between Nx80 and O2 is arbitary and (IMO) irrelvent, since this is not how I (and I believe others) would use O2.
The people who want to use pure O2 generally wish to do so as a shallow decompression gas, after using another deeper decompression gas eg Nx50. Under these circumstances, Nx50+Nx80 is NOT faster than Nx50+O2.
BSAC previously made "recommendations" for max ppO2 of 1.4 and max Fo2 Nx80. They attempt to provide a retrospective "justification" of their choice. Whilst, we have found an interpretation of BSAC's statement which is not factually false, it is of trivial relevence. However, due to its abiguity, the statement also covers a great many other situations; in this more relevent situations it is false. [The cynic in me suggests that this ambiguity might be deliberate.]
Since it does not appear possible to justify Nx80, as part of the "best" decompression gas selection, (which could support a reccommended max ppO2 1.4), the recommended max deco ppO2, 1.4 compared to 1.6, needs to be looked at on it's own merits, [elsewhere in the thread].
David Martin
David Martin
30-03-2004, 12:33
:=Oh well...it looks like BSAC have no intention of reconsidering their position on deco gases. Nice of them to tell us formally.
The BSAC statement fails to acknowledge that the diver at rest (ie decompressing) is at less risk of an oxygen toxicity event and can therefore make use of a higher ppO2. In contrast, both of the technical agencies from which I hold qualifications (one of which the BSAC claims to "recognise") allow ppO2=1.6 bar for decompression on this basis. These limits are commonly used throughout the world, whether in the warms waters of Florida or the "harsher conditions of northern Europe" - including Scandinavia, whose conditions can be considerably less favourable than are our own.
To add to the above, BSAC's statement mentions the restrictions imposed by US agencies, reducing the max ppO2 according to conditions.
I am well aware of such recommendations to reduce the ppO2 for the *bottom phase*, which appear not to concern BSAC (and also for not using ppO2 1.6 for deep deco (eg below 36m) but am not aware of any such restrictions being suggests for shallow decompression stops, in particular the 6m stop, where a ppO2 of 1.6, permitting the use of pure O2 is particularly useful.
Divers from "US agencies" dive in colder waters than the Florida coast; is anyone aware of such agencies restricting deco ppO2 1.6 for "northern European conditions"? Or has BSAC yet again failed to appreciate the difference between working and decompression phases of a dive?
David Martin
I just think that BSAC is just being so insular as to ignore the fact that they have members in places other than the UK
Dave
J Abbott
31-03-2004, 10:24
Divers from "US agencies" dive in colder waters than the Florida coast; is anyone aware of such agencies restricting deco ppO2 1.6 for "northern European conditions"? Or has BSAC yet again failed to appreciate the difference between working and decompression phases of a dive?
At ERD level and above IANTD used to teach a reduction from the 1.6bar ppO2 of 0.025% for each of extreme cold and extreme work for the decompression phase of the dive. Such that an extremely cold and extremely hard working decompression would leave you with a recommended ppO2 of 1.55bar (1.6 - 0.025 - 0.025). TDI don't make such a recommendation for the decompression part of the dive and I don't know if the IANTD teaching is still current.
Cheers
John
David Martin
31-03-2004, 12:01
:=Divers from "US agencies" dive in colder waters than the Florida coast; is anyone aware of such agencies restricting deco ppO2 1.6 for "northern European conditions"? Or has BSAC yet again failed to appreciate the difference between working and decompression phases of a dive?
:=
At ERD level and above IANTD used to teach a reduction from the 1.6bar ppO2 of 0.025% for each of extreme cold and extreme work for the decompression phase of the dive. Such that an extremely cold and extremely hard working decompression would leave you with a recommended ppO2 of 1.55bar (1.6 - 0.025 - 0.025). TDI don't make such a recommendation for the decompression part of the dive and I don't know if the IANTD teaching is still current.
I couldn't find such a recommendation when I looked at the current standards (found via google), but since I don't hold IANTD qualifications, I wouldn't claim to be a definative authority on them! Do you have a current (preferably online) reference?
I'm a little suprised to see the conecpt of "extreme work for the decompression phase of the dive" - My understanding is that there should be no/negligable work in the decompression phase! (The reason for permitting the higher ppO2 [1.6] compared to the working phase 1.4 [or less].)
However, what doesn't surprise me is the silence from "HQ". There appears to be no-one commenting upon this thread, justifying the statement, or providing references for the claims within the recent statement.
David Martin
andycarroll
31-03-2004, 13:12
I'm a little suprised to see the conecpt of "extreme work for the decompression phase of the dive" - My understanding is that there should be no/negligable work in the decompression phase! (The reason for permitting the higher ppO2 [1.6] compared to the working phase 1.4 [or less].)
However, what doesn't surprise me is the silence from "HQ". There appears to be no-one commenting upon this thread, justifying the statement, or providing references for the claims within the recent statement.
David Martin
Hi
I suppose a 'hard working' deco is viewed a little different from a hard working dive.
One example might be an overhead and you are still swimming during deco to exit the overhead rather than just rising as you do, but then we are back to the 'Floridian Cavers' :0)
Regarding the statement, at least it is now possible to propose it be changed, so the release may be a good thing in the long run. I understand the proposed new NDO is an Inspiration diver, which might also help.
I know the argument is being based on 80% against 100%, but AFAIK it is common to dive mix in the UK with two stages, one of 32% and one of 80%.
From dive reports on other sites it is common practice to switch to 32% at 40 mtrs (1.6po2). Using 50% it is common practice to switch at 21mtrs(1.6po2). This is common practice and happening all over the UK, and has been for a number of years. I have not read of any major increase in Oxygen Toxicity events due to this 'bad practice', however, there does seem to be an alarming increase in deaths whilst diving deep on air, you know 50 mtrs or so. I can't wait for the statement defending that decision!!
Kindest Regards
Andy
Nigel Hewitt
31-03-2004, 13:20
> (The reason for permitting the higher ppO2 [1.6] compared to the working phase 1.4 [or less].)
That matches my first IANTD course. That was ages ago but the most recent one (this month) involved using pure oxygen, or as pure as you can get it, as a bail out at 6 meters.
> However, what doesn't surprise me is the silence from "HQ". There appears to be no-one commenting upon this thread, justifying the statement, or providing references for the claims within the recent statement.
I think the problem is that it is not an HQ problem. They have a technical committee who make the decisions and that's it. You can't give somebody the job of setting standards and then pick and choose which standards you want. HQ's job is to print it and disseminate it which they have done. If you were to complain that you couldn't find the information somebody would come straight back and point you to it.
nigelH
> They have a technical committee who make the decisions and
> that's it. You can't give somebody the job of setting standards
> and then pick and choose which standards you want.
That's true, but you *can* expect such a technical committee to enter into discussion when their advice differs from that of everyone else in the known world. If the position is defensible, defend it. If it isn't defensible...
And therein lies the problem. This issue has been hanging around for *years*. It has demonstrably cost BSAC membership. Yet every time BSAC has been asked by its membership to discuss the matter, the response has been a flat "no". This time around, it's no different - no-one's interested in a historical statement of what BSAC has said, we're interested in BSAC reconsidering its bizarre position.
Vic.
Nigel Hewitt
31-03-2004, 14:20
And therein lies the problem. This issue has been hanging around for *years*. It has demonstrably cost BSAC membership. Yet every time BSAC has been asked by its membership to discuss the matter, the response has been a flat "no".
Agreed. The technical committee seem to be accountable, on technical matters, to nobody but themselves. That must rest on the elected council because, as Sartre observes, in choosing your advisors you choose your advice. I must confess it annoys me but it doesn't effect my diving.
The BSAC people I dive with are sensible people and the only club dives I go on are inshore so don't involve me side-slinging 100% so no problems. If they want to come well offshore it won't be a club dive and, since I am trained to use 100%, if it is a reasonable mix for the dive I will bring it.
BSAC is just being left behind by many of us - even relative newbies like me. It's rather sad. If BSAC finally just becomes a club for people who like teaching scuba British diving will be the poorer.
nigelH
J Abbott
31-03-2004, 14:30
I couldn't find such a recommendation when I looked at the current standards (found via google), but since I don't hold IANTD qualifications, I wouldn't claim to be a definative authority on them! Do you have a current (preferably online) reference?
Yes. Try the Link or search for 'Teaching Oxygen Tracking' with google.
I teach TDI not IANTD. I understand the logic in calling for a reduction in ppO2 but it seems a bit specious. If I am doing a 6m stop in extreme cold am I going to use a 98.4% mix rahter than O2?. I think not. Nor would I plan for a stop depth of 5.75m to reduce the ppO2.
The source of most of the teaching material is NOAA. I don't have access to their manual but would be interested in what it has to say.
John
:=At ERD level and above IANTD used to teach a reduction from the 1.6bar ppO2 of 0.025% for each of extreme cold and extreme work for the decompression phase of the dive.
I'm a little suprised to see the conecpt of "extreme work for the decompression phase of the dive"
What I was taught by IANTD was the general concept that work, temperature and time are risk factors that must be considered when planning max PPO. So if your decompressing in extremely cold water and/or fixed in a very heavy current, you might consider reducing the PPO from 1.6. Basically what I was told was assess the risks and be sensible.
In normal circumstances the decompression is far more relaxed than the bottom phase and 1.6 is fine. My IANTD Advanced NX course taught a 50% gas switch at 20m and
Technnical Nx an 80% gas switch at 10m. Both were completed in 'normal' UK tidal conditions.
Personally I think IANTD have it about right. They teach a consistent set of rules and a progressive approach to implementing them. This is a far cry from the BSAC approach which appears to teach specific limits with scant regard for the actual risks involved.
However, what doesn't surprise me is the silence from "HQ". There appears to be no-one commenting upon this thread, justifying the statement, or providing references for the claims within the recent statement.
IMVHO the recent statement is not 'that bad'. The general recommendation of a 1.4 max ppo, is sensible enough for the majority of Advanced NX divers. The problem is that experienced ERD divers, who have been trained and assessed elsewhere, do not want to be held to it.
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