What is a deco dive?

[sijomon]

Hi,

This question might seem a little stupid, especially as I am about to start my Dive Leader training, but;

What is a deco dive?

Now obviously I know the formal deffinition of a deco dive as given during BSAC training; "a decompression dive is any dive with a mandatory decompression stop", however is that a mandatory decompression stop as given by the BSAC88 tables (what I was taught to dive by) or as given by a dive computer (what I am coming to realise everyone *actually* dives against - currently I only really use mine as a dive timer and depth gauge).

I ask because I was talking to some divers from another club recentley (not BSAC) and they said they allways avoided deco dives (according to their computers), wereas many of my dives (maybe 50%) are deco dives (according to the tables). I have also read many forum posts, magazine articles that distinguish between deco and non-deco diving with respect to specific safety concerns, or intervals before excersise etc etc.

If the difference betwee the BSAC88 tables and the average diving computer were minimal this migth be a none issue but they often seem to be poles appart. As an example, I regularly dive Capernwary, a typical dive might be ~15 meters average depth, and maybe 19 meters max depth (probably only for a few moments) for total dive time of around 40 minutes. Now on air the BSAC88 tables give a mandatory decompression stop of 1 minute at 6 meters for this dive (assuming it's the first dive of the day). Therefore I would classify this dive as a deco dive. I have never, either at capernwary, nor on any of my other dives, some of which have been 30+ meters, had my dive computer report a mandatory deco stop. So on tables I regularly dive 'deco' dives, on computers I never do, a big difference. So we come back to my question, for the purposes of communicating with other divers and diving proffessioanls, understanding magazine articles, forum posts etc "What is a deco dive?"

Any information (please be gentle this is my first post on these forum) greatfully recieved.

Simon.

[Edward]

Hi Simon,

First off there is no stupid question (except the one you don’t ask).

Someone will come along soon and explain this more elegantly.

When planning your diving make a decision whether you are going to use Tables or a computer, once made stick to that mod for your series of dives. You have already discovered its near impossible to change from one to other.

If your selected dive planning mod requires you to undertake a ‘deco’ stop (not the safety stop many computers display) then you are doing a ‘deco’ dive; alternatively, if you overstay your welcome and a stop is required.

HTH

Edward

[AllynCarter]

I'm PADI trained (shhh! Don't tell anyone :) ) so not familiar with the BSAC 88 tables, but I've always assumed that the reason dive computers seem to be rather more lenient with the regard to deco is that they record an accurate multi-level profile, whilst tables are forced to assume worse case scenarios and always round to deeper depths than the actual dive profile.

[PeteM]

Any information (please be gentle this is my first post on these forum) greatfully recieved.

A deco dive is a dive where the planning method you have chosen indicates compulsory stop

As you have seen tables can give very different results to computers (mainly due to the square profile effect in the examples you quote) so you need to make your choice before getting in the water then follow it - and for the level of diving you are doing not get worried that another methodology gives different results

[sijomon]

Ok the answers so far seem sensible, thank you :) however the gist of my question is how is the term 'deco dive' understood more objectively. It might help understand my issue if I give a concrete example:

I am going for a days diving in a couple of weeks, for 2 two dives, which in all likely-hood will both be 'deco dives' as far as BSAC88 table go, and non-deco dives as far as my computer goes. The following day I am planning on running a half marathon. I think "hang on this might be dangerous from a DCI point of view", so off I go to the internet, the BSAC forums, the DAN forums etc and find several intelligent replies and links to articles. However these replies and articles all make a significant distinction bewteen the time recomended between diving and excersise for a deco dive and a non-deco dive. So relating this back to my initial question, and just making it a little more specific; "What is a deco dive with respect to excersising after diving?".

Essentailly, without a universally accepted defintion of a deco dive, aren't the distinctions made in articles, fairly meaningless? Without each and every article on diving safety that involves a distinction between deco and non-deco dives giving the exact profiles of the divers involved in the realted studies, how is one suppose to universally interpret the term "deco dive" and therfore sensibly apply the results of these artciles/studies to one's own diving?

To go back to the example I first gave, that of a standard dive in Capernwray, according to one article I read, if I used my computer I could go running 4 hrs after I got out of the water (non-deco dive), if I used my tables I would have to wait 6 hours (deco dive). Now I'm fairly sure my body doesn't know what planning method I used, so one of those times is wrong as far as that article is concerned. How do I tell which?

Again thanks for your time in helping clear this up for me.

[Edward]

Simon,

Forget tables v computers as different tables or different computers can give you a deco or non-deco dive profile – when planning.

As Pete stated pick or choose one planning method and stick to it, for example the 88 Tables will get you back to Surface Code A after 16 hours no matter what diving you have been doing. My Suunto may take 48 hours to completely clear if I’ve been seriously pushing the envelope, but I would have been so far outside the 88 Tables there is no comparison.

One thing to remember is the models used by Tables or computer are only that and can not guarantee your specific decompression requirements.

My question to you is; which is more important the 2 dives or the ˝ marathon?

Regards

Edward

[Ed Howarth]

Every body (and I mean "body") is different to every other body, your circumstances before a dive and after a dive is different (hydration, alcohol, sleep, temperature, stress, tiredness etc) is different from every other person, your dive profile will be different to your buddy's, however hard you try to keep together. So all a decompression model can do is have a bit of an educated guess at what your nirogen loading will be in the vast complexity of tissues in your body.

It's very much a grey area. Think of it as doing a risk assessment of whether you're going to get a bend or not. It will range from 1 in a millionth chance (never zero), to pretty well, as near as damn, a certainty.

Now, you do not want to look at odds whilst you're on a dive; If your computer (or tables) said 1 in 10 on the screen, I would poo myself. 1 in 100, I'd not be too happy, 1 in 1000....mmmm I might risk it, but most of us would much rather have a friendly yes/no answer. Good old binary, something our brains can understand. So when my tables or computer says "Do deco", I do. When it says "You've finished your deco, out you come", I stay a bit longer (I'm old and wise:rolleyes: ), and then come out. Even if I am being told it is a non deco dive, I still do some. I'm a coward. Or maybe I'm just trying to slew the odds in my favour.

Until we all have a transparent bodies, with litmus type tissues that you can see turn from red to blue as the nitrogen is released, deco will always be someone's opinion. Think of deco as more of a sliding scale of nitrogen loading, bubbles and risk and it will help your understanding.

Ed

[sijomon]

Again, very sensible, cogent answers, thank you.

So am I right in saying that the terms 'deco' and 'non-deco' are essentially meaningless in that I can't say to you 'I did a deco dive' and have you draw any firm, objective, conclusions about my dive from that statment?

[Ed Howarth]

Again, very sensible, cogent answers, thank you.

So am I right in saying that the terms 'deco' and 'non-deco' are essentially meaningless in that I can't say to you 'I did a deco dive' and have you draw any firm, objective, conclusions about my dive from that statment?
Others may disagree, but I would say absolutely correct!

I sometimes do a really short training dive with a couple of trainees where they do a Controlled Buoyant Lift on each other, or an Alternative Source Ascent. Maybe from 20 m to 6m twice. It might take us 10 minutes in total, but I make sure we all do a 5 minute stop before surfacing. OK, I make it part of the training - practicing buoyance and messing about with reg switching maybe, but I take the view that "I am in deco". No computer or table would tell me so, but the risk of DCI is undeniably higher because of the provocative profile. So I redress the balance of risk by doing "deco".

Ed

[Edward]

Again, very sensible, cogent answers, thank you.

So am I right in saying that the terms 'deco' and 'non-deco' are essentially meaningless in that I can't say to you 'I did a deco dive' and have you draw any firm, objective, conclusions about my dive from that statment?

Every dive is a deco dive, just some have stops.

Edward

[sijomon]

Ok, I think I have the answers I needed. Thanks for the time to help me get this clear.

I think I will take the phrase "Every dive is a deco dive, just some have stops." away with me.

Incidentally, I always do a 3 min stop at 6 and often do one at 9 - regardless of whether my planning method shows no-deco - I also tend to assume the worst and go with whatever advice is most paranoid. Its nice to see I'm not the only extra cautious one ;)

Thanks again.

Happy diving :D

[SUNDODGER]

I'm liking all the responses from both Eds so far. I agree and always do a stop of some description around the 6m mark - even better if you can carry on the dive from 6m to the surface for another 5 minutes or so - while having a 'stop' and slow ascent.

I also used to know a chap who worked in RNPL and advocated 'panting' on the final ascent - the rationale being that there was a greater gaseous exchange. His philosophy and I have never heard anyone else suggest it. But I figure, hey, I'm on the way up, I've still got 100 bars (usually!!) why not?

[Nigel Hewitt]

I think I will take the phrase "Every dive is a deco dive, just some have stops." away with me.
Be careful. It's a lovely sound bite but what it says is that the term 'deco dive' is meaningless and therefore useless.

I count a deco dive as one where I do deco stops.
I count deco stops as ones that come out of my computers.
My computers are not set to be conservative about stops as I will add slow ascents and extra time as I see fit but I want the computer to be honest not just bang in extra time for fun. If something goes wrong I want the computer to tell me I missed X minutes of stops not X minutes most of which was was imaginary. Missing stops I was planning to add on isn't missing stops in my book.

But then I'm a heretic who thinks BSAC88s are wacko (Back in '88 BSAC divers were tough and bent meant lying in the bottom of the boat foaming at the mouth and twitching - we've got a lot softer over the last 22 years) so you need to ignore me.

[sijomon]

I guess the conclusion I am taking form this is that the terms "deco/non-deco" are all but meaningless since you can't communicate anything by using them in a conversation with another diver, and the inability of a term to transfer meaning = meaningless.

[garethwoodruff]

Be careful. It's a lovely sound bite but what it says is that the term 'deco dive' is meaningless and therefore useless.



But then I'm a heretic who thinks BSAC88s are wacko (Back in '88 BSAC divers were tough and bent meant lying in the bottom of the boat foaming at the mouth and twitching - we've got a lot softer over the last 22 years) so you need to ignore me.

Awsome statement on the 88's :)

[garethwoodruff]

Personally I would be looking at the way you approach the dive, dive planning, maybe deep stops, rule of thirds, bail out provision..........

If you decide on using the tables, i will have a chat afterwards about how it went etc............if computer driven, use the planning software etc...

Any fool can jump in and do some deco, planning takes a little thinking :)

good luck with your dive leader course

[Janos]

I'm not so sure that 'every dive is a deco dive'. A deco dive is one where you have mandatory deco stops. However, different people have different attitudes towards conservatism. Plus tables assume a square profile which your computer isn't.

So it is unsurprising that your tables give you a mandatory stop whereas a computer doesn't. Neither is "right" - it's down to the individual diver as to the level of risk they are happy to take.

Janos

[Lanny]

Although it could be argued that if you have any sort of on-gassing at depth, then some level of decompression will occur on the way to the surface and every dive is therefore a deco dive. I would agree with Janos that this is something of a pedantic approach and the generally accepted definition of a deco dive is one that involves planned decompression stops.

If the OP's computer profiles are more aggressive than the BSAC 88's then I would suggest that he is doing a multi-level dive and slow ascent. Nigel's views on the 88s are spot on. Although they are as safe as the other tables out there for "no deco" dives, you would have to be barking mad to dive any profiles "right of the line" on BSAC 88s. Comparing a deco dive profile on 88s to a desktop deco programme's profile is an instructive and potentially scary experience.

[sijomon]

I think we may have wandered off the point a little here, which is to be expected with any forum thread to be fair. My original post's aim was to prompt discussion on how little information is actually conveyed if one diver says to another that he 'doesn't do deco dives' or something similar, or how difficult it is to correctly apply advice that says one thing for deco dives and one thign for non-deco.

[OneDragons]

When using 'normal' assent rates then the difference between deco and none deco is very real. However the line between the two is not distinct and is more a grey area.
(The reason I mention a normal assent is, a fast assent from any depth can induce DCI as your body does 'decompress' on every dive.)

So a 10 minute dive to 20m is very different to a 40 minute dive to 50m. One is clearly a deco dive the other isn't as one profile allows you to return to the surface straight away and the other does not. The problem is it is impossible to say 'exactly' when a non deco dive becomes a deco dive for each person, hence with the different methods of monitoring N loading you will get different definitions of when one becomes the other.

Or to put it another way there is a very real and distinct line between a deco and non deco dive which is defined by your physiology reacting to gas and pressure, but you have no definitive way to say where that line is.