Empirical evidence on deco stops.

[Finless]

Does anyone know of any empirical evidence concerning best deco mixes for air/EANx diving.

Is it better to get on a low mix (50% O2) sooner or wait and do a richer mix later (I use 80% O2).

Given the fact that divers seem to prefer either one or the other would suggest that there are plenty of divers available for post dive bubble checks? Or, alternatively, evidence gained in the fashion similar to Mr Pyle and his deep stops is acceptable.

[Ainsleyberryman]

By best do you mean fastest deco ? And what type of diving do you mean - Trimix for a bottom gas ? - which would dictate different type of deco as opposed to not so deep Air dives.

When I plan deco I look for a short hang time but do so with a 20% safety margin in my software. I also have to consider using O2 or not based upon expected surface conditions, boat cover, etc. Also how many tanks I can carry would dictate the gasses available.

I also look to swop from one deco gas to the next when the PPO2 drops below 1 ata to maximise the O2. I also look to ensure Im not going to be narked if i switch from Tmx to Air/Nitrox on my first deep stop.

Id also look to keep my CNS low and bailout options.

However, it should be possible to develop an algorithm that takes care of the simple stuff, like PP02's, run times, gas switches, total deco.

Not much help, more questions Im afraid.

[Finless]

Originally Posted by Ainsleyberryman

By best do you mean fastest deco ? And what type of diving do you mean - Trimix for a bottom gas ? - which would dictate different type of deco as opposed to not so deep Air dives.

Not much help, more questions Im afraid.

Air/EANx diving only - all I currently do.

Better = is there any evidence to suggest it is best to get on lower O2 mix sooner or a higher O2 mix later to give the "best" results.

"Best" results = your choice of what is the meaning of "best" (safer/less tired etc).

Example 1 of a good answer = a scientist has run one of those Doppler(?) tests to check bubble formation/flow in the blood and has definitively said - "for diver X diving up to 50 mtrs on air/EANx there is clear evidence that starting your deco on X% O2 is better than...."

Example 2 of a good answer = Richard Pyle/thousands(?) of dives/discovered he felt better doing deep stops (stopped to do something as a result of the testing/sampling he was doing) and realising he felt better/less tired post diving. Any similar anecdotal evidence?

[And]

Hi

Nobody doing Doppler studies can say 'definitely' to anything. Just like Oxygen toxicity, it cannot be predicted. We cannot say how many bubbles equals a bend and also the same person doing the same dive on different days can have totally different bubble readings.

Anyway, didn't know you could deco from air .

Andy

[Ainsleyberryman]

I think it would not be possible to get empirical evidence as everyones physiology is so different as are the other factors involved - previous diving, fitness, hydration, exertion, familiarisation, stress etc. Try DAN for info.

In an ideal world you would try to eliminate the Nitrogen as fast as possible while balancing this with toxing on too much O2 and too many deco tanks.

If I were you id get some deco software and simulate a simple dive and then compare lots of different mixes and see how that affects certain things like:

Run times
PPO2
CNS
OTU
Gas switches
Stop depths.
Number of tanks
Narcosis
Surface conditions

I have found on my typical dives on Air to about 50m a deco mix of 70% give me the shortest run time and most flexability. For deeper stuff on trimix it gets more complicated and there are lots more things to consider and lots more opinions to think about.

[Finless]

Originally Posted by And

Anyway, didn't know you could deco from air .

Andy

Andy,

And there I was, thinking EVERY dive was a deco dive. Just goes to show ..... eh

Bryan

[Finless]

I am not explaining the question well enough + I suspect that I already know the answer.

Given "any" test, where two different methods are applied to achieve the same end, one would suppose that one of the methods used must be either the "same as", or "better than", the other method.

To extend my arguement further, take 2 identical divers who have done identical dive profiles (both past the NDC limits) up to the point of ascent at which point they have identical gas loading of their bodies. Diver 1 uses a 50% deco mix and diver 2 uses an 80% deco mix. Both ascend using the exact profiles as prescribed by the tables they are using (both diver are using the same tables). Is there any evidence to suggest which is the best/safest profile?

I assume the standard answer will be that one option may get you out of the water quicker BUT apart from that there is no difference to your post dive state?

Is there any empirical OR anecdotal evidence to suggest that getting on your deco mix at a deeper depth rather than waiting to a shallower depth/richer O2 mix is better or worse (excluding time variations on the deco stop). Better than the other could be judged by being less tired or scientifically proven to have less bubbles rushing round your system or ........

Simply put - which is best - decoing on a "leaner" mix and starting your deco sooner or ...... (exclude exit time from the water as method of deciding).

Is the exit time really the only difference assuming two identical divers/profiles prior to reaching deco stops?

[nigelH]

Originally Posted by Finless

To extend my arguement further, take 2 identical divers who have done identical dive profiles (both past the NDC limits) up to the point of ascent at which point they have identical gas loading of their bodies. Diver 1 uses a 50% deco mix and diver 2 uses an 80% deco mix. Both ascend using the exact profiles as prescribed by the tables they are using (both diver are using the same tables). Is there any evidence to suggest which is the best/safest profile?

If they are using the same tables then both divers should be reducing their body inert gas loadings by the same rules so for each stop depth they should leave with the same tissue saturations just at different times. There is an argument that the controlling (worst case) compartment might be different but if they have done the same bottom time/bottom mix I don't expect it.
I would say equal risk until you get to extreme cases.

nigelH

[The Purist]

I used to do the 36/80 thing, then decided to give 50/100 a go and personally I felt a lot better (more alert, less knackered) after the 50/100 deco, but I'm not a deco-racer and so use a very conservative ascent profile. I used the same s/w for planning everything, and ran the same settings in terms of conservatism for each gas combination.

Also you may want to look at the newer deco models (RGBM and VPM-B) - they appear to have a very good safety record in terms of bends 'inside the table'. The general concept of these is (roughly) that the deep stops actually benefit you by collapsing the tiny bubbles before they get significant, so by the time you hit the shallow stops you're a whole lot cleaner than the old models indicate, meaning you have less shallow stops to do.

The 'real' DIR teams have a lot of very useful stuff to say on deco - they have consistently pushed the boundaries of the established models and have got away with it. A quiet word with someone who actually walks the walk may well prove fruitful for you.

When you start your ascent there are a million and one 'safe' ascent profiles that you could use - some fast, some slow, some conservative, some agressive. What suits you depends on your own fitness, attitude to risk, equipment choices, buddy, what you had for breakfast etc. If there's a certain profile that you do regularly then have a play with different options - keep the bottom time & total in-water time around the same and play with different gasses and ascent profiles. IF you feel noticable better after any particular combination, then IF you want to be a bit quicker try tweaking the settings back a bit to see if you feel any different. Do all of this really gently and slowly - feeling 'different' should be a subtle thing, not staggering around howling from the pain in your elbows!

[Finless]

Originally Posted by The Purist

Do all of this really gently and slowly - feeling 'different' should be a subtle thing, not staggering around howling from the pain in your elbows!

Yes, I think I would prefer to avoid that.

The aim for me is to feel less tired in the evenings after a dive.

Also, I prefer it when other people do the testing and I get to reap the benefits later.