Re-Evaluating My GF

[stuartv]

I don't really subscribe to the oxygen window theory that staying at a high PPO2 accelerates off gassing.

Huh? Wha'chu talkin' 'bout, Willis?

Are you suggesting that doing your deco on your bottom gas is just as fast (for deco) as using deco gases with a high ppO2? I think I MUST be misunderstanding what you are trying to say.

 

[J-Vo]

Huh? Wha'chu talkin' 'bout, Willis?

Are you suggesting that doing your deco on your bottom gas is just as fast (for deco) as using deco gases with a high ppO2? I think I MUST be misunderstanding what you are trying to say.

Ya, I think we have a miss understanding. What I mean is that staying at 70' at PPO2 of 1.6 rather than going up to my 60' stop does not accelerate my N2 off loading.

 

[J-Vo]

From memory there is a lot in this thread, but don't know if the studies are/were published:

My DCS Hit

I finally made it through this. All I can say is wow, just wow.

 

[boulderjohn]

I don't really subscribe to the oxygen window theory that staying at a high PPO2 accelerates off gassing.

Are you suggesting that doing your deco on your bottom gas is just as fast (for deco) as using deco gases with a high ppO2? I think I MUST be misunderstanding what you are trying to say.

I believe he is referring to an old and very much debunked theory that when PPO2 is highest, the biological processes involving oxygen being converted to CO2 create an "oxygen vacancy" that essentially creates more room for nitrogen to leave the tissues and thus speed up decompression. This theory is not talking about the simple gradient difference created by the relative PPO2s of nitrogen.

This theory was first embraced by people in the DIR community, led by George Irvine. The deco processes for GUE and UTD used to accommodate this belief by extending the time for the deepest stops after a gas switch. Thus, in contrast to all other decompression plans, their 70 foot stop with the switch to 50% and the 60 foot stop would be longer than the 50 and 40 foot stops. This created what was called the "S-Curve" profile that was a key aspect of Ratio Deco.

From the start, people pointed out that it violated Dalton's Law. When I read the study, I was stunned--the conclusion leaps out in one paragraph with nothing preceding it that I could see that would lead to it. When I was a UTD diver and was required to follow that profile, I argued against it. Mark Powell flat out said it was wrong in his first edition of Deco for Divers. Eventually, GUE also decided that the theory was wrong, but it originally kept the S-curve in its teaching because it had apparently been working for them. (I still have JJ's explanation of that.) It eventually moved to a linear ascent in that part of the ascent profile. At the time I left UTD, Andrew was also saying that the theory might not be correct, but he was still using the S-Curve at the time of the recent Spisni study.

For many people, the theory was close to sacred for many years. I am surprised when I run into people who still believe in it, but they do exist.

 

[stuartv]

Ya, I think we have a miss understanding. What I mean is that staying at 70' at PPO2 of 1.6 rather than going up to my 60' stop does not accelerate my N2 off loading.

Ah. Gotcha.

 

[stuartv]

I believe he is referring to an old and very much debunked theory that when PPO2 is highest, the biological processes involving oxygen being converted to CO2 create an "oxygen vacancy" that essentially creates more room for nitrogen to leave the tissues and thus speed up decompression. This theory is not talking about the simple gradient difference created by the relative PPO2s of nitrogen.

This theory was first embraced by people in the DIR community, led by George Irvine. The deco processes for GUE and UTD used to accommodate this belief by extending the time for the deepest stops after a gas switch. Thus, in contrast to all other decompression plans, their 70 foot stop with the switch to 50% and the 60 foot stop would be longer than the 50 and 40 foot stops. This created what was called the "S-Curve" profile that was a key aspect of Ratio Deco.

From the start, people pointed out that it violated Dalton's Law. When I read the study, I was stunned--the conclusion leaps out in one paragraph with nothing preceding it that I could see that would lead to it. When I was a UTD diver and was required to follow that profile, I argued against it. Mark Powell flat out said it was wrong in his first edition of Deco for Divers. Eventually, GUE also decided that the theory was wrong, but it originally kept the S-curve in its teaching because it had apparently been working for them. (I still have JJ's explanation of that.) It eventually moved to a linear ascent in that part of the ascent profile. At the time I left UTD, Andrew was also saying that the theory might not be correct, but he was still using the S-Curve at the time of the recent Spisni study.

For many people, the theory was close to sacred for many years. I am surprised when I run into people who still believe in it, but they do exist.

Right. I was having a brain-fart earlier. Thank you for the detailed explanation, though. It has been a while since I read that part of Deco for Divers.

 

[leadduck]

IIRC Mark Powell writes in Deco for Divers that the term "oxygen window" is used inconsistently in the community. The only actual effect of oxygen to speed up decompression is, that it reduces the amount of inert gases (N2, He) in the inspired gas. Whereas the other idea, that the pPO2 difference between arterial and venous blood would help decompressing, was nonsense.

Nevertheless, staying longer right after a gas switch could still make sense. Let me compare three profiles with depth, bottom time, deco gases, and total runtime all the same:

(a) GF50/85
depth stop run gas
----------------------------------
↓ 60 2 2 18/45
┄ 60 28 30
↑ 30 3 33
┄ 27 2 35
┄ 24 3 38
↑ 21 1 39
┄ 21 2 41 EAN50
┄ 18 3 44
┄ 15 3 47
┄ 12 6 53
┄ 9 7 60
↑ 6 1 61
┄ 6 27 88 O2
----------------------------------

(b) GF77/77
depth stop run gas
----------------------------------
↓ 60 2 2 18/45
┄ 60 28 30
↑ 24 4 34
┄ 24 1 35
↑ 21 1 36
┄ 21 2 38 EAN50
┄ 18 2 40
┄ 15 4 44
┄ 12 5 49
┄ 9 8 57
↑ 6 1 58
┄ 6 30 88 O2
----------------------------------

(c) GF65/85 XX

depth stop run gas
----------------------------------
↓ 60 2 2 18/45
┄ 60 28 30
↑ 27 4 34
┄ 27 2 36
↑ 21 1 37
┄ 21 9 46 EAN50
┄ 12 13 59
↑ 6 1 60
┄ 6 28 88 O2
----------------------------------

The last one (c) is a strangely modified one: it always stays under the ceiling of the GF65/85 model, but ascends to the ceiling only if doing so increases off-gassing significantly. It will stay at the current depth as long as current off-gassing speed is considered OK. The effect of this extra rule is simply, that it'll overstay after the gas switch at 21m until the 18m and 15m stops clear, and then also waits at the 12m stop until the 9m stop clears.

If we compare the integral supersaturation of the three profiles, we get:

The difference between 50/85 and 77/77 is as expected: shifting decompression stress between slow and fast compartments. Comparing the modified profile 65/85X to 50/85 however is interesting: every compartment has lower ISS in the modified profile. Its larger GFlow is better for the slow compartments, but it also reduces ISS of the fast and medium compartments by staying further below the ceiling while on EAN50.

 

[J-Vo]

Interesting.

Could you not accomplish the same result by simply lowering your GFHI?

 

[rsingler]

It will stay at the current depth as long as current off-gassing speed is considered OK.

How do you measure that?

 

[leadduck]

How do you measure that?

This example used this rule: compare the off-gassing gradient (p_tissue - p_inspired) at the current depth with the gradient at the ceiling, for every compartment and for every inert gas (N2 and He). If there's any compartment and inert gas that has less than 0.5bar off-gassing gradient at the current depth, and is 0.4bar or more greater at the ceiling than at the current depth, then ascend to the ceiling, else stay. Always ascend if this allows switching the gas.

This is of course heuristic and a matter of tuning. There may be better rules, ideas are welcome. The main takeaway should be: no matter how you pick your GF, the rule "always ascend to the ceiling" is probably not optimal. That means also that "flying the curve" is not a good idea actually; counter-intuitively, going in 20ft steps may be better than going in 10ft steps with EAN deco.