Re-Evaluating My GF

[rsingler]

@Dr Simon Mitchell ?

 

[rjack321]

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.

basically the exact opposite of the recent thread asking "why don't we ascent in 1ft steps"

 

[dmaziuk]

basically the exact opposite of the recent thread asking "why don't we ascent in 1ft steps"

I don't think that one has gas switches though. It's an interesting question because by strictly Haldanean model if you ride your chosen M-value line continuously, you're off-gassing most efficiently the whole time. Until you change the gas.

 

[stuartv]

I don't think that one has gas switches though. It's an interesting question because by strictly Haldanean model if you ride your chosen M-value line continuously, you're off-gassing most efficiently the whole time. Until you change the gas.

Even with gas switches, you’re still off-gassing at max efficiency.

 

[Jay]

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.

Just have to say it's nice having you back!

 

[dmaziuk]

Even with gas switches, you’re still off-gassing at max efficiency.

Hmmm... I wonder if integral supersaturation is the correct measure here... That graph does look counter-intuitive.

 

[TrimixToo]

Just have to say it's nice having you back!

+1

 

[Celt]

That seems aggressive, out of interest, what is the basis for your profile?

reading back through some of the posts the dive run times seemed very long so i checked a similar dive i had on air and that was the plan i used with IDECO PRO it didn't really bother me because the run time was 3 min longer than my tables gave me.

 

[Celt]

and the gases/switches are?

what about this seems aggressive?
This plan is basically GFs 40ish/70 for 30mins at 52m with 50% and 100% for deco. ~40/70 is not typically called aggressive.
The 22 and 15m times are a bit short so I don't know how this profile was derived. But there's ample O2 time so it'll likely work

No gas switch, Was just looking back at some of the run times and they seemed very long so i checked a similar dive i had and that was the plan i used from IDECO PRO although i used a lot less gas than the plan showed.

 

[scubadada]

my run time for a 30 min bottom time at 52m is 83min
52m/30min
42m/1min
32m/1min
22m/1min
15m/1min
12m/4min
9m/7min
6m/12min
3m/23min
back in the boat

That seems aggressive, out of interest, what is the basis for your profile?

and the gases/switches are?

what about this seems aggressive?
This plan is basically GFs 40ish/70 for 30mins at 52m with 50% and 100% for deco. ~40/70 is not typically called aggressive.
The 22 and 15m times are a bit short so I don't know how this profile was derived. But there's ample O2 time so it'll likely work

reading back through some of the posts the dive run times seemed very long so i checked a similar dive i had on air and that was the plan i used with IDECO PRO it didn't really bother me because the run time was 3 min longer than my tables gave me.

No gas switch, Was just looking back at some of the run times and they seemed very long so i checked a similar dive i had and that was the plan i used from IDECO PRO although i used a lot less gas than the plan showed.

So, I guess my question wasn't so stupid after all. Using MultiDeco, I got a run time of 90 minutes with Buhlmann ZH-L16C 100/100 and 113 minutes with VPM-B nominal.