Article on deep stops
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atdotde
Contributor
I blogged some comments on the idea of depth independent M-values here: Short comment on Doolette’s “GRADIENT FACTORS IN A POST-DEEP STOPS WORLD” – The Theoretical Diver
Yes it should matter a lot. The question whether your first deepest stop is too deep due to loading slow compartments depends a lot on whether you do that stop on 18/45, air or EAN50.
Therefore any strategy of choosing GF_low independent from gases used doesn't make much sense to me.
Basking Ridge Diver
Contributor
I know right? But it does not matter... which is why I asked the question...
"Short answer, yes (agnostic).
Long answer would be an account of the uncertainties around all of this stuff - irrespective of the gases. I am probably going to put a comment on the current thread later.
The theory would be that because helium is allegedly a faster diffusing gas, you should consider selecting a lower GF low (and therefore forcing deeper stops) on a deeper dive with high helium content, so that you avoid too much supersaturation in the fast tissues early in your ascent. However there is doubt about the actual differences in gas exchange kinetics between helium and nitrogen in the tissues that matter - Doolette's work published a couple of years ago in Journal of Applied Physiology, see here:
Moreover, the comparative studies on deep stops to date suggest that the deeper stops prescribed by bubble models (largely for the above purpose) are probably too deep for optimal efficiency. What I can't tell you is how far you should back away from these deep stops. We can take guesses, but that is substantially what they are: guesses.
Warm regards,
Simon"
LiteHedded
Contributor
70/85 here. working fine so far...
50/70 here.
Plenty of experience with 85 high and I can't do that. 80 high is my max and its borderline. 70/70 seemed a bit too narrow of a offgassing window to provide, so I've stuck with 50 as my max low and 70 when I want a conservative high, although I could do 75 high.
JonG1
Contributor
I reliably get mild symptoms at 85 high on a single OC dive.
Sometimes but rarely symptomatic at 80
Nothing at 75 high BUT I now do far longer dives on CCR, in colder water, and sometimes at altitude than the preponderance of my 12+yrs of OC trimix/deco diving has been.
The 70 or 75 high is a common setting to compensate for that uncertainty - and consequences of a "unexplained" hit. In the ocean or Great Lakes with EMS relatively nearby I usually do 75 and feel fine. For really remote stuff with serious consequences I back it off to 70 high. A more detailed version is here:
Narc'ed Diving, just launched NARC'ED TV!!!
E.g Last week was 3hrs (50mins of which was deco) at 540m (1700ft) elevation in 5C (41F) water on 18/45 dil. 400m of uphill hiking and 1.5hr drive by 4WD from a highway accessible to an ambulance. Once down to the highway, I was probably 18-24hrs away from the nearest chamber depending on the ferry schedule. Evacuation would be a monumental exercise that could jeopardize perceptions of cave diving in this area for years. Backing off to a high of 70 was an easy choice even if 75 would likely have been "fine".
JonG1
Contributor
Thanks rjack thought i'd read about it somewhere but couldn't recall where.
Out of interest on CCR do you elevate ppo2 on deco above your core setpoint or stay at 6m instead of 3m for last stop or stay on loop with high o2 on surfacing?
Out of interest on CCR do you elevate ppo2 on deco above your core setpoint or stay at 6m instead of 3m for last stop or stay on loop with high o2 on surfacing?
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