Effect of gas density while deep diving

[rjack321]

But it's a very small population at risk:
IF you're a non-responder to elevated CO2, and
IF you're at 130', and
IF you're unexpectedly forced to exert yourself with otherwise low-resistance equipment (open circuit), then you are starting to be at risk for gas density issues.

@Kfay
And at 130ft the CO2 you retain when you work hard on OC probably isn't going to kill you. More narced yes. But if you aren't in a cave or a wreck and you're going to huff and puff and go through your limited gas supply faster trying to blow off the Co2 which most of the time is just going to cut your dive short before you go up.

In a cave or wreck, your minor Co2 hit, compounded narcosis from the Co2, and decreasing gas supply is going to be a problem quick. Which is why "deep air" has been a known risk factor in cave diving fatalities since Exley first published a "Blueprint for Survival" way back in 1979.
https://nsscds.org/wp-content/uploads/2018/05/Blueprint-for-Survival.pdf

 

[FreeFlyFreak]

And the research he quotes indicate that gas density becomes borderline already somewhere between 30m/100' and 40m/130' unless you have some He in your mix.

Alert Diver | Performance Under Pressure

 

[jgttrey]

I'm having a hard time imagining anyone doing a retrospective analysis of OC diver's gas density similar to what Anthony and Mitchell published in 2015.
The pool of dives is immense and the outcomes are not as clear.

I agree that it would be hard to pull together a retrospective analysis and that's not really what I had in mind. The data they reference on pp. 72-73 of their paper from "QinetiQ" was from lab test dives and seems to have been much of the foundation for the 5.2/6.2g/L recommendation. If I'm reading it correctly, the data shown in this paper, and on the bar chart, is for rebreather dives only, but there is similar data for OC dives that is "nearly identical." I'm intrigued because I would think that while the effect might be identical, the inflection point might be different.

Obviously, probably not of interest/money for a big study of deep OC Trimix gas density effects, but perhaps there is more data to be mined from existing sources. My only point is that I wouldn't be surprised if the density recommendations were slightly different for OC.

 

[rjack321]

Alert Diver | Performance Under Pressure

What I find sad is that it took 4 years for this work to make it from here:
https://www.omao.noaa.gov/sites/def...rs and Scientific Diving Proceedings 2016.pdf
before it's finally in Alert Diver.

 

[rsingler]

What I find sad is that it took 4 years...

Wouldn't it be nice if Deep Stops had disappeared that quickly...

 

[FreeFlyFreak]

@Kfay
And at 130ft the CO2 you retain when you work hard on OC probably isn't going to kill you. More narced yes. But if you aren't in a cave or a wreck and you're going to huff and puff and go through your limited gas supply faster trying to blow off the Co2 which most of the time is just going to cut your dive short before you go up.

In a cave or wreck, your minor Co2 hit, compounded narcosis from the Co2, and decreasing gas supply is going to be a problem quick. Which is why "deep air" has been a known risk factor in cave diving fatalities since Exley first published a "Blueprint for Survival" way back in 1979.
https://nsscds.org/wp-content/uploads/2018/05/Blueprint-for-Survival.pdf

Thanks for posting the link.
Heard about it.
Well worth the read.

 

[seeker242]

This seems kinda neglected by training agencies like PADI

Not just PADI, TDI also no mention of density.

 

[АлександрД]

Are you really talking about breathing gas density for non professional divers???
Also about "feeling" of gas density near 40m (120ft) depths???
Please, read this Forgotten experiment "Neon 400" Here really was a problems.
And forget about "problems" with gas density for others, than commercial deep divers..

 

[grantctobin]

Are you really talking about breathing gas density for non professional divers???
Also about "feeling" of gas density near 40m (120ft) depths???
Please, read this Forgotten experiment "Neon 400" Here really was a problems.
And forget about "problems" with gas density for others, than commercial deep divers..

Please don’t espouse such unsafe nonsense. Gas density is a HUGE concern in deeper than recreational depths on both OC and CCR.

 

[АлександрД]

In compare of 40 atm test with Neon (ρ=0.9) and use of Helium for standard mixes (ρ=0.17) - it is nothing (please, see smile at the end of my message)
Neon has weight of 36 g/l at 400m.
Helium has 6.8 g/l at 40m.
Five time bigger. And "human rabbits" has staying alive at the end.

P.S. Overbalanced Aqualung Legend (and Apeks then) was developed exactly to help with bigger gas mix density.