Effect of gas density while deep diving

[jgttrey]

Thanks that makes sense. But doesn't that raise the question of how gas densities were calculated in the QinetiQ database?

The correctness in "absolute" terms is not as important as making sure our gas density planning is based on a calculation that is apples-to-apples the same with the way densities were calculated in the dataset. It's not a question of which is right: they just need to be the same.

 

[KenGordon]

OC is <probably> more forgiving. Assuming everything is working properly. Of course something like a FFM can radically change that answer - and nobody has any hard data. Even the CC data isn't a super bright line.

From talks given by Simon, particularly with reference to people dying at extreme depth, I get the impression that WOB reflects an issue rather than being the issue. The limiting factor is getting the gas in and out of the alveoli and that is eventually prevented by Bernoulli effects, which in turn depend on the gas density.

 

[rjack321]

From talks given by Simon, particularly with reference to people dying at extreme depth, I get the impression that WOB reflects an issue rather than being the issue. The limiting factor is getting the gas in and out of the alveoli and that is eventually prevented by Bernoulli effects, which in turn depend on the gas density.

that would be far beyond 5 or 6 or even 8 gm/L

the issue is how best to draw a line in OC when the published population of divers was on CCR. 5.2 gm/L on OC is going to have less propensity to have a CO2 than 8. How much less nobody really know

 

[KenGordon]

that would be far beyond 5 or 6 or even 8 gm/L

the issue is how best to draw a line in OC when the published population of divers was on CCR. 5.2 gm/L on OC is going to have less propensity to have a CO2 than 8. How much less nobody really know

So is there another reason gas density matters?

My understanding is that as density increases this effect gets worse and eventually you die, but meanwhile gas exchange is compromised. I am assuming that the compromised gas exchange is what matters, so you can’t get rid of co2 or get o2 into the blood as effectively.

 

[jgttrey]

Ross's response (he mentioned being banned, but was kind enough to respond):


The sample equation is off. 39.9m is not 4.9 ATA - that is divers
rounding at work. 39.9m of salt water, at sea level, is 4.96 ATA =
(39.9/10.078) + 1. This slight variation in ATA values used, is a
common error in "diver rounded" calculations.

Gas density is easily seen at sites like this:
Gases - Densities

The Alert Diver values agree with those above, but they list the STP
values at 0° C (32F). However, our program uses the 20°C values, which
are more realistic for human consumption. If there was a 30°C set of
values available, we would use those instead.

Air @ 20°C = 1.205 g/l/ATA
1.205 * 4.96 = 5.97 == 6.0.

The water vapor content and the gas temperature at the restriction
point, is debatable, and they both affect the calculation.

There is no rounding of data in our program, but different
interpretations of the problem and a gasses density gives a slightly
different answer.


Please post this in full.


Regards
--
Ross Hemingway
HHS Software
Kingston ON Canada
HHS Software Corp.

Thanks that makes sense. But doesn't that raise the question of how gas densities were calculated in the QinetiQ database?

The correctness in "absolute" terms is not as important as making sure our gas density planning is based on a calculation that is apples-to-apples the same with the way densities were calculated in the dataset. It's not a question of which is right: they just need to be the same.

Further to this....

Reviewing the Mitchell and Anthony paper, I don't see a specific reference to how QinetiQ calculated gas density -- i.e., were the densities for the dives in the database calculated at "Standard Temperature" of 0°C or the "Normal Temperature" of 20°C that is used in MultiDeco? However, immediately after the discussion of the QinetiQ data (but not specifically related to it), the paper has a table for gas densities at Standard Temperature and Pressure. So, my "soft" assumption is that the QinetiQ densities were calculated the same way - at 0°C reference.

The difference is material as there is about a 6.8% difference, with densities calculated at 20°C (per MultiDeco) being lower. Thus, it would seem that if you are using MultiDeco to calculate gas densities, you'd need to adjust the target you are shooting for to something like 4.9-5.8 g/L. Or, just don't use MultiDeco for this and do the calculation manually with the spreadsheet. The comparison needs to be apples to apples.

If I had a request, it would be that MultiDeco allow users to tell MultiDeco to use Standard Temperature for gas density. I'd also like to see (on the mobile version) gas density reported in the "Best Mix" tool rather than having to actually generate a dive plan, then a report, before seeing the density.

Anyway, I hope someone will gut check me on the above as it seems to be a material issue and I am far from comfortable in my thinking.