Effect of gas density while deep diving

[Swamp_monster]

You can play around with the values in here Gas density guidelines

But to achieve 5.2g/L density at 13 ATA / 400ft you are looking at using a gas in the ~80% helium range.

While mildly applicable at even the 130ft recreational limits, if you aren't doing 250+ft dives I wouldn't expect you to have ever really been introduced to this issue before.

Thanks for the info, I am still curious about the training agencies. For example, PADI's tec 50 courses you will be going about 6 ata where you are not certified to use helium yet or at least not required to be. This is above the safe limit of 6-7 g/l (7.78 g/l)... No matter what blend you are diving above 21% it seems if you are not using helium you will be too dense for safety. How is this allowed? Is this because air density related to co2 retention is new to diving or some other reason?

 

[rjack321]

Thanks for the info, I am still curious about the training agencies. For example, PADI's tec 50 courses you will be going about 6 ata where you are not certified to use helium yet or at least not required to be. This is above the safe limit of 6-7 g/l (7.78 g/l)... No matter what blend you are diving above 21% it seems if you are not using helium you will be too dense for safety. How is this allowed? Is this because air density related to co2 retention is new to diving or some other reason?

Maybe you should take this up with PADI? You should also discuss with PSAI, I don't know how often their "level 7" course is taught but it is 240ft on air.
PSAI Narcosis Management®

Oh and their insurance carriers since ultimately they are the ones defining Tec50 and PSAI Level 7 as "safe enough". Or perhaps its just that income exceeds claims, so they continue to be profitable.

 

[rsingler]

Dr. Simon Mitchell explains the challenge of gas density

 

[Swamp_monster]

Maybe you should take this up with PADI? You should also discuss with PSAI, I don't know how often their "level 7" course is taught but it is 240ft on air.
PSAI Narcosis Management®

Oh and their insurance carriers since ultimately they are the ones defining Tec50 and PSAI Level 7 as "safe enough". Or perhaps its just that income exceeds claims, so they continue to be profitable.

So you think they are aware of the issue and neglect it? I wonder if anyone has stats on how common this is in regards to diving accidents for technical or even deep recreational dives. I am not sure of this, but could it be that it may not be that much of an issue for the agency's to care about it?

 

[jgttrey]

The gas density issue has become much more prominent and standards for gas planning have not caught up.

However, I believe the issue is of much greater importance to rebreather divers that OC divers. You'll see the original paper was in that context, as well as the study that noted the spike in CO2 issues once you get above 6.0g/L was in rebreather divers. Gas density impacts OC divers for sure, but it's likely much more an issue on CCR where that dense gas is also having to go through the scrubber and WOB is already an issue. The problem is compounded.

I ran a ton of dive plans after reading this paper and what came out of it for me seemed to be that planning CCR trimix dives with typical DIL mixes designed to keep the END at <100, you often hit near the 6.2 g/L upper limit, or above. Usually not horrifically awful, but at or above the recommended maximum. Having enough helium to deal with the narcosis gets you close, but not all the way. I would never do deep dives on air DIL. The upshot is that on CCR gas density concerns would lead to a higher FO2 than traditional gas planning rules would call for. For example, 10/50 becomes a 200' DIL, ideally, because it's 5.2 g/L at that depth. It would max out at 255' when the density hits 6.2g/L.

Of course, boosting the FHe to address density issues then creates a cascade of other considerations - higher helium loading forcing deeper stops and longer deco, making ICD issues more prominent in your planning, etc. It's a complicated subject but not one that is really relevant to recreation diving or, IMHO, to something a little deeper if on OC.

 

[rsingler]

The problem with the training agencies is the time lag between recognition of an issue, and change in the curriculum. Additionally, the deaths that have occurred are always outside the recreational range, further decreasing the potentially affected population for a training agency which exists to make money. Finally, tec is not a heavy focus for PADI in particular.
When you then add in the huge body of experienced divers who have dived successfully to depths beyond 6g/l but do not understand the physiology, you have an additional headwind against change. They deny the risk, because they do not understand it. I think the Dr. Simon Mitchell video above nicely explains the physiology, and is well worth the hour's length.

 

[Swamp_monster]

The problem with the training agencies is the time lag between recognition of an issue, and change in the curriculum. Additionally, the deaths that have occurred are always outside the recreational range, further decreasing the potentially affected population for a training agency which exists to make money. Finally, tec is not a heavy focus for PADI in particular.
When you then add in the huge body of experienced divers who have dived successfully to depths beyond 6g/l but do not understand the physiology, you have an additional headwind against change. They deny the risk, because they do not understand it. I think the Dr. Simon Mitchell video above nicely explains the physiology, and is well worth the hour's length.

Yes, I watched the video last night actually and it did help me understand the concept more. I am posting here for additional info.

 

[rsingler]

Since you've seen the video, let's go back to your OP and see how we can tie it to your questions...

The thicker the air the harder it is for the body to 'process' and if working too hard the body will be making more co2 and will not be able to replace it with 02 fast enough leading to slow suffocation. If my understanding is completely off please correct me and let me know.

* The thicker the air, the harder it is to move. It's not harder to "process," except perhaps for rebreather divers who may see channeling in their CO2 absorber and other phenomena. CO2 is very diffusible, and the body can generally absorb it and release it if it is delivered to the lungs. The problem is that
a) in some divers, they are unable to breathe out well enough at the alveolar level due to small airways collapse, and
b) some divers are not triggered to breathe harder as CO2 rises, even if they could. This is the key takeaway from Dr. Mitchell's video, because these divers don't know in advance that they may be non-responders to elevated CO2.

* It's not a function of suffocation. There are plenty of oxygen molecules at depth. 21% O2 at 5ata is the same number of molecules as 100% oxygen at the surface.
Instead, they problem is a direct toxicity of carbon dioxide itself.

* Finally, effort indeed plays a key role, as elevated production of carbon dioxide in the face of the other two factors above may lead to a fatal blood carbon dioxide level.

 

[Swamp_monster]

Since you've seen the video, let's go back to your OP and see how we can tie it to your questions...
* The thicker the air, the harder it is to move. It's not harder to "process," except perhaps for rebreather divers who may see channeling in their CO2 absorber and other phenomena. CO2 is very diffusible, and the body can generally absorb it and release it if it is delivered to the lungs. The problem is that
a) in some divers, they are unable to breathe out well enough at the alveolar level due to small airways collapse, and
b) some divers are not triggered to breathe harder as CO2 rises, even if they could. This is the key takeaway from Dr. Mitchell's video, because these divers don't know in advance that they may be non-responders to elevated CO2.

* It's not a function of suffocation. There are plenty of oxygen molecules at depth. It is a direct toxicity of carbon dioxide itself.

* Finally, effort indeed plays a key role, as elevated production of carbon dioxide in the face of the other two factors above may lead to a fatal blood carbon dioxide level.

Thanks for the clarification! I feel that it was worth posting this on here for a better understanding.

- This is very off-topic but the technology already exists to measure the blood oxygen level. (check out this https://bit.ly/35KBz0s ). I wonder how difficult/valid it would be for a company to build a computer with something like this integrated.

 

[rjack321]

Thanks for the clarification! I feel that it was worth posting this on here for a better understanding.

- This is very off-topic but the technology already exists to measure the blood oxygen level. (check out this https://bit.ly/35KBz0s ). I wonder how difficult/valid it would be for a company to build a computer with something like this integrated.

Maintaining sufficient O2 saturation has nothing to do with the gas density problem.
Its all about elimination of CO2