Deco dive with divers on different back gas

[LFMarm]

If your CCR is functioning properly, there should also be negligible CO2, but that's not the issue. Nor is it the OC inhalation effort (regs have venturi assist, "over balanced" designs, etc.). The issue is the additional effort to move the denser gas out of the lungs has a tendency to shortchange the expired volume. (IIRC, the vital capacity was cut by ~50% in tests at the 5.2 g/L density level.) That leads to more CO2 remaining in the lungs and the associated problems thereof.

I think that threshold is 6.2 g/L. 5.2g/L is considered very good.

https://www.omao.noaa.gov/sites/default/files/documents/Rebreathers%20and%20Scientific%20Diving%20Proceedings%202016.pdf

 

[crofrog]

Digging into that link...

The linked paper is: Rebreathers and Scientific Diving - Workshop Proceedings


Is carbon dioxide retention as big an issue in open circuit diving compared with rebreather diving? One of the features of open circuit is each breath is from a clean source of "unbreathed" gas, so no CO2. Also OC generally has a very low work of breathing (assuming the second stage regulator is correctly used -- lever in breathe mode, cracking resistance correct, serviced, etc.).

Am not disrespecting the research at all, am just bringing it to the current topic of lightweight decompression on open circuit in the 45m/150ft range. For all of us diving beyond this level, often well beyond, then we'd be taking into account the gas densities and, as we're using rebreathers, high helium percentages are preferable.



BTW: great link, interesting paper.

Did you read it?

 

[inquis]

I think that threshold is 6.2 g/L. 5.2g/L is considered very good.

Yes, but that bar graph is different from vital capacity. The 5.2 g/L is the recommended limit. (Yes, it's a little weird they make 2 recommendations for limits.)

Apparently, my memory is doing OK. From this DAN writeup:

One study indicated that at a depth of 100 feet, the maximum volume of gas that you can inhale and exhale in one minute (maximum voluntary ventilation) is approximately half of what it would be at the surface.​

(Density of air at 100 ft is 5.2 g/L.)

I can understand that 50% ventilation penalty causing the performance degradation depicted in that bar graph. I don't know the vital capacity reduction at 6 g/L, but the more severe failure rate in the bar chart would suggest a nonlinear impact.

 

[Blackcrusader]

I think the ccr discourse has ventured off too far from what the op asked. What he asked is not unsafe if planned for

 

[Boarderguy]

Just did a 100fsw dive today. Buddy had 32 and I was on air. Same ocean, same dive plan, different gasses. My deco obligation was 26 minutes after faffing around trying to find my lost go pro. We didn't die and nobody impounded our equipment.

 

[Blackcrusader]

Just did a 100fsw dive today. Buddy had 32 and I was on air. Same ocean, same dive plan, different gasses. My deco obligation was 26 minutes after faffing around trying to find my lost go pro. We didn't die and nobody impounded our equipment.

It's a miracle

 

[belorn]

I would emphasize the importance of communication during the dive. There is always slight variation even if divers use the same gas and same GF, and having different gases mean higher variation. Communicate a plan ahead of the dive that fulfill the conditions of both divers, and continuously update each other during the dive.
Earlier in this thread there was this notion that if the EAN32 diver set their computer to air, then they could see the expected deco schedule of the air diver. This seems like a dangerous assumption.