Counterdiffusion (high-nitrogen-content gas after high-helium-content gas)

[J-Vo]

My understanding is it used to be an issue when divers would switch from TRIMIX to deep air(think 200' stop) for deco. If you stick to standard gasses at standard depths it shouldn't be an issue. This means using a 21/35 on the way up, not switching from 10/70 to 36 at 100' after spending 30 minutes under 300'.

 

[Dsix36]

When I was doing deeper dives with long bottom times we would graduate the helium % in the BO tanks to help mitigate the counter diffusion risk.

This one time at band camp when things went sideways and our 5 hour dive turned into a 10 hour dive, I did do a complete diluent swap and flush to remove the 10/70 trimix and went to straight 32%. Was is smart? Probably not, but I needed to reduce my deco time and this removed several hours. I think I got lucky to not experience any side effects from it. Definitely not something that I would recommend doing though.

 

[inquis]

remove the 10/70 trimix and went to straight 32%. Was is smart? Probably not

Not super dangerous, as I understand it. Again, inner-ear DCS is the issue, and it had quite a long time to off-gas prior to the 32% switch. @Dr Simon Mitchell's explanations cite the danger when the inner ear is very near critical saturation -- IBCD is the "straw that breaks the camel's back". By the time you reach 130 ft, there's plenty of room for that straw.

 

[divingx]

Hah--maybe that is it!!

I *think* there is some validity to the IBCD issue but I cannot state what it is. Its one of those things I heard about and believed...maybe.

Probably because of superficial ICD. It happens when the breathing gas has lower diffusivity than the surrounding gas.

 

[rjack321]

Why would anyone ever inflate with helium in the first place??? Never heard of that.

Not applicable in a drysuit... but in a diving bell you're surrounded by trimix, much deeper, the whole atmosphere can be switched out by the operator - the whole IBCD "issue" is a by product of commercial diving operations not recreational