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Comment by slau | Hacker News Reader

I don’t entirely disagree with you, however him switching to his emergency cylinder could still reduce his core temperature very quickly, no?

The heliox mixture supplied by the ship/bell is most likely heated/insulated (by the same warm water heating his suit). Once he switches to the emergency reserve, the 2-300 bar gas gets expanded to 10 bar, massively cooling it down. That cylinder was most likely already at 7-10°C (whatever the ambient temp was), so the heliox is probably coming out at just above freezing. Thoughts?

I haven’t dived trimix yet, but I know many in my club have indicated it’s easier to feel cold when diving OC trimix, and recommend it as yet another reason to only dive trimix on CCR.

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No. Divers lose much more heat through conduction than respiration. And the emergency reserve gas only lasted a few minutes so once that was exhausted he wasn't breathing at all.

A rebreather helps a little because the gas stays warmer but it isn't a huge factor. I've been using open circuit trimix for 20+ years in the cold waters off of California. It's fine as long as you have a separate tank for drysuit inflation. Divers may "feel" colder when breathing trimix but this is an illusion due to the reduction in gas narcosis: one of the effects of narcosis is to make you feel warmer even when you're hypothermic. Helium is an excellent thermal conductor but the heat capacity is actually lower than nitrogen so all else being equal you're not losing body heat any faster by breathing trimix.

There's lots of mythology in diving. Even a lot of technical diving instructors seem to be unclear on the basics.

Interesting. Maybe I got confused because I remember watching Simon Mitchell’s talk[1] about the hydrogen dives with Richard Harris. He mentioned that the Comex Hydra tests revealed hydrogen was significantly cold to breathe.

If I’m not mistaken, hydrogen has a lower heat capacity than helium. So how come helium doesn’t feel cold to dive? Could it also have to do with the actual percentages?

Mitchell mentions that for their 245m dive they’re using 4% O2, 91% H and 5% N2. Could it be that most average trimix doesn’t incur a temp penalty because when we’re diving 30/70 it just doesn’t have that much of an affect?

[1]: https://m.youtube.com/watch?v=skL5EQa8DFY

I don't know. I have no experience with hydrogen mixes and wouldn't presume to contradict Dr. Mitchell but this might again come down to a difference between perception versus actual measured body heat loss.

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