Hi guys-
First, it is difficult to apply some of these references I have seen offered in regard to helium use. Commercial diving depths and exposures are not applicable to "recreational" type dives.
HPNS is not a major issue for technical dives. Forget about that...
Some misconceptions:
Helium is not bad in regard to decompression. In reality, you will incur a *less* severe decompression penalty from helium than nitrogen. I know what the references say and what the deco programs spit out. They are wrong. Helium is very friendly. Most deco programs will penalize you for higher helium (versus nitrogen) content in your mix which is converse what should happen.
Helium does not necessarily play a big role in "cooling" you down, with the exception as a insulating gas. Yes, you do feel cooler at depth, but I think this is largely due to the fact that you are not impaired by narcosis. I remember diving wet, on air, off VA and MD to 180' and not having any issues with temperature. However, diving to 180' on mix, wet, off FL, in temperatures 30 degrees greater than VA, I can stil catch a chill.
Heliox is not used due to cost and logistics of pumping. Primarily, the second issue. You need a booster pump to get the needed pressures of a heliox mix and not every person or shop has access to a booster.
FWIW, we have been using heliox 50/50 (along with 100% O2) for decompression with great results. Totally eliminating nitrogen during deco is the way to go.
Cheers,
Mike
Association of Underwater Explorers
http://www.mikey.net/aue