Ignoring the He?

[Mark Chase]

Following several recent threads on decompresion it croped up several times that He was ignored in the profile. I asumed this was just on the deco gas but ANdy informed me that they also ignored the He in the back gas.

Now strange but true the He actualy reduces the deco comitment on mixes up to 30% so the question is, are you lot ignoring the He in the back gas to pad out the stops to cover the He in the deco mix or it is OK ot add the He in the back gas and ignore the He just in the deco mix??

Cant find any data on this so hoping you can fill in the blanks

ATB

Mark Chase

[Dominic]

AIUI, there's two big schools of thought about He:

It's a fast gas, so your tissues fill up with it quicker and so it takes longer to offgas than N

It's a fast gas, so offgasses quicker and so takes less time than N

And then there's the intermediate of "It goes in quicker, comes out quicker, so it all depends on depths and mixes really"

You might want to put this one on the DecoWeenie mailing list - Phi could probably explain the rationales better than I can

[jasondrake]

The bottom line is no-one really knows. The whole perfusion/diffusion thing is involved as well as He diffuses faster than nitrogen but is less soluble, so it's half time will depend on whether diffusion or perfusion is the controlling factor - again something no-one really knows much about. And the birth and death of He bubbles is a complete mystery, as for nitrogen, so again it's all a bit of a suck-it-and-see. In a few hundred bend cases time there might be a bit more to go on.