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I went back this weekend and reviewed Buhlmann's treatment
of HeliOx and trimix, and something funky struck
me about the way HeliOx and trimix differ.  Under Buhlmann,
Heliox is subject to accellerated decompression using
Nitrox and O2, just like trimix is.

Buhlmann treats inspired gasses independantly for calculating
half-times, then uses the proportion of dissolved gasses
to weight the a/b values for calculating Ptol, the ceiling.

Once you inspire some helium, you do this weighting for the
rest of the dive.  That's fine for trimix, but for
HeliOx,  what ends up happening is that as the saturation
level of helium rapidly drops, the ambient saturation of N2 in the 
tissues (from being on the surface), starts to play a 
proportionatly larger and larger role in 
determining a/b weightings.  This makes Nitrox and O2
work to accellerate deco, because they are reducing
the N2 gradient and treating He like N2- even though the N2 gradient is 
trivial, and has actually fallen during the Heliox dive.  

I, and apparently everybody else, have assumed that
you use the trimix weighting scheme to treat Heliox,
but if you think about the nature of a halftime model,
decompressing on ANY O2 and/or N2 mixture should give you
very similar deco profiles.  

In fact, if you don't apply the trimix weighting scheme for 
Heliox, and treat the gasses sequentially, Helium with Nitrox/O2
deco generally has faster deco profiles than either air/Nitrox/O2
or trimix/Nitrox/O2, because once you get off the HeliOx and 
onto ANY mixture not containing helium, you have instantly
maximized the off-gassing gradient.

Why shouldn't Heliox/Nitrox/O2 be sequenced instead 
of treated throughout the dive under the weighting 
scheme of trimix, since N2 levels are *way* sub-critical?  

Could someone explain why and how Buhlmann came to this 
treatment of Heliox, or if in fact he did?

Thanks,

-Will

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