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"George P. Wentland" wrote:

> All nonsense aside, how about reducing decompression times with heliox
> mixtures. Replacing say 32% NITROX with 32% oxygen/68% helium and reducing
> decompression times as the result of eliminating Nitrogen in the deco mix.
> While helium also gets absorbed by the tissues it also off gases at a higher
> rate.
>
> Good Diving by George

You wont have much luck with that (unless you may be in a saturation situation
-- but who the f* does saturation diving on air?!).

Bubble physics says a bubble grows when the partial pressure gradient G
across the bubble surface is positive. This can be because:

  a) the ambitent pressure is recuded (remeber we are accending)

  b) the tissue tension increases and this is the case when you switch from
  a heavier mix to a lighter one (helium enters the bubble much faster than
  nitrogen can leave it)

  c) P_Skin or P_Elastic drop -- these are of no concern for this discussion
  (they depend on other factors like bubble size)

Research on _isobaric_ counter diffusion showed that a switch from nitrogen
to helium did lead to bubbles.

Now to the Neohaldaenean models:

With a gas swich from nitrogen to helium you increase the pressure gradient
of the nitrogen between tissue and blood. Sure this forces out the nitrogen
much faster, but at the same time you are ongasing helium. The problem now
is that you are saturating the slower compartments much more than what would
have been possible with nitrogen. Although the tolerated supersaturation of
helium is higher than for nitrogen the slow compartments still have quite small
supersaturation limits. That means you might be shortening your deeper deco
stops but you will pay the price for this at the shallower stops.

It's amazing to see that both models indicate that you'd better not to switsch
from nitrogen to helium mixes while accending!
My advise is better put the helium in your bottom gas.

Regards, Frank

P.S. Doing deco on helium based mixes while having breathed he-based deep
mixes is another story!


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