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Hi Andy,

> If you are forced to pull your deep stops on the Helium mix instead of
> non-helium you could end up with a significantly longer decompression. 
> Wind and wave action, as well as concern of hypothermia often limit
> practical decompression times. 

I have to disagree with you here.  I know that, by looking only at 
neo-haldanian conceptions of decompression practice, what you just stated 
is very sound.  However, I am of the increasing belief that these 
conceptions inadequately describe the sorts of things that increase or 
decrease probability of DCI.  For example, as a decidedly non-authority 
figure who has no business espousing this stuff, it is in my opinion that 
several additional deep decompression stops on bottom mix, not accounted 
for by the deco algorithm (i.e., calculate deco as if you did not make 
these stops at all), would DECREASE the probability of DCI (whereas the 
neo-Haldanian view would suggest an increase in DCI probability).

More specific to the discussion at hand, I also believe the difference in
DCI probability if one had to do the "required" deep stops on bottom gas
instead of intermediate deco gas is miniscule compared to the variability
induced by our chaotic physiology. 

Compartment-based models of decompression tend to give people the idea 
that it's better to get the hell out of deep water quickly and onto a 
decompression gas quickly.  Quite frankly, I think this idea is what has 
gotten a lot of people bent.  The compartment-based models are designed 
to minimize the *formation* of bubbles.  Trouble is (as we all know) the 
bubbles are always there after a decompression dive.  Decompression should 
instead be conducted in a manner that minimizes the *growth* of bubbles (or, 
conversely, maximizes the dissolution of bubbles).  This means a greater 
emphasis on deep stops, and a relatively lesser emphasis on what diluent 
gas is breathed at what depth.

Besides, the deep stops are much shorter anyway, so their contribution to 
the absorbtion and elimination of dissolved gas molecules is probably not as 
substantial as their contribution to preventing bubble growth.

> Apparently, you consider this position flawed as you don't agree with the
concept of 
> accelerated decompression by switching inert gases. I am not qualified to
judge the 
> validity of your viewpoint, but I have not seen anyone else challenge the
concept 
> therefore I would like the input from other members of the list, possibly
referring me to 
> specific studies AGAINST accelerated decompression. 

I'm not entirely clear about how you are defining "accelerated 
decompression".  If you simply mean switching from a lighter gas to a 
heavier gas during the course of decompression, then I think most of us 
(including George, who decompresses on nitrogen/oxygen mixtures following 
trimix dives) would agree that gas switching on decompression is 
beneficial. What we might disagree on is the magnitude of importance of 
this particular aspect of decompression diving relative to the importance 
of other aspects of decompression diving.

As for specific studies, there are a couple (I don't have them right now, 
but I can get them for you if you like).  Ask Chris Parrett for a copy of 
Weinke's bibliography. Other than that, I can only offer you the wisdom I 
have acquired over many hundreds of decompression dives, most successful, 
some not-so-successful.

Aloha,
Rich

Richard Pyle
deepreef@bi*.bi*.ha*.or*
*******************************************************************
"WHATEVER happens to you when you willingly go underwater is
COMPLETELY and ENTIRELY your own responsibility! If you cannot
accept this responsibility, stay out of the water!"
*******************************************************************

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