On4/3/99 7:58 AM, Adri KC Haine wrote:

>Basically what you are saying here is that when using helium in your mix 
>(and only then), you can shorten the time of the shallower stops by doing 
>deep stops. This has been proven by the WKPP by still being alive and 
>unbent, right?

Adri -

That's my understanding, but I defer to Jess Armantrout's post for the 
particulars (Trout - thanks for the detailed explanation).  Realize that 
most of my information comes from the same source via the cocoanut 
telegraph -- my trimix instructor was Errol Kalayci, a WKPP member, and 
he telephoned George Irvine during the class for the "revised" deco 
theory.  Now that I've been diving it for about a year, I'm trying to 
figure out from a physiological/ theoretical perspective *why* it works 
-- partly because I enjoy the challenge and partly because I don't enjoy 
being a test subject!

While we're talking specifically about the effect of deep stops while 
diving trimix (at least as far as shortening the time of the shallower 
shops is concerned), Erik Baker's article on deep stops makes a 
convincing case that adding deep stops will reduce the likelihood of DCS 
on any decompression dive regardless of the gasses used.  This is true 
for two reasons: 1) deep stops force you to slow your ascent rate; and 2) 
at the start of deco, the pressure differential between compartment 
pressures and ambient pressure will be lower. (straight Buhlman model 
stops start too shallow.)

>While, OTOH, every Buhlmann based software ( I use zplan a lot) doesn't 
>take that into account and adds time to the shallow stops when doing deep 
>stops , by taking the unproven and probably wrong Buhlmann figures for 
>Helium  on- and offgassing. Am I correct in assuming this? 

That's my understanding.

>So, is there any chance some software is incorporating  some "real" 
>compartimental  "a" and "b" values for Helium?

At this point, your *guess* is literally as good as mine. Probably not 
any time soon because they're not known. I don't know if anyone is doing 
the kind of empirical research needed to refine the "a" and "b" values 
for helium.  Comex and Bill Hamilton probably have better ideas, but 
they're not common knowledge, and I don't even think the U.S. Navy is 
doing this kind of trimix research right now.  

>Or should we depart from Buhlmanian thinking altogether and incorporate 
>other, more complex algorithms? VPM, bubble stuff ,gradient factors, etc..

Who knows? Buhlman's work has proved itself to be reliable for the most 
part and (except for the helium algorithm) was well tested.  All the rest 
is educated guesswork that may model decompression better in some ways 
and less well in others. What amazes me about mix diving (and deco diving 
in general) is that we're really flying by the seat of our pants.  
Basically, what works is what works.  To me, that's a very sobering 
thought. 



Best regards --

Bill

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