On8/28/00 11:38 PM, Paul Braunbehrens wrote:

>Pardon my ignorance, but if the 50 and 100 % plan is so much better, 
>why do you have to stay in the water longer?  Intuitively, that seems 
>inconsistent (I'm just asking, not trolling for flames).
>-- 
>Paul Braunbehrens mailto:Bakalite@ba*.co*
>http://www.daw-mac.com Mailing list for digital audio on the mac


Paul -

I don't think I really want to do the research necessary to do a point by 
point analysis of the flaws of the whole 80/20 thread, but your question 
is an easy place to start and Allyson has done a lot of the legwork.  

The best deco isn't necessarily the one that gets you out of the water 
fastest; it's the one that gets you out of the water the most 
efficiently: as quickly as possible with the lowest reasonable risk of 
injury -- either short term or long term. As Allyson stated in her 
original post, even though using EAN 36 and 80/20 does get you out of the 
water a few minutes faster (under one set of assumptions), you leave the 
water with several compartments closer to their M-values than if you use 
50/50 and 02. The closer you are to an M-value, the higher your risk of 
DCS -- both clinical and subclinical -- in that tissue.  Because M-values 
are theoretical and our tolerance to overpressurization in a given tissue 
compartment varies from day-to-day, due to changes in hydration, injury, 
water temperature, and conditioning, among other things, you really don't 
want to shave the envelope all the time. 

Deco programs and tables don't just model your optimal decompression 
profile, they model your risk tolerance. You could get longer bottom 
times on straight Navy tables instead of using a deco program, but you 
don't because you don't want to accept the 10% DCS risk built into the 
original Navy tables. I could get out of the water 3 minutes faster using 
80/20 but I don't because it's riskier than using 50/50 and 02 and 
decoing a few minutes longer.  It's riskier because you leave the water 
at a higher level of overpressurization in certain tissue compartments. 
(BTW, This is precisely what gradient factors "pad")

Now in the real world, you may not feel a difference on 99 dives out of 
100, but what you're not feeling are the long-term effects of the 
repeated subclinical DCS that you get when you push the M-value envelope 
-- which manifests itself in the form of long bone necrosis and brain 
abnormalities. Since I intend to keep diving for a long time, I plan to 
avoid that. 

To make a long story short (now you can see why I really don't want to 
get too involved in this thread) the real comparison between using EAN 36 
and 80/20 deco and 50/50 and 100% O2 deco isn't -- and shouldn't be -- 
made at the level of which one gets you out of the water fastest on a 30 
minute bounce dive. It should be made by looking behind the run time and 
examining the saturation levels of each tissue compartment at the end of 
each deco stop to see which set of deco gasses is causing the least 
damage to your body and putting you at the least risk for both short term 
DCS and long term damage. 

What I think you'll find is that using a combination of back gas, 50/50 
and 02 does this better than EAN 36 and 80/20 on most of the bounce dives 
that we do. My understanding from George Irvine and Bill Mee's "Baker's 
Dozen" post is that they've found both from experience, computer 
modelling and post-dive doppler testing that this is certainly the case 
for the near-saturation diving that they do at Wakulla.

That's my take on this issue. Those of you out there with a better grasp 
of decompression theory and deco modelling than I have, please feel free 
to correct any errors.




Best regards --

Bill

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