On3/31/99 11:23 AM, Cam Banks wrote:

>Can someone explain to me how deep-decompression stops work?  It seems
>on the face of it that on a deep deco stop (70-100 fsw) you would
>absorbing more N2 or He into your body in the absolute sense.


Cam -

Real good question. I've been diving deep stops of various types for a 
while now, and have recently started looking into the theory behind them. 
 From what I can tell -- and I am NOT a hyperbaric expert -- the 
different answers you're getting from Mike Melendez and Jess Armentrout 
reflect the difference between theory and practice -- and with mix, 
that's a big gap because the theory isn't accurate.

THEORY -- First, for a very good explanation of the theory behind deep 
stops, read Erik Baker's  article "Clearing up the Confusion about Deep 
Stops" in the latest issue of Immersed. You can also find it at 
http://www.abysmal.com/technical/technical-clearingdeep.html  

The most important parts of the article are the tissue loading charts.  
They're hard to read, but worth it.  What Baker finds is that based on 
Buhlman's models for N2 and HE, deep stops effectively reduce the 
pressure gradient (difference) between tissue compartment pressures and 
ambient pressures on ascent and therefore reduce your DCS risk.  This 
appears to be because the first stop in a strict Buhlman model is too 
shallow and therefore creates too extreme a pressure difference between 
compartment pressures and ambient.  (I'm skipping a lot - read the 
article.)  

Baker also models Rich Pyle style deep stops and finds -- based on 
Buhlman's *theory* -- that while Pyle's method fixes the problem of a 
steep overpressure gradient between fast compartment pressures and 
ambient pressure on ascent, it increases gas loading in slower 
compartments and brings them close their M values at shallower stops. 
(Which might help explain "slobitis" -- fat bends that are the most 
commonly reported minor DCS symptom after long mix dives.) The solution 
in Baker's model and in the DECOM tables that Mike Melendez ran is to add 
time at the shallow stops -- producing a longer overall run time.

PRACTICE -- The problem with this theory is that it seems to be wrong -- 
at least in part.  Buhlman's helium algorithm was experimental -- it 
wasn't tested and it makes some assumptions about the speed that helium 
goes in and out of tissue that seem to be contradicted by experience.  
Jess Armentrout is speaking from this experience and you'll find messages 
from George Irvine to this effect in the archives. 

As I understand it (and George/Bill/Jess please correct me where I'm 
wrong), they've found that helium goes in *and* out of tissue a lot 
faster than nitrogen, so at your deep stops you're off-gassing helium a 
lot faster than you're taking in nitrogen.  Plus, the counterdiffusion of 
HE at the deep stops may slow the uptake of additional N2.  

The net result is that your helium appears to be out faster and deeper 
than the models predict and you're on-gassing nitrogen from the deep and 
intermediate stops more slowly, so you should be able to get out of the 
water sooner. In fact, because the bulk of your deco time is going to be 
spent on high N2 deco mixes (like 50/50), you don't want to overdo your 
intermediate hangs, because you may have to deco from your deco at the 
shallow stops.

Now here's the real problem -- no one (except maybe the WKPP) really 
knows how it all fits together or can model it accurately.  As much as 
I'd like to have confidence in my Decom or Voyager schedules, they're 
really just good implementations of Buhlman's model, which is itself just 
a best-guess fit for helium. 

Also, each piece of software implements conservatism factors differently 
(Does anyone know if any of the software packages use Baker's linear 
gradient factor function to adjust conservatism?) and, as Ed Thalmann 
pointed out the latest March/April 99 Alert Diver, we're the test 
subjects. (For more info on this, see Erik Baker's article, Understanding 
M Values in the Winter 98 Immersed or at the Abyss website.) 

Now take what I've just written with a grain of salt, this is just the 
best picture *I've* been able to develop of the subject -- and I'm just 
an ordinary joe diver.  Bottom line -- deep stops are proved in theory 
and practice to reduce DCS -- particularly in more critical fast tissues. 
Exactly how to implement them and how to adjust your shallow deco stops 
(longer or shorter) is educated guess work based on experience. My 
practice is to run Decom with deep stops on a very low conservatism 
setting and then add even deeper stops per the WKPP model implemented in 
Zplan. For the bounce dives that I do, I know I'm overdoing my shallow 
deco (I tend to agree with Jess Armentrout) but I also take George 
Irvine's advise and limit my deco obligation on ocean dives, so the run 
time difference is minimal.  Best advice: be careful because this is just 
theory and you're the test subject.



Best regards --

Bill

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