Bingo.

-----Original Message-----
From: Bill Wolk <BillWolk@ea*.ne*>
To: Cam Banks <cam@ca*.co*>; Techdiver Mailing List
<techdiver@aquanaut.com>
Date: Friday, April 02, 1999 4:45 PM
Subject: Re: Deep deco question


>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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