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Bill, 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? 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? So, is there any chance some software is incorporating some "real" compartimental "a" and "b" values for Helium? Or should we depart from Buhlmanian thinking altogether and incorporate other, more complex algorithms? VPM, bubble stuff ,gradient factors, etc.. Any thoughts? rgrds --- Adri KC Haine darkangel@en*.co* "Killer factors in technical diving are complacency, attitude, oxygen toxicity, exceeding personal limits, ignorance and complexity." Mike Menduno - EuroTek 95 On Fri, 2 Apr 1999 17:41:12 Bill Wolk wrote: >On3/31/99 11:23 AM, Cam Banks wrote: > < stuff snipped> >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 -- <stuff snipped> >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 > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > __________________________________________________________________ Get your own free England E-mail address at http://www.england.com -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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