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On Fri, 24 Jul 1998, Kevin Connell wrote: > Will - I really know nothing about rebreathers but isn't open circuit > oxygen quicker offgassing? > > Ie/ isn't the rebreather going to start giving you the inerts back through > the loop that is coming out of your lungs in the first place, causing the > p. pressure gradient to drop? You'd think so, but the actual amount of gas that you off-gass is apparently miniscule, and doesn't affect the PO2 enough to even register. I always had this image that off-gassing involved a couple liters of gas or something, but apparently it's *way* less than that. What *is* important is making sure you fully purge the loop several times when switching to pure O2. Regards, -Will > > > > > At 10:43 AM 7/24/98 -0400, "William M. Smithers" <will@tr*.co*> wrote: > > >You would probably burn through about 25 cu. ft of > >O2 in the process, if you include a 7-8 hour deco. > > > >Regards, > > > >-Will > > > > > > > > > > > >> ---------- > >> > From: William M. Smithers <will@tr*.co*> > >> > To: CHKBOONE@ao*.co*; techdiver@aquanaut.com > >> > Subject: Re: Helium Deco / Wil > >> > Date: Thursday, July 23, 1998 9:14 PM > >> > > >> > > >> > On Thu, 23 Jul 1998, William M. Smithers wrote: > >> > > >> > > Incidentally, that's also why Heliox is a much safer gas > >> > > for repetetive multi-day diving - it clear almost completely > >> > > out of your system overnight, unlike N2, which takes > >> > > 36 hours or so. > >> > > >> > BTW, for this and several other good reasons, I've been > >> > diving with NO nitrogen anywhere in the bottom > >> > and deco mixes for about seven months now - I dive > >> > Heliox10 end-to-end, with pure O2 for deco starting > >> > at 20ft. And I feel *great* after the dives. Unless > >> > I'm cruising a reef at 80fsw on Nitrox with an AL80, it's the devil > >> > gas for me. > >> > > >> > The extra hang time is shorter than you'd think. > >> > Compared to an appropriate trimix, you're typically looking > >> > at an extra 10 minutes or so for an hour-long deco. > >> > > >> > Of course, diving heliox can start to get more than a bit > >> > pricey, unless you're on a closed circuit rebreather. ;) > >> > (I typically burn through all of four cubic feet of > >> > heliox for a typical two or three hour dive to > 250fsw). > >> > > >> > Regards, > >> > > >> > -Will > >> > > >> > > >> > > >> > > > >> > > > Regardless of the rates of diffusion of two different gasses such > >> as He > >> > > > and N2, given a fixed period of time for on-gassing and a symmetrical > >> cycle > >> > > > each gas should off-gas completely in the same period of time. > >> > > > If at the same time you pore red marbles into a bucket at a rate > >> of 50 per > >> > > > second and blue marbles into the same bucket at a rate of 100 per > >> second for > >> > > > 10 seconds then pore them out at the same rates for 10 seconds the > >> bucket > >> > > > should be empty of both groups of marbles at the same moment. > >> > > > If, however, they are each pored in at different rates that > >> decrease > >> > > > exponentially and then out at linear rates (or close on both counts) > >> then > >> > > > there can be a divergence of the points in time at which each is > >> totally clear > >> > > > of the bucket. > >> > > > This would be true of all tissues, not just the slow ones, and > >> this is > >> > > > what I suspect is happening but I am looking for confirmation and a > >> more > >> > > > thorough description of the process. I am not sure that the > >> exponential / > >> > > > linear model is valid or uniformly applicable. > >> > > > > >> > > > I can see possible merrit in your suggestion if helium diffuses > >> to points > >> > > > beyond the influence of the blood stream but this would seem to make > >> the > >> > > > process dangerously unpredictable. I agree that the use of helium > >> would > >> > > > probably shift the controlling tissue to a slower one for a portion > >> of the > >> > > > dive. > >> > > > Actually you are making two dives and two decompressions when > >> using trimix > >> > > > and I suspect that the procedures we customarily use are not the most > >> > > > effective way to manage them. I suspect that we are making a > >> descending > >> > > > excursion on nitrogen at the first gas switch so that we can blow off > >> helium > >> > > > earlier in the deco schedule than may be necessary. > >> > > > Something like this may be behind George's insistance that 50/50 > >> is a > >> > > > better deco mix than 36 - because it delays that descending excursion > >> on > >> > > > nitrogen rather than jumping the helium deco gun and paying for it > >> throughout > >> > > > the rest of the deco by having to dump the nitrogen you took on to do > >> it. > >> > > > I may be way off base but if I can get a few questions cleared up > >> I might > >> > > > be able to shorten and improve the effectiveness of my deco > >> procedures. > >> > > > This post is the first question. > >> > > > > >> > > > I appreciate your response, Wil, and I hope that the explanation > >> turns out > >> > > > to be as simple as you suggest but I fear that I am in for some big > >> headaches > >> > > > over the next few days, especially if I can get Bill Mee, Eric > >> Maiken, or John > >> > > > Crea to bite on this. > >> > > > > >> > > > Does the above seem reasonable or is my brain so fogged with > >> thinking > >> > > > about this stuff that I am missing something obvious ? > >> > > > > >> > > > Chuck Boone > >> > > > > >> > > > > >> > > > > >> > > > -- > >> > > > Send mail for the `techdiver' mailing list to > >> `techdiver@aquanaut.com'. > >> > > > Send subscribe/unsubscribe requests to > >> `techdiver-request@aquanaut.com'. > >> > > > > >> > > > >> > > -- > >> > > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > >> > > Send subscribe/unsubscribe requests to > >> `techdiver-request@aquanaut.com'. > >> > > > >> > -- > >> > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > >> > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > >> -- > >> Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > >> Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > >> > >-- > >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > > > > > -------------------------------------------------- > Kevin Connell <kevin@nw*.co*> > > Northwest Labor Systems > http://www.nwls.com > Lake Stevens, WA > > -------------------------------------------------- > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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