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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'.
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