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I think Rich's answer is technically correct, but then we are dealing in a situation where the obvious has suddenly turned Mondo Bizzaro by the injection of unrelated silliness. On Sun, 20 Oct 1996, Richard Pyle <deepreef@bi*.bi*.ha*.or*> wrote: > >I think George's answer was closer to the mark, although Mike's answer >probably explains the rationale behind why the differences exist as they >do. > >Rich > >On Sun, 20 Oct 1996, Scott Cherf wrote: > >> At 7:05 PM 10/19/96, Mike Severns wrote: >> >> >I think that you are not taking into consideration that on closed circuit >> >(CC) >the PPO2 (the set pont determines the PPO2) is not changing but the >> >O2% is. It >grows less as you decend and greater as you ascend. >> >> Yes exactly, that's what I was getting at. Above about 70fsw, the O2 fraction >> in the breathing loop is higher than air when the setpoint is .7 ATA. So >> I'd expect that the CCMGUBA tables would show longer NDL's at depths shallower >> than 70 when compared to air tables, and shorter NDL's below 70. >> >> >At a .7 set point on a CC unit you would be breathing 28% O2 at 50 feet, 31% at >> >40 feet,37% at 30 feet and so on, as compared to a constant 21% on OC. The O2% >> >is increasing as you get shallower. On open circuit it remains the same. >> >Therefore you are eliminating N2 at a faster rate at a .7 set point on a CC >> >system than you are on an OC system. >> >> Interesting idea. You think the tables are accounting for more efficient >> offgassing during a normal ascent, and that this offsets the increased >> ongassing at depth? >> >> I'd pretty much decided that it was just a difference in the 'conservativeness' >> of the tables, and that the SRL tables were likely to be more conservative >> than the Navy tables since they'd been developed for a different user group >> with different operational methods (i.e. no onsite recompression chamber). >> >> I hadn't thought about the increased offgassing efficiency, mostly because >> I'd assumed it would be an inconsequential effect at a 60fpm ascent rate, >> but from 70' I guess it does work out to more than a minute breathing higher >> and higher O2 fractions until you hit 70% at the surface. >> >> Regards, >> Scott. >> >> >> -- >> Send mail for the `techdiver' mailing list to `techdiver@terra.net'. >> Send subscription/archive requests to `techdiver-request@terra.net'. >> > >Richard Pyle >deepreef@bi*.bi*.ha*.or* >******************************************************************* >"WHATEVER happens to you when you willingly go underwater is >COMPLETELY and ENTIRELY your own responsibility! If you cannot >accept this responsibility, stay out of the water!" >******************************************************************* > >-- >Send mail for the `techdiver' mailing list to `techdiver@terra.net'. >Send subscription/archive requests to `techdiver-request@terra.net'. > > George M. Irvine III DIR WKPP 1400 SE 11 ST Ft Lauderdale, FL 33316 954-493-6655 FAX 6698 Email gmiiii@in*.co*
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