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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!" *******************************************************************
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