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Hi John: > During the management of exposures to high PO2's, we are all taught about > the need for air breaks (ie, 25 minutes on oxygen at depth, interspaced > with 5 minute intervals on air). Which brings up my question: we are also taught that after we've received a full " %cns-dose" that we can plan repetitive dive %cns by letting the first dose fall-off with a 90 min 1/2-time during the surface interval. yet, on decompressions with a lot of time spent between say 100 and 20 ft on mixes with low fO2 (as air), the %cns accumulated during these stages is low. shouldn't account for fall-off of bottom-dose me made? ie: instead of simply adding the magnitudes of "instantaneous" doses to give the total (integrated) dose, it seems account for the time that the dose occurred should be included (green's function). at least this would cause our calculations to be consistent for diving and surface interval (after all, the surface IS your shallowest deco stop....). true, it is good to be conservative in estimating %cns while you are in the water--a convulsion on the surface would probably only be embarrassing rather than life-threatening. another important reason why fall- off should not be calculated for in-water stages is the possibility of the role of CO2. effects due to "retaining," emersion, work of breathing, work, etc all lessen dramaticly on the surface. maybe this allows the "liberal" (whoops) surface treatment of %cns. none-the-less decompression safety encourages us to try and keep ppO2 high. game theory? > However, I have noticed that the major emphasis/focus is on the use > of airbreaks during the shallow decom stops (where 100% oxgyen is most > often used). it's odd isn't it? noaa, the usn, etc seem to find a greater cns danger due to high-ppO2 in mixed gas at depth (rather than pure O2). i think exposure limits (ppO2/time) are ~.2 to .5 ata higher for a given time on pure-O2 compared to gas. this could also have been a measure of usd conshelf#2 work-of-breathing though.... >Since we are concerned about exposures to elevated PO2's, > it seems to me that we should be considering the use of "airbreaks" > during the deep decom stops (ie, take an "air break" prior to starting > onto nitrox during decompression). so, there should be a fall-off included for in-water %cns calculations? if so, should the fall-off be exponential (as the 1/2-time deal seems to imply?). i think duke's statistical models give a linear fall-off of %cns risk for low ppo2. regards, em _____________________________________________________________.sig Eric Maiken email: eapg243@ea*.oa*.uc*.ed* Dept. of Physics o: 714 824-6621 U of California fax: 714 824 2175 Irvine, CA 92715-4575
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