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Wrolf asks about O2 tox calcuations on a typical Doria dive. I ran through some as a lunchtime exercise. First, one has to define "typical Doria dive." I am speaking second hand here, but I understand that since the Doria has such a high profile (254 fsw max depth, 170 fsw to the side) that a "typical" dive can be anywhere from 195 fsw max (touring the Promenade deck) to 225 fsw (hunting for second class china). Second, one has to compute cumulative O2 tox for the whole dive. Randy Bohrer has developed an approximation for this which will be used in the Dive-Rite nitrox computer (the Bridge). The procedure is to divide a dive into time increments, determine the PO2 a diver is breathing in each increment based on FO2 in breathing mix and depth, and then compute the fraction of the NOAA CNS tox limit for that PO2 which the increment constitutes. E.g., if you are breathing air at 218 fsw, then your PO2 is 1.6 ATAs with a NOAA limit of 45 min, and 10 min at this depth would be 0.22 of the CNS limit. One then sums the fractional limits over time increments to get the total CNS exposure. This should be a conservative model, since going from high PO2 to low PO2 should actually reduce cumulative O2 effects, whereas in Randy's model the exposure function does not decrease with time regardless of PO2. Calculation: NOAA limits for PO2 can be found in your nitrox manual. I happen to have one here, but don't have the Navy tables handy (the ones in the manual only go to 150 fsw), so the following calculations are based on a guess at the actual hang time required: 200 fsw / 20 min bottom time 3 minutes to ascend to 50 fsw 50 fsw 1 min 40 fse 3 min 30 fsw 8 min 20 fsw 15 min (on O2) 10 fsw 30 min (on O2) Your PO2s, NOAA CNS limits, and fractional limits are as follows: Run Time PO2 NOAA Fract. limit Cum. Limit 0-23 1.47 120 .19 .19 24 .53 720+ .001 .19 27 .46 720+ .004 .20 35 .40 720+ .01 .21 50 1.6 45 .33 .54 80 1.3 180 .17 .71 Here I have included ascent in the bottom time, and have rounded PO2s up when looking up the NOAA limit, for conservativism. I should emphasize that the deco stops I have listed are purely hypothetical, but shouldn't be too far off. The total cumulative CNS exposure on this dive is thus 0.71. Dives which do not exceed 1.00 are assumed not to exceed NOAA limits, hence this hypothetical dive does not exceed NOAA limits. Disclaimer: the procedure I have outlined is as Randy briefed it at the 1993 Boston Sea Rovers show, and as I have discussed it with him subsequently. It should be a conservative approach to O2 safety, but I (and I assume Randy also) cannot be responsible for any consequences of its use. John jheimann@sc*.gt*.co*
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