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You wrote: > > >>The resetting of the clock in 12 h with a t1/2 of 90 min does not >>follow, assuming that it takes 6 t1/2 to reach 98+% desaturation, as is >>the assumption with N2 offgassing. One would expect that a 9 h >>desaturation would be required to reset with t1/2 = 90. >> >>Comments? > >Magic! > >No, after 6 half-time, you reach approximately 98.6%. However, the math >still apply beyond that percentage. If you apply the formula, you'll never >reach 100% (or in our case, 0% residual CNS). After a 12 hours surface >interval, applying the exponential decay formula, the residual CNS% becomes >0.39625% (which is assumed to be zero). > >If you check the residual CNS% after a 9 hours SI, it should be 1.5625%. > >However, him not sure it was proved that the CNS% is resetted at an >exponential decay rate. The NOAA guideline is simply to wait 12 hours. > >Michel Therrien >m.therrien@ne*.qc*.ca* > > I'm quite aware of the asymptotic nature of an exponential decay. What the question was asking: Why is the clock reset in 12 h for a 90 min t1/2? This issue comes up frequently and a 'logical' answer has yet to be produced. In the case of N2, the USN tables reset the clock (as in repetitive diving) at 12 h because the controlling compartment, for repet dives on Standard Air Tables, is chosen as the 120 min compartment. THus - either NOAA advocates some intrinsic difference between N2 clearing & O2 clearing, or the ~ 1.5% residual O2 after 6 half times (the N2 case) is considered significant. Again - comments on the issue! Esat Atikkan
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