Dan Sumners
Registered
My BSAC Sports Diver manual states, with regard to CNS oxygen uptake, "For every two hours on the surface breathing air, the CNS % is reduced by half".
However, the recently published (and highly accessible - I recommend both it and his 'The Six Skills and Other Discussions: Creative Solutions for Technical Divers', even if, like me, you're nowhere near to being a 'technical' diver) 'Staying alive: risk management techniques for advanced scuba diving' by Steve Lewis (Doppler), states "...some divers believe that if a diver surfaces with a CNS loading of X per cent, after 90 minutes on the surface that loading will have dropped to 1/2 X per cent.
"This is not a technique suggested or recommended in any NOAA manual that I know of and was characterised as 'poor science' by Dr Bill Hamilton [who helped prepare the NOAA oxygen exposure limits].
"The only CNS loading tied to a 90 minute half time in NOAA publications are exposures delivering an oxygen partial pressure of 1.6 bar/ata. These are the ONLY [his emphasis] exposures given a half time of any sort.
"If we check out the single dive limit for 1.6 bar (45 minutes) and compare it to the daily limit (150 minutes), it becomes apparent something different goes on compared to what happens with lesser ppO2s [eg single exposure for 1.5 bar is 120 mins and 24 hour is 180 mins]."
I think - but am not sure - BSAC isn't the only agency that teaches the half life, and a quick search online shows many believe it to be the case, as one would expect if it's being taught. However, BSAC does seem to be the only agency that says the half life is 2 hours, rather than 90 minutes.
Can anyone shed any further light? Does anyone have any counterevidence to Steve Lewis's claim? Do you know on what evidence BSAC or any other agency bases its claim, if any? Or are the agencies passing on potentially dangerous misinformation?
However, the recently published (and highly accessible - I recommend both it and his 'The Six Skills and Other Discussions: Creative Solutions for Technical Divers', even if, like me, you're nowhere near to being a 'technical' diver) 'Staying alive: risk management techniques for advanced scuba diving' by Steve Lewis (Doppler), states "...some divers believe that if a diver surfaces with a CNS loading of X per cent, after 90 minutes on the surface that loading will have dropped to 1/2 X per cent.
"This is not a technique suggested or recommended in any NOAA manual that I know of and was characterised as 'poor science' by Dr Bill Hamilton [who helped prepare the NOAA oxygen exposure limits].
"The only CNS loading tied to a 90 minute half time in NOAA publications are exposures delivering an oxygen partial pressure of 1.6 bar/ata. These are the ONLY [his emphasis] exposures given a half time of any sort.
"If we check out the single dive limit for 1.6 bar (45 minutes) and compare it to the daily limit (150 minutes), it becomes apparent something different goes on compared to what happens with lesser ppO2s [eg single exposure for 1.5 bar is 120 mins and 24 hour is 180 mins]."
I think - but am not sure - BSAC isn't the only agency that teaches the half life, and a quick search online shows many believe it to be the case, as one would expect if it's being taught. However, BSAC does seem to be the only agency that says the half life is 2 hours, rather than 90 minutes.
Can anyone shed any further light? Does anyone have any counterevidence to Steve Lewis's claim? Do you know on what evidence BSAC or any other agency bases its claim, if any? Or are the agencies passing on potentially dangerous misinformation?
