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On Wednesday, 1 May 1996 Dennis Pierce wrote: >> because it doesn't use oxygen as the source - most closed units do - if >> you observe the MOD of your nitrox mixture (typically 112' for EAN32%). >i think you need to go back to your books again. >> This depends on what you consider your safe pPO2. I am a sport diver so >> while others might consider a pPO2 of 1.4 ATA (112 fsw) acceptable on >> EAN32%, my own choice is to use an MOD of 100 fsw for this gas. i.e., a >> pO2 of 1.29 ATA. However most nitrox courses teach 112 fsw as the >> absolute MOD for EAN32%. >sure you arent talking about 36% ____________Reply separator_____________________________ Dennis, EAN32% @ 100'fsw = 1.29 ATA pPO2, @ 112'fsw = 1.40 ATA pPO2 EAN36% @ 100'fsw = 1.45 ATA pPO2, @ 112'fsw = 1.58 ATA pPO2 The NOAA table stops at 1.6 ATA and implies this as the maximal permissable pPO2 exposure, but this limit has much more to do with pulmonary toxicity than CNS toxicity, despite the term CNS clock. Convulsions have been reported at exposures to pPO2s of 1.3 ATA (US Navy Divers Manual - 1995 revision). While it may be a matter of choice or risk acceptance for the informed diver, recreational nitrox courses do not customarily teach divers to plan a decompression dive. For this reason pulmonary toxicity is rarely an issue for these divers to consider. The NOAA Nitrox I table gives a 20 min NDL @ 130'. This is a pPO2 of 1.59 ATA and 25 mins shorter than the NOAA single exposure limit of 45 mins. Even doing multiple dives in a day, most divers keeping to the NDL limits would find it difficult to exceed the NOAA 24 hour exposure limit of 2.5 hours. So CNS safety is the issue: why would you teach a student to dive at a pPO2 which could result in an low-frequency, high risk event? i.e., a convulsion and probable/certain death. For the recreational nitrox diver, who does not have to accept a higher pPO2 to speed up decompression, it doesn't make sense to me to assume this risk for the sake of going a few feet deeper. I stand by what I said and was taught (Steve Hoffman, PSA was my instructor): A prudent MOD for recreational nitrox divers using EAN32 is 100', i.e., a pPO2 of <1.3 ATA. PSA recommends a limit of 1.4 ATA for "dives involving thermal stress or physical exertion, becasue these increase the risk of O2 toxicity". Do IANTD or TDI recommend differently? Regards, Peter Heseltine
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