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> A question I asked on my Nitrox course (IANTD Technical Nitrox) was "Why is > the limit 40% O2?". Is there (WRT equipment) a difference between diving a > Nitrox mix near the limit and diving air at a greater depth? The ppO2 could > be the same in each case, and as far as i know O2 is the only thing that > affects combustion - the N2 is just there to get in the way. So, should air > divers be taking more care. or are Nitrox divers just adding an extra > element of caution. Or is it perhaps that the FO2 may be more important for > combustion that the ppO2. > > The course instructor sort of bluffed his way round it, passing the buck to > the equipment manufacturers. Can anybody enlighten me? I talked to a person responsible for mixing Nitrox and O2 cleaning equipment. His answers were as follows: 1) NOAA initially established the 40% guideline. I have the NOAA "Diving for Scienct & Technology" manual and am trying to find the reference. 2) FO2 is the consideration, not PO2. PO2 will vary with depth, FO2 does not. High FO2 will cause deterioration of many components such as Teflon many metals at an accelerated rate. PO2 is more of a physiological concern. 3) Adiabatic heating can result in the generation of temperatures sufficient to partially combust hydrocarbons. The greater the concentation of hydrocarbons (fuel) and O2 (oxidizer) the greater the risk of an explosion. Also, small scale combustion still produces significant quanities of CO. These answers kinda make sense, though we are no closer to the physics of what happens at 40%. I will continue looking around. :~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~: :Patrick Murphy : murphy@un*.sl*.un*.co* : :Unisys, Government Systems Division :"I would kill for her! : :McLean, VA : I would die for her! : :(703) 847-2545 : Either way, what bliss!" : : Opinions expressed are mine alone : Gomez Addams. The Addams Family : ~~~~~~~~~~~~~~~~~~~The score: Opinions-3 A**holes-1~~~~~~~~~~~~~~~~~~~~~~~~~
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