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ddoolett@me*.ad*.ed*.au* writes: >I am prompted by the current discussion about course proliferation to >ask what use is nitrox other than as a decompression gas? Here in >South Australia we are isolated fom the fads in the U.S.A., additionally, >ANDI and IANTD are only starting in Australia and the closest training >facility is 1600km (1000 miles) away. However, most cave diving in Actually only 800km away - there is at least one IANTD facility in Melbourne now :-) >Australia occurs in South Australia and there is a small group of us who >use trimix in some of our cave dives. Having designed the decompression >protocols for these dives, I fully undestand the benefits of low inert >gas partial pressure/inert gas switching for decompression, I also >understand the theory for using a lower inert gas percentage as a bottom >mix - so please don't reply with a primer on decompression theory. >My impression, possibly incorrect, is that nitrox, with a higher >PO2 than air, is being used as a bottom mix to either reduce decompression >obligation or increase the decompression safety margin. If this is so, >is the minor reduction in tissue inert gas loading by using nitrox in this >way worth the risk of inadvertantly diving on a potentially oxygen toxic >mixture, by for instance, accidentally using the wrong cylinder (even >though they are meant to be colour coded and labelled). In the recreational Nitrox community I believe that the most common mixture being promoted is NOAA Nitrox I (32% O2). This mixture reaches a PPO2 of 1.6 at 40m which is the recreational depth limit. At this PPO2, the O2 toxicity limit for CNS toxicity is 45 minutes (DCIEM manual). As the recreational use of nitrox is strictly no-decompression, then the NDL will be far more restrictive than the O2 toxicity limit. Hence, an O2 hit with Nitrox I in recreational profiles is extremely unlikely. If the only gasses being used are Air and Nitrox I, then the consequences of a swapped tank aren't quite so serious - well, DCS (which may occur if one is diving Nitrox tables with air) IS serious, but not as bad as an O2 hit!. There is the issue of accidental depth violations potentially being more serious. However, if the violation is significant, the diver is likely to do their best to correct it ASAP and so it would have to be extreme to casue an O2 hit in the few tens of seconds taken to correct the situation. I believe the main risk is sloppy mixing procedures. If a fill station fails to test the mix after filling the cylinder, or their O2 gauge has a significant error, then the risk could be quite high. If Nitrox becomes mainstream, then the need to get fills from reputable suppliers will become even more important. The main negative with Nitrox as I see it is the additional cost. The cost to keep at least the tank O2 clean and the cost of the fills (around 3 times the cost of air in Melbourne). If the cost was similar to air, then I think it would be worth considering for the added safety margin or extended bottom time. See my next message on the IANTD system for comment on other aspects of technical training. David. ______________________________________________________________________________ David Giddy, | Voice: +61 3 253 6388 Telstra Corporation, | Fax: +61 3 253 6144 P.O. Box 249, Clayton, Victoria 3168, AUSTRALIA | Net: d.giddy@tr*.oz*.au* X400: g=david s=giddy ou=trl o=telecom prmd=telecom006 admd=telememo c=au ------------------------------------------------------------------------------
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