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Tab asks about finding one's limits and about gas mixing. As for limits - a good test is fear. If you do a dive to 100 fsw and don't feel comfortable there, you have probably exceeded your limit. On my first deep (>130') coldwater dive I was so worried that I barely slept the night before. In retrospect I wasn't really ready to do it, and was lucky it went smoothly. Unfortunately, some divers don't know enough about the risks to be afraid (e.g., the guy whose 160' dive on a single 60 raised so much ire on this list). As for gas mixing, most people just use the ideal gas law. Helium is the only gas in current technical use for which nonideal behavior may be significant. On the other hand, according to my trimix instructor (Randy Bohrer), decompression schedules for commonly used trimixes (e.g., 17/50) are relatively tolerant of small errors in He/N fraction. Moreover, since PO2 is the only thing anyone measures (He analyzers being too costly for sport divers), not that much accuracy can be expected in the He/N mix (+/- 5% is good). Given these facts, nonideal gas behavior of He is lost in the noise. WARNING - if you are new at this you should at very least become familiar with oxygen cleaning and safety rules before you even mix nitrox. Better yet, take a nitrox course; these are now widely available. You will avoid learning from the costly and sometimes fatal mistakes of others. Homebrew nitrox and trimix can be done safely, but you should be aware of the physiological concerns, which include DCS and drowning from CNS tox-induced seizures, and physical concerns, which include fire or explosion from exposing O2-incompatible diving gear to high pressures of O2. Jheimann@sc*.gt*.co*
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