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Wrolf recently posted in favor of "poor man's mix," which is trimix blended from air and helium. While he is correct that this mix is easy to blend and analyze, I would disagree that it is an attractive bottom mix. I had a discussion with Randy Bohrer, my trimix instructor, on this subject, since I am as cheap as anyone else and would rather do things myself. First, what's good about poor man's mix: it doesn't require use of high pressure pure O2, so the mixing and pumping equipment doesn't have to be clean. No Haskel booster, clean tanks, or L-factor air filters required. Moreover, it only involves mixing two components, so errors in mixing can be corrected. The trouble with trimix is that there is no inexpensive He analyzer available. If you screw up when mixing trimix using only air and He, you can compute the FHe directly from the measured FO2 of the mix. If you screw up when mixing He, air, and O2, then you don't know from the FO2 alone how much He is in your mix. Now for the bad news: it's hard to get enough He in the mix to get an adequate reduction in narcosis. If you keep the FO2 in the mix at or above 16%, which is a very good idea since otherwise you can't breath the mix at the surface, then the maximum FHe in the mix is 24%. Now the maximum operating depth of 16/24 trimix is 255 fsw (assuming max PO2 of 1.4 ATA). At this depth, the equivalent narcotic depth (END) is 185 fsw (by coincidence, you get the same answer whether or not you assume O2 is narcotic). Cosidering the operational complexity of trimix diving, a 130 fsw maximum END is more desirable. I should note that it is possible to make more user-friendly mixes by mixing nitrox and He. For example, 18/50 mix can be blended from equal parts of EAN 36 and He. If the nitrox is accurately blended and analyzed first, then the FHe can be computed (just as with poor man's mix) directly from the analyzed FO2 in the final mix, and adjusted accordingly. Depending on how you make or buy your nitrox, you may not need any clean tanks for this approach. John jheimann@sc*.gt*.co*
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