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> So you carry another bottle of nitrox to deco with in order flush the > loop. Air, actually. The nitrox I breathe is a mixture of air diluent and pure O2 to bring the PO2 up to setpoint. > How does the use of the trimix instead of the heliox effect the ppN2. It > seems, it isn't constant anymore. Until we get a good sensor for helium and/or nitrogen, the PN2 will always have some noise in it when mixing air with heleiox or trimix. But given the whole slew of other variables that affect DCS, I'm not too concerned about minot fluctiuations of the N2:He ratio in the loop. To answer your question, I usually have two air cylinders, two O2 cylinders, one heliox, one trimix. I'll do the dive as outlined in my last message - air to 100 feet, heliox below that. If I ascend from depth, then decide to descent again (rare, but it happens), I'll switch to the trimix instead of heliox, to replace some of the nitrox that was lost on the ascent. This is not critical - I could just descend again on heliox and have a lower PN2. The computers will handle it either way (I indicate to it what I'm doing via the menu system), but I find it hard to concentrate at great depths if the PN2 is too low. It all depends on what I plan to do, and feel like doing, on the course of the dive. I carry all the gas I need with me to do these switches. This is not a watse, because it all serves as my initial OC bailout supply. > Another tangent! > > A while back you mentioned getting the jitters in the upper 300's and was > unsure why. You said it might be some sort of proto-HPNS, but then > continued to say it was kinda shallow for this. Two things: 1) Do you > think the constant ppN2 may have an effect on that. I only get the "jitters" on heliox. About 100 feet of N2 seems to be enough for me to feel relaxed on all the dives I do. I don't think it matters too much whether I'm doing constant PN2 of constant FN2. > If N2 helps in > reducing the symptoms of HPNS, then a higher ppN2 might help, but if you > lost that N2 in the loop, then symptoms might start to show. Right, as in my example above. That's why I like to have the trimix on hand if I go shallowish, loose substantial N2 molecules, then re-descend. If it's a short bounce on the descent, then I usually don't bother. But if I'm going back down to stay for a while, and depending on the depth change involved, I might switch to trimix to "replenish" the lost N2. > 2) I saw in > the navy manual the other day, that HPNS starts to show signs of symptoms > around 400' to 500'. So, considering the varibility of human physiology, > why couldn't this start to show up shallower in certain individuals or > conditions? I suppose it could. Maybe I'm super-suceptible. Or maybe it shows up in me because of the boku-fast descents. But with about 100 feet of N2 in the loop, I haven't had any problems at all, even past 400. > How much data does the navy have documented at these depth > ranges with these kinda gas mixtures? I bet they don't have much with > constant ppN2, but a lot with other gas mixes. Any navy guys out there > have any comments? I'd like to know that too. Ed Thalmann gave a great talk on this at the first rebreather forum. Basically, it's another one of those "nobody knows what the hell is going on" situations. > Please elaborate as much as possible? Let me know if I missed anything above. Rich
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