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On Mon, 19 Jun 1995, 418 wrote: > When I first heard of blending nitrox using the partial pressure method, I was > told that this leads to stratification in the tank. My experience in chemistry > tells me that this is impossible, O2 and N2 being miscible in any proportion > and the densities being very similar (1.43 vs 1.25 g/l). In fact I think it > would be very difficult to maintain an O2 and N2 gradient! > > But we still hear this argument quite often and, in Canada at least, there are > many individuals (and maybe even a certification agency) that believe that > stratification can happen and will cause O2 toxicity problems!? > > Although I'm convinced that this stratification thing was invented by Nitrox > detractors, I would like to know if someone has ever demonstrated (published?) > that it does not happen. We're thinking about mixing Nitrox at our workplace > and someone has already brought up the stratification story. Is there a paper > somewhere or maybe the proceedings of a conference that will help me prove that > is not a problem? I've never had this happen when blending nitrox. I always get the predicted mix right after I fill the tank from the compressor. I've even done some tests where I invert the cylinder after filling; and I still get the correct mix. The same applies to trimix. However, I *DO* get stratification when I mix heliox (particularly when I start with O2 and top-off with He). The first time I discovered this I blended 21% heliox by adding 380psi of oxygen and 1620 psi of helium. I *knew* the O2 was in there because I had just put it in there. When I analyzed it, however, I got only 2% O2. I then turned the tank upside down and the analyzer said 98% O2. I laid the tank on its side with the analyzer still hooked in, and it slowly oscillated back and forth from high to low, as if the gas was rocking back and forth inside the cylinder (pretty remarkable to watch, actually). If I thop the heliox off with air to get trimix, the layering seems to be gone by the time I finish filling with air. There was one experience a friend of mine had, however, where he used a trimix cylinder immediately after filling, where we think it might have stayed stratifeid a little while. He descended to about 270 feet head-down, and about 30 seconds after he uprighted himself on the bottom, he suddenly got very light-headed and almost blacked out. He started swimming horizontally and his head cleared up right away. Physics only says that stratified gas layers can't be a state of *equilibrium*, but a state of disequilibrium can last for some (perhaps long) period of time. I wouldn't worry about it for nitrox, because O2 and N2 have very similar densities. I wouldn't worry about it for trimix, because I've never seen it happen, especially if a day or more passes between filling and using. I would, however, watch out for heliox, which you're likely to use if/when you move towards rebreathers for deep diving. Some people bend the valve stem pipe at an angle to help mixing. I doubt if that helps any, but I don't think it would hurt. Aloha, Rich
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