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On Fri, 15 Nov 1996, Richard Pyle wrote: > Number 2 is far & away the simplest solution. That's why I usually blend > my heliox with 10% O2. > > > This leds to another > > question. How much O2 do you put in > > the bottom mix for a breather dive like this and is this a consideration > > for your bottom mix? Of coarse the other bottle would be used for the near > > the surface intially and bailout to get you through the shallow water on > > the way up. > > Usually 10% - the main thing to consider is that you want to be able to > breathe directly from your diluent in OC mode if the need arises. In > other words, you'll want to mix your bottom-diluent the same way you > would mix your bottom gas of an OC dive. > This makes much more sense, and This is what my original instinct told me. But again, I can't believe after all this time I haven't thought of this problem. This is a definite product of a lack of field experience. I have always simplified this by dismissing the O2 consumption rate. I didn't sit down and do the math to see how long it would take to bring the O2 down far enough. I guess I'll bring this up in my paper, there's another page. > > I'm not sure how to answer. It's not really a real-world problem because > it seems silly to do a rebreather dive greater than 200 feet with only one > diluent. Agreed!! But I was trying to simplify it by making a generalization. And it doesn't work like that. You can't do that when comparing deep and shallow dives. I know better. Why? If you're deeper than this, 1) you'll want some helium to > cut the narcosis; and 2) you'll want some nitrogen to cut the deco time. > The easiest source of nitrogen is air, and it's just as easy to make > heliox-10 as it is to make heliox-21. If you must use only one diluent, > then use the heliox-10 (or trimix with a low FO2). There are very few > situations that you'll want to breathe the heliox shallower than about > 20-30 feet (where the O2 may be too low) - besides, at those depths, you can > always breathe the O2 in OC mode. > > > Does the O2 really spike too > > high? > > In this sense, it is a real-world problem. It happend to John once on a > dive to 240 feet. I had made him a heliox-10 mix, but then we didn't dive > for a few months. During that time, the helium differentially leaked out > until the gas was something like heliox-27 (contrary to what some Einteins > will tell you, differential leakage of helium CAN happen!). He switched to > heliox on the descent, his PO2 spiked when he got deep, and he couldn't > bring it back down again. When I figured out what was wrong, I told John > to flush his loop with air to bring the PO2 down, and we aborted the dive. > This is a perfect example of the discussion. This example explains everything by itself, besides the marvelous explanation from you! Thanks for clearing that up. > > This seems like a basic problem , but I keep talking myself into > > circles and making it complicated. Yeah this was a very basic question, but I got confused. > > Let me know if I didn't explain anything clearly enough. > Crystal clear! Aloha Tony
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