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> Something along these lines, I have thought of before and maybe > got an answer but can't remember. Do you have to carry at least two > cylinders to do a deep mix dive? (Just as a hypothetical bounce dive > assuming no LONGTERM bailout needed) Let me clarify this as it isn't > coming > out right. Rob and I had a disscussion about this and this is what I came > up with. The problem: Assuming you are using only a gas with 21% of O2 > as a > diluent and go deep, say >300'. If you desend fast like you have > reported, then the intial O2 would spike greater than setpoint and > wouldn't come down until metabolised. Right? If you only had one diluent gas, with 20% O2 (I rounded it off for easy math), and your setpoint was 1.4, then you could descend to 198 feet and flush the loop with diluent and have a PO2 of 1.4. The only way to go deeper than this without raising the PO2 is to sit at 198 feet and burn off the O2 to bring the PO2 down (you'd want to leave the solenoid disabled so the rig wouldn't inject any more O2 into the loop). After the PO2 drops, You'd add more diluent to bring up the volume. It would also bring up the PO2 some, so you'd have to keep burning the O2. Your rate of descent would be limited by the rate at which your body burned up the O2. In my experience, it would take you about a half hour to get to 300'. > Would this not happen on a > slow decent? My guess is that on a slow decent, no problem, the O2 > shouldn't go too high because of the time getting down would be longer > giving more time for the body to bring down the O2. It depends on how slow your descent is. It takes me about 30-40 minutes to bring the PO2 down from 1.4 to 0.4 just be metabolism - this means about 3 to 4 minutes per 0.1 atm. At 300 feet, the PO2 of a 20% mix would be 2.0. To avoid exceeding 1.4, it would take me about 18-24 minutes to get down there. > However, on a fast > decent, the body couldn't keep up. Once, down this wouldn't be a problem > as the setpoint would be easily maintained. It would only be easy to maintain if you were a superb rebreather diver - everytime you clear your mask, or change depth, you'd have to add diluent. Since the input PO2 of the diluent is so high at depth, you'd cause a rise in the PO2 everytime a little diluent was added. > This would only be a problem > during a fast decent on a deep dive. Now, I'm guessing there are two > solutions: 1) Stop for a minute on the way down, More like 15 or 20 minutes; or bring an excercise bike. > therefore slowing the > decent, or 2) use a diluent with a low O2 in order to keep the O2 from > rising to much as well as a shallow diluent. 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. > I have always thought and advocated in my paper that the > diluent should have at least 21% O2 for bailout reasons, but I never > considered having two diluent bottles just to get down fast and coming > right back up in an emergencybailout; therefore, requiring two > bottles(one with > 21% O2 and the other less) to complete the bounce dive. Is this a real > world problem or just a theoretical problem. 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. 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 seems like a basic problem , but I keep talking myself into > circles and making it complicated. Let me know if I didn't explain anything clearly enough. > P.S. I didn't send this over tekdiver cause I thought it was more of a > personal question, but you can reply back through tekdiver if you think > pepole would be interested in the discussion. O.K., it might be of general interest. Aloha, Rich
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