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On Wed, 29 Jun 1994, Terry McCracken wrote: > When you shift to nitrx on the decompression stop are you not, as far as > partial pressure of nitrogen is concerned actually at a shallower depth and > may have just exceeded you decomperssion ceiling.Could this not cause DCS by > causing too much nitrogen release. > Maby it is my misunderstanding but it appears to me that you are using > relative pressures for a nitrox dive but absolute for decompression diving. I *think* I understand your confusion, and I'll try to explain it. There are two factors involved: partial pressure of inspired nitrogen (PPN2) which depends both on the mixture breathed and the depth; and ambient pressure (which, of course, dependends only on the depth). The rate of nitrogen uptake into your system depends only on the inspired PPN2. By using nitrox, you reduce the inspired PPN2 at a given depth, and thus reduce the rate of nitrogen uptake compared to air at the same depth: this is where the EAD concept comes from (This much, I realize, you already understand). So just to make sure we stay in synch: A diver breathing nitrox-36 at a depth of 100 feet is breathing a PPN2 of about 2.58 ATM, which is the same PPN2 that a diver on air would breathe at about 75 feet. Since both divers are uptaking nitrogen at the same rates, both divers have the same no-decompression time limits (nitrox diver at 100' has same NDL as air diver at 75') Again, sorry for the review of stuff you already understand, but I just wanted to make sure we were in synch... As long as the NDL is not exceeded, things are pretty straightforward. It gets a bit more involved, however, when the NDL is exceeded. To understand what's happening, it helps to have a different perspective on what at decompression ceiling really is: Let's assume a diver has exceeded the NDL. That means that the diver has absorbed a certain amount of nitrogen in excess of what will remain dissolved in the blood/tissues when the diver goes to the surface (i.e, if the diver went directly to the surface, the excess nitrogen would come out of solution in the form of bubbles, and the diver would be bent - it's actually much more complicated than this at the physiological level, but I won't go into that now). O.K. back to the scenario at hand... This diver who has exceeded his/her NDL now has a "ceiling" depth. The ceiling is the MINIMUM depth at which the AMBIENT PRESSURE is sufficient to prevent the dissolved nitrogen from coming out of solution in the form of bubbles. Think of the ambient pressure as forcing the dissolved nitrogen to stay dissolved. Now: because the diver who spent X minutes at 100' breathing nitrox-36 has the SAME AMOUNT of excess nitrogen in his/her blood/tissues as the diver who spent the same X minutes of time breathing air at 75', BOTH divers have the SAME ceiling depth. Because the ceiling is the depth at which the AMBIENT PRESSURE is sufficient to keep that excess amount of nitrogen in solution, it doesn't matter what the diver is breathing. Now lets look at the second component of a decompression stop: the duration of the stop. O.K.: both divers arrive at their ceiling with the same amount of excess nitrogen in their blood/tissues. Lets say both divers are following the same air decompression schedule: By the EAD concept, both divers follow the schedule for X minutes at 75 feet. Those air decompression schedules assume that the diver will be breathing air at the decompression stops (to keep it simple, let's say there is a single decompression stop required). Let's say that, according to the tables, the diver needs to spend 10 minutes at the decompression stop before safely ascending to the surface. This means that it takes 10 minutes for the excess nitrogen to come out of the blood through the lungs. However, just as nitrogen uptake rates depend on the inspired PPN2, nitrogen off-gasing rates also depend on PPN2: The less N2 in the inhaled breath, the steeper the N2 concentration gradient from the blood to the lungs, and the faster the rate at which nitrogen molecules move from the blood to the lungs, etc. In other words, the diver breathing nitrox is off-gasing faster than the diver breathing air; so theoretically, the nitrox diver could surface sooner than the air-breathing diver. This is why divers DECOMPRESSING on nitrox while following EAD air tables are probably less likely to get bent than air divers following the same schedule; hence an extra decompression safety factor exists when a nitrox diver follows EAD decompression shedules. Oh, jeez....I guess I went on a bit long there, and I probably confused the issue more than clarified it. Actually, it's all a very dynamic process, much more complicated than I just explained. But, given this "simplistic" explanation, let me summarize: Decompression "Ceiling" DEPTHS are determined by the AMBIENT PRESSURE required to prevent excess nitrogen in the blood & tissues from coming out in the form of bubbles, and thus are not affected by what the diver is breathing - only by the amount of excess dissolved nitrogen. NDL's and decompression TIMES are determined by RATES of nitrogen uptake/off-gasing, which are a function of inspired PPN2, and thus ARE affected by the breathing mixture. Whew....thanks for sticking with it if you made it this far! Let me know how much more confused you are now than you were before you asked the question.... Aloha, Rich
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