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> Now for a short inert gas digression: > At sea level, a human will have approxemately 1.5 liters of Nitrogen > (N2) dissolved in the body tissues. Why? Because air is mostly made up of > N2. > > What happens if we start breathing pure oxygen? > > The body is saturated with N2 and there is no N2 in the gas we breathe. > N2 will start to diffuse out of the tissues into the gas. If this gas is > the O2 in the rebreather bag the O2 will be diluted by N2 because the > scrubber will only remove CO2 and nothing else. Over time, the O2 will > be diluted by N2. If the flow of O2 in the bag is impeded for some > reason, (corroded valve, empty O2 bottle etc) the volume of the bag will > *NOT* decrease, there will be no increase in resistance to signal the > diver. Instead, the concentration of O2 will decrease until the diver > becomes unconcious. Remember that lack of oxygen (hypoxia) is very > insidious, the victim will hardly notice anything before (s)he passes out. O.K., reality-check time. Let's say Jane-average-diver has 1.5 liters dissolved N2 in the body when saturated on air at sea level (I've heard the value 1 liter stated for average, but for the sake of argument, let's say it's 1.5 liters). The question we are addressing: When breathing on an oxygen rebreather, where the initial concentration of oxygen in the breathing loop is near 100%, is it *possible* for the loop gas to ever become hypoxic without periodic flushing by the diver? The naive answer is that this is impossible as long as the total minimum breathing loop volume exceeds about 1.88 liters. Why? Because if all 1.5 liters of the dissolved N2 is transferred to the breathing loop, then then there would need to be at least 0.38 liter made-up by "something", which in the case of an O2 rebreather would be oxygen (that works out to 21%). So, as long as the total minimum loop volume (which equals the volume of your lungs at average full inhalation, plus the volume of all the hoses, plus the non-displaced scrubber canister) is at least 1.88 liters, hypoxia would be impossible. I think if you add Jane-average-diver's lung volume to the minimum conceivable hose and canister volumes, you'll find it difficult to design a rebreather with less than 1.88 liters total minumum loop volume. But this is the naive answer. First of all, it's naive because it applies only at sea level (assumes 20% O2 gas fraction required to avoid hypoxia). But in fact, at depth, the O2 fraction would need to be even lower to induce hypoxic PO2, because the ambient pressure is greater. Sure there is the concern of the equivalent of "shallow water blackout" when returning to the surface, but this shouldn't really be a concern on an O2 rebreather. Why? Because as a diver descends, the volume of the loop must be made up by "something", and if O2 is the only gas attached to the unit, then that "something" must be oxygen. During the ascent, this excess gas must be vented, but since the N2 and O2 would be very-well mixed during the ascent, both N2 ans O2 would be vented off. Thus, your fraction of O2 would be *greater* after a descent and ascent than it would be if you just sat at sea level. More importantly, the "1.88 liter" answer is naive because the 1.5 liters will never transfer completely into the breathing loop. N2 will move from the tissues to the loop only until the PN2 in the loop is at equilibrium with the dissolved PN2 in the blood & tissues. From our above premise (1.5 liters), we're saying that when the dissolved PN2 is 0.79 (equilubrium at sea level), there is 1.5 liters of N2 in the blood & tissues. Put a diver on an O2 rebreather and what happens? N2 moves from the tissues to the breathing loop, and the PN2 in the breathing loop starts to climb. At what PN2 will the breathing loop be when the tissues and loop are at equilibrium? Well, the PN2 will CERTAINLY be less than .79, because we know that at a PN2 equilibrium of .79, the body has 1.5 liters of N2 dissolved in it, but since the body will have LESS N2 dissolved in it (because the N2 is moving from the body to the breathing loop), then the equilibrium PN2 will be LESS than 0.79 atm. If the PN2 is less than 0.79, then the PO2 MUST be greater than 0.21 (because we only have O2 and N2 to work with). Moreover, it takes a helluva long time to reach equilibrium, so the loop PN2 in the real world would be smaller still. If you sit down and work out the numbers, you'll find it's not all that easy to achieve a hypoxic state on an O2 rebreather (underwater at least - I'm not sure about mountain climbing where you have a decreasing ambient pressure). At the very least, you'd have to start with a breathing loop full of air, rather than near-100% oxygen. Second, you'd need an initial loop volume of air that is considerably greater than the operating and/or minimum loop volume of the rebreather (so that you have more N2 molecules to work with). Third, you'd have to be a damn good rebreather diver to avoid venting off any of the loop gas (which would ultimately mean a net loss of N2 from the system). The danger is starting with a high FN2 in the loop at the surface *AND* having an initial loop volume that is considerably greater than the minimum operating loop volume *AND* working in very shallow water. Of course, you could also easily run into problems if the input gas was something other than 100% O2 (as in a semi-closed rebreather). Aloha, Rich
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