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> I've discussed this question with a couple of dive buddies and we couldn't reach > a consensus. Assuming you have a fully closed rebreather, and ignoring venting > of the breathing loop, what would happen to the pressure in the loop as O2 is being metabolized if no dilutent or external O2 were added? Would the pressure > remain the same or would it decrease? The simple answer is that the loop volume would decrease, but the pressure wouldn't change. The loop volume = your lung volume plus the breathing hoses, plus canister, plus counterlungs. From the diver's perspective, you would eventually start to "bottom out" the counterlungs on a full inhalation. The pressure in the loop would remain the same because the water pressure is acting directly on your lungs and on the counterlungs. If you completely bottomed-out the counterlungs but continued to "suck" with your lungs, I suppose you could draw a slight negative pressure inside the loop (relative to ambient), but that would only be momentary. The complicated answer is that the pressure in the loop is equal to the pressure at the "deepest" part of the "collapsable" loop. "Collapsable" loop means portions of the loop that are flexible and compress when ambient pressure and internal pressure are out of balance. Lets assume your lungs and the rebreather counterlung(s) are the only collapsable sections of the loop. For illustrative purposes, imagine a rebreather which had counterlungs that hung 33 feet deeper than the diver. If the diver is at 33fsw, then the pressure in the loop at all points (both the lungs and the counterlungs and all the plumbing in-between) is 3 ata (not for very long, though, because the diver's lungs would quickly rupture). On the other hand, if the counterlungs were at the same depth as the diver's lungs, but the CO2 scrubber canister was at 66fsw (connected by rigid plumbing), then the pressure in the entire loop (including the canister) would be 2ata. Same is true if the canister was on the surface while the diver was at 33fsw. In the real-world, this is why counterlung placement with respect to diver's lungs is critical on a rebreather. As an interesting twist, the pressure inside the diver's lungs is equal to the pressure inside the rebreather loop *only* if the airway is kept open. If the diver locks down the airway in the throat and relaxes the diaphragm and chest muscles, then tissue tension of the lungs and surrounding body leads to a slight increase in the pressure inside the alveoli and lung passages. Boyle's law says that that means the volume is slightly decreased. When you get really good at rebreather buoyancy control you'll notice a very subtle buoyancy shift when you lock your airway. If I take a breath and hold it with my chest muscles and diaphragm (keeping my airway open), then my buoyancy is the same as it is when I'm breathing normally. But if I hold my breath by locking my airway, I become slightly more negative. Like I said, this is VERY subtle. > Also, if the pressure remains the same, > what would the makeup of gasses in the loop look like assuming the CO2 is being > absorbed by a scrubber? There is no makeup - the volume decreases as the O2 is metabolized and the CO2 is bound by the absorbent. > Thanks in advance for no flames and intelligent > responses. You're welcome, but I think you're being idealistic. I have a hunch that you will get a barage of responses form the bonhead crowd saying that you asked a stupid question (I consider "stupid question" to be an oxymoron). They'll probably accuse me of being the choir master on the hog ranch. But that's O.K. - if you and I and 95% of the subscribers on this list can see the humor in the comments from the bonehead gallery, but don't take any of it personally, then we can have information exchange *and* a good laugh all at the same time. Aloha, Rich
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