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On Wed, 29 May 1996, Carl Heinzl wrote: > > >But isn't the CO2 being pulled out of the loop by the absorbent canister? > > That brings up an interesting point - can the scrubber pretty much > absorb ALL the CO2 being exhaled, leaving none to return in the input > or does *some* fraction return in the breathing loop? If so, what > fraction returns, and, is this a function of scrubber lifetime (i.e. > how far along you are in the usable life of the scrubber) AND, what is > the function of time to predict CO2 retention in the loop? Carl! What the hell are you doing up so late?!?! ;-) In fully-closed rebreathers, the cannister has to pull out as much CO2 as the diver produces, or there would be a net increase in CO2 in the loop. It's a rate-driven thing. When a volume of absorbent in a rebreather is "used up", it's only "used up" in the sense that the RATE of CO2 removal has dropped below CO2 production. If you consider that a high workload diver is cranking out CO2 as much as ten times as fast as a low workload diver, then it follows that a cannister that is "used up" at high workloads is not "used up" at low workloads. I have noticed this when pushing a cannister past its rated life (intentionally). The first symptom is shortness of breath at high workloads. But a cannister that gives me such symptoms at high workloads will sustain me for as much as 2 or 3 more hours at rest. But I digress. Your question is valid, because I don't know how SCR rebreathers in general design their cannisters, and at what rate of Co2 production they consider the absorbent "used up". There are two levels to consider here. The first is that x% of the Co2 produced is lost in the discharges gas, so therefore a given volume of absorbent should last x% longer on a SCR than it does on a fully closed unit. The second level has to do with how the discharged CO2 "buffers" the rate at which Co2 is reaching the absorbent. When you consider than most rebreather manufacturers do not rate their cannisters for different lifetimes depending on workload (even though workload could potentially have a ten-fold affect on cannister life), then I think the Co2 lost in the discharge of a SCR is comparitively trivial. Aloha, Rich
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