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> But, this is a steady state equation, what about during periods of > peak activity and CO2 production? Will it still remove ALL the CO2 > then? If it's designed right it should. > And hence my question... So, from what YOU are saying a "fresh" > canister can remove ALL the CO2 from a "high workload diver". > Shouldn't this be a measurable and stated quantity? And, as such, it > could be graphed over time if CO2 absorbancy is only a function of the > integral of how much CO2 has already been absorbed. I'd be curiosu to > see the equation of state that shows this value... It's a helluva complicated process. I used to think it was simple, but now I understand how complex it is. I don't understand the complexities themselves, just that it is more complex than most people realize. Yes, a "fresh" canister should be able to handle 100% of the production (I think it's impossible to oull out 100% of the CO2 molucules - it's hard to do 100% of *anything* with gases - but it should take virtually 100% of the CO2 your body produces at a high workload). I agree, it would be VERY interesting to see the curve of canister life. At some point on the curve the canister can no longer support the highest workload. At some later point on the curve, it cannot handle a lower workload, and so on. I think the conservative thing would be to say a cnnister is "used up" when it cannot handle the rate of production at highest workload. But it's even more complictaed, because based on my understanding, it's not a situation where: "bang", once you've exceeded the rate of production the canister can handle, the CO2 builds up right away. Rather, it's a gradual increase in the inspired PCO2. I *think* that as the PCO2 increases in the loop, more CO2 molecules are bound to the absorbent binding sites. This is a tough one to communicate in ASCII, but the way to think of it is this: suppose you've got x number of binding sites remaining in the absorbent, and you're at the point where the rate of total volume binding is less than the rate of total volume produced. The absorbent is not completely used up (i.e., there are still binding sites available), but there are just not enough binding sites to keep up with production. The PCO2 in the loop will increase, which means the PCO2 entering the canister is higher, which means more total number of CO2 molecules are being bound in a given pass (if I understand my chemistry correctly, the rate of binding is in part a function of the PCO2 exposed to the binding sites - maybe someone on the list who understands this better can elaborate). Thus, the rate of PCO2 increase in the inhalation is "buffered" to some extent by a slight increase in the rate of CO2 binding in the canister. Now, this brings up just one other twist in the equation. At high workload, your RMV is increased. This means two things with regard to the canister: 1) the total volume of gas passing over the canister is increased, and 2) the lag time in the canister is decreased. Obviously, these things affect the efficiency of the absorbent. Another twist is that higher CO2 production means more CO2 is being bound, which means more heat is produced (exothermic reaction), which means the average temperature of the absorbent is higher -- and we all know that temperature can affect the efficiency of the absorbent. Like I said, it's a really complicated process, most of which is over my head. The point is, it's very difficult to pick a certain spot on the curve and say: "There! That's where the absorbent is used up." The tricky part for the manufacturers to determine how to rate the life of the canister. It's hard for them to have to pick a number of "X" hours for a given canister volume and absorbent type to account for all people at all workloads. As is obvious, workload will affect canister life. Furthermore, different people have different metabolic characteristics. At identical (average) workloads, I get twice the duration out of one canister charge that my diving partner John does. The best and probably simplest way to measure how much of a canister is "used up" is to keep track of how much O2 you've burned. Ideally, you can figure out all the O2 you metabolize, and discount the amount lost during loop venting. (But don't forget to add the O2 contributed by the diluent in addition to the O2 supply!) For obvious reasons, this method works better for fully closed systems than semi-closed. Another way to do it is to put in a CO2 sensor. This won't help you predict the remaining life of your canister, but it will probably give you early warning of an increasing loop PCO2 before physiological symptoms alert you to the problem. > So, Rich, are YOU in bed yet? It's only 11:45 there now :^) It was only 8:40 when I wrote the message. I went to bed at 1:30, and Cara got me up at 6:45... Aloha, Rich
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