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>Posted on 29 May 1996 at 19:49:11 by Richard Pyle [CO2 absorption in the scrubber] >It's a helluva complicated process. I used to think it was simple, but >now I understand how complex it is. I like to make models :-). The rate of reaction (CO2 removal) _locally_ has to be proportional to the probability of a CO2 molecule being in the neighborhood of an absorption site. This probability is the product of the two particle densities (# molecules/unit of volume). Write this density as [] and we find the rate to run at k x [CO2] x [scrubber]. (I keep k and [scrubber] separate as [scrubber] drops as CO2 molecules are scrubbed). k is a number between 0 and 1 and typically doubles for every 8'K increase in temperature. The extremes can be described thus I think: If k is large then CO2 is scrubbed quickly and there is hardly any CO2 molecules in the gas at the scrubber's exit port. I.e., the CO2 is scrubbed quickly in a short piston shaped region that travels up the scrubber container. This also keeps the CO2 level in the breathing gas very low irrespective of the divers work load until the scrubber is about spent. If k is small then the CO2 molecules travels a long way down the fresh scrubber tube and visits many scrubber sites before it bonds. Some CO2 even travel through. In this case the residual level of CO2 depends on the work load and progressively gets higher throughout the life of the scrubber. (Assuming a constant breathing rate this means that as [scrubber] falls linearily in time, [CO2] increases proportionally in order to keep the product constant). In the model of the piston shaped scaped active region, this means the piston is extends beyond the physical length of the scrubber tube. Observe that to define when an inefficient scrubber is spent involves estimating max permissible [CO2] in the gas the diver breathes in. In the high rate scrubber the transition from low [CO2] to high [CO2] takes place within a short timespan. A measurement of [CO2] (or ppCO2) at the exit port would tell us where on the scale between 0 and 1 we find k for a given scrubber material. >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. What do you think of using an array of temperature sensors ? John
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