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Date: Thu, 30 May 1996 11:40:41 -1000 (HST)
From: Richard Pyle <deepreef@bi*.bi*.Ha*.Or*>
To: "Peter N.R. Heseltine" <heseltin@hs*.us*.ed*>
Cc: Jim Bembanaste <lungs@ic*.ne*>, Mike Cochran <mcochran@ne*.co*>,
gasmixers@ao*.co*, techdiver@terra.net
Subject: Re: Physiologic safety parameters for SC rebreathers

> By far the greatest limitation to CO2 extraction by any canister system is
> getting the CO2 (expired) gas to actually make contact with active
> absorbent pellets. You can calculate that theoretic amount of CO2 that
> will be absorbed by the weight and chemical composition of the absorbent,
> but channeling (which may be realted to how the canister is filled) and
> canister design greatly affect the "life" of the absorbent.

When you say "amount of CO2" above, do you mean the total number of CO2 
molecules, or do you mean a RATE of absorbtion (i.e., number of molecules 
per given unit of time)?

> Drager has a rubber gasket that is placed in the middle of the canister
> half way through the fill process to limit the channeling that may occur
> at the absorbent/canister interface. This doesn't prevent channeling in
> the material itself. Dragersorb is different in consistancy from Sodasorb
> etc., with less dust and different sized pellets. I think these factors
> have a far greater effect on the life of a canister than variance between
> divers in CO2 production given that you abide by the manufacturer's
> usually very conservative 'sorb use tables. Dragersorb is about 2.7 L per
> fill and is rated for (I think) four hours.

I agree that the size/shape/packing of the pellets or granules of 
absorbent, and the size/shape/design of the canister have a huge 
influence on absorbtion efficiency, but I was saying that if you hold 
those things constant, you can still get a two to tenfold difference in 
actual canister duration depending on workload and diver metabolizm.  How 
can a manufacturer give a rating for a canister life in terms of hours 
of diving when there can be such a huge variation in consumption?

My suggestion is to key canister life to O2 consumed (when possible), 
rather than hours diving.

Rich

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