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>> >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?!?! ;-)

Late???  it's only 2:40 here ! :^)

>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

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?

>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

I knew that :^)

>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

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...

>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.

Sure, again, this is obvious.

>But I digress.

That's ok :^)

>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

Well, again, as you know, this depends on the unit...  Some units,
such as the Odyssey, that exhaust LESS gas at greater depths must have
scrubber design constraints very close to those of the fully closed
units...

>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.

I would agree - even assuming that 20% of the gas is discharged each
cycle, this is only a 20% change in CO2 being absorbed, as you pointed
out, pretty minor compared to the 10x change associated with workload.

So, Rich, are YOU in bed yet?  It's only 11:45 there now :^)

I still have 3 or so more hours of work tonight, just thought I'd drop
in here for a little bit of mental relief :^)

-Carl-

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