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> I don't think the change in bag volume is going to have much of an effect
> on the CO2 concentration, other than as Rich points out that you are
> effectively venting/purging the bag. I'm sure he is right that the venting
> of the bag outweighs the any change in pCO2.

This is basic math, folks.  Lets say you have a rebreather loop full of 
gas.  Let's say that 1% of the gas molecules are CO2.  At 90m, the PCO2 
in the breathing loop is 0.1 ATM/Bar.  That's like breathing 10% Co2 at 
sealevel - that's bad.  If you ascend, and don't allow the rebreather 
loop to expand (i.e., the excess gas on ascent is vented into the water), 
when you get to 10m your PCo2 will be 0.02 ATM/Bar. That's like breathing 
2% CO2 at sea level - not too bad.  Is this magic?  Smoke & Mirrors? No - 
it's simple physics (assuming ideal gas laws, and all those other caveats).

Now, in the real world, at least two other factors are at play. First is, 
your body is probably still producing CO2 during the ascent (unless 
you're already dead). Assuming you began your ascent because of 
hypercapnia symptoms, and assuming those symptoms are a result of CO2 
buildup in the loop, and assuming this CO2 buildup is a result of a 
failing canister, then there is reason to believe that the rate at which 
you are producing CO2 on the ascent is greater than the rate at which the 
canister is pulling it out. This will cause the FCO2 in the loop to rise 
during the ascent.  The second factor is that on constant-PO2 
rebreathers, the solenoid starts firing away like mad during the ascent 
to try to keep the PO2 up (remember, the PO2 in the loop is dropping on 
the ascent just like the partial pressure of all the other gasses).  This 
means more (presumably 100%) O2 is being added to the loop. This will 
cause the FCO2 in the loop to drop.

Neither of the above factors, however, are likely to affect the PCO2 in
the loop NEARLY as much as the drop in ambient pressure during the ascent 
will.  And remember - it is the PCO2 that we're worried about, not so 
much the FCO2.

> *But*, there variable likely to have a profound effect on scrubber
> efficiency is: the deeper you are the more concentrated is the CO2 per
> liter flowing over the scrubber surface.

I don't follow you - please explain why increased depth leads to 
increased PCO2 (concentration of CO2) in the loop (except for the 
increased work of breathing factor leading to higher workload and more 
produced CO2).

> If you are close to scrubber
> "capacity" at depth, as you rise the "dwell time" of the CO2 molecules is
> effectively increased on the scrubber bed. I would guess that this would
> be why the scrubber appears to work better ass you rise and also reduces
> your hypercapnic symptoms.

Why is the dwell time increased as I get my "ass" out of deep water?

> So my *hypothesis* is that the reduction in hypercapnic symptoms noted on
> ascent when using a scrubber near the limit is due to (a) venting of CO2
> form the bag 

Yup, I agree this would result.  In Mike's case (the one that started 
this thread), however, I have a hunch that that if the symptoms he 
experienced at depth were, in fact, hypercapnia, they were probably due 
to his breathing pattern, rather than failing absorbent (i.e., 
inefficient CO2 elimination at the *lungs*, not at the canister).  That's 
the only reason I can think of that would account for the fact that a 
subsequent high workload in shallow water did not lead to exacerbated 
hypercapnia symptoms.

> (2) some contribution by gas concentration and flow
> over/through the scrubber. And that the shorter life of teh canister has
> something to do with the per minute capacity of the canister to absorb at
> depth, not its total theoretic absorbtion capacity. Which, by the way, is
> maybe why smaller cannisters won't work as well as you think they might.
> Hmmm.

I still don't think I understand what the basis of this argument is.

For general consumption, here's a trick you can try at home.

Assumptions: 1) Using a rebreather, 2) maintaining relatively constant
loop volume, 3) relatively constant temperature, 4) ideal gas laws (the
last one is a safe assumption given that we're talking a couple hundred
psi at most). 

We all know that for any gas in the breathing loop, Pgas = Fgas * ambient 
pressure.  We should also all know, the formula PV=nRT (which works given 
assumption #4).  "P" is ambient pressure, "V" is relatively constant 
(assumption #2), R is constant (by definition), T is constant (assumption 
#3).

That leaves us with P~=n.

So what?  The "trick" to remember from this is that the "partial pressure" 
of a gas is proportional to "n" above (number of molecules).  It's sort of
like a direct measure of "n", only in different units.  Here's what I 
mean: suppose you have a a rebreather that maintains a constant PO2 of 
1.2 atm.  This means that if you start at 30 feet, then go to 500 feet 
(or whatever depth), if the PO2 is 1.2 at both depths, then the number of 
O2 molecules in the breathing loop is the same at both depths.  The only 
difference is that at 500 feet there is a shitload of other molecules 
(mostly helium, one would hope) in the loop also. This means that 
"partial pressure" is a measure of the absolute concentration of a gas, 
whereas "fraction" is a measure of the *relative* concentration of a gas 
(i.e., relative to the denominator of all the gases combined).

The reason I describe this "trick" is to nail home my point about PCO2.  
If you start with a certain PCO2 in a rebreather loop at a depth of 200 
feet (say), and then start to ascend, the gas in the loop will expand and 
vent out of the over-pressure relief valve (or nose, or wherever).  Some 
of the molecules in this vented gas will be CO2.  This means that the "n" 
of CO2 molecules in the loop drops (net loss of CO2 molecules), and it also 
means (as detailed above) that the PCO2 drops.

Now let's take the reverse.  Suppose a diver starts at 20 feet with a 
certain PCO2 in the loop, and then descends to 200 feet.  The volume of the 
loop is held constant by the addition of diluent and O2.  Unless there is 
an appreciable amount of CO2 in the diluent or O2 supply cylinders, then 
the PCO2 will NOT increase on the descent, because the "n" will not 
increase (where would the addition of CO2 molecules come from?).  Ya with me?

O.K., now you say "But wait, Rich, thee IS an increase in the number of
CO2 molecules, because the my canister is failing, and my body is cranking
CO2 out faster than the absorbent is pulling it out."  Well, I'll buy that
- there would be an increase in PCO2 in the loop in that case - BUT THIS
IS NOT RELAVENT TO DEPTH! You would get exactly the same increase in PCO2
if you stayed at 20 feet as you would if you descended to 200 (or 500, or 
wherever).

Yes, there are some indirect (and mostly trivial) affects of depth.  One 
of them is that the total gas mixture is denser at depth.  This would 
have a two-fold effect; 1) it would increase your work of breathing, and 
therefore increase the CO2 production at a given exertion level; and 2) 
it would cause more efficient cooling of the absorbent (and we all know 
that absorbents don't work as well when cold).

However, I sa 1) these are relatively trivial for the discussion at hand, 
and 2) do not apply if a diver switches from nitrox in shallow water to 
heliox in deep water.

O.K., nuff said.

Rich

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