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>I understood that part, I was just thinking that if the "threshold" point 
>of pH or whatever it is that triggers the indicator is relatively sharp0 
>and not *directly* associated with bound CO2, then it might be the case 
>that the line between colored and uncolored absorbent is just the point 
>where that threshold is.  In other words, there may still be a cline in 
>binding potential of the absorbent, with some of the purple stuff 
>capbable of binding CO2, and some of the non-purple stuff already bound 
>to CO2 (but not enough to push it over the threshold of the indicator).

I agree with most of what you say here.  I suppost the issue is the 
steepness of the the "cline" in absorbent potential.  Here's a few 
arguments for believing that it might be "steep":

1.  We do notice that upstream from the boundary the sofnolime is cold 
    and downstream it is hot suggesting that much less reaction is 
    occuring upstream from the boundary.  

2.  If you believe 1 indicates the absorbent above the boundary is 
    effectively inactive then the fact that the boundary can move 
    more than half way down the stack without signs of hypercapnia
    suggest that less than half the thickness of scrubber is needed
    for adequate scrubbing.

3.  My sodasorb manual shows a double in series scrubber system for 
    anesthesia.  When the upstream scrubber is exhausted is is replaced 
    by the down stream scrubber and a fresh scrubber installed down stream.  
    I'll have to check this cause it's been a while since I read it.  
    It looks like they expect the down stream scrubber to be fresher 
    than the upstream one.  

4.  The input PCO2 is probably around .03 ATA.  The output PCO2 is probably
    < 0.001 ATA.  This is a 30 fold decrease in PCO2.  If you believe
    my half length argument then the scrubber contains >5 half lengths.
    This would mean that 75% of the scrubbing is being done by the first
    40% of the absorbent.



Barrie

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