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Wil,

In a message dated 98-07-23 11:26:19 EDT, you write:

<< > Why does adding helium to a mix increase the deco time if it
diffuses so
much
 > faster than nitrogen ?
 
 Because it gets into the deeper tissues faster, and the deeper
 tissues take longer to off-gas.
 
 Regards,
 
 -Wil >>

---------------------------------------------------
    By deeper tissues I assume you mean slower tissues.    I would think that
if helium loads these tissues for a period of 25 minutes it should offload in
about 25 minutes unless this is a non symmetrical process, which I'm sure it
is (some realistic variation of exponential / linear perhaps).    
Without a change in perfusion of these slower tissues I can presently see no
reason for helium not to diffuse out with the same vigor with which it
diffused in.   

    Regardless of the rates of diffusion of two different gasses such as He
and N2, given a fixed period of time for on-gassing and a symmetrical cycle
each gas should off-gas completely in the same period of time. 
    If at the same time you pore red marbles into a bucket at a rate of 50 per
second and blue marbles into the same bucket at a rate of 100 per second for
10 seconds then pore them out at the same rates for 10 seconds the bucket
should be empty of both groups of marbles at the same moment. 
    If, however, they are each pored in at different rates that decrease
exponentially and then out at linear rates (or close on both counts) then
there can be a divergence of the points in time at which each is totally clear
of the bucket.
    This would be true of all tissues, not just the slow ones, and this is
what I suspect is happening but I am looking for confirmation and a more
thorough description of the process.     I am not sure that the exponential /
linear model is valid or uniformly applicable.  

    I can see possible merrit in your suggestion if helium diffuses to points
beyond the influence of the blood stream but this would seem to make the
process dangerously unpredictable.    I agree that the use of helium would
probably shift the controlling tissue to a slower one for a portion of the
dive.
   Actually you are making two dives and two decompressions when using trimix
and I suspect that the procedures we customarily use are not the most
effective way to manage them.    I suspect that we are making a descending
excursion on nitrogen at the first gas switch so that we can blow off helium
earlier in the deco schedule than may be necessary.
    Something like this may be behind George's insistance that 50/50 is a
better deco mix than 36 - because it delays that descending excursion on
nitrogen rather than jumping the helium deco gun and paying for it throughout
the rest of the deco by having to dump the nitrogen you took on to do it.    
    I may be way off base but if I can get a few questions cleared up I might
be able to shorten and improve the effectiveness of my deco procedures.
This post is the first question.

    I appreciate your response, Wil, and I hope that the explanation turns out
to be as simple as you suggest but I fear that I am in for some big headaches
over the next few days, especially if I can get Bill Mee, Eric Maiken, or John
Crea to bite on this.

    Does the above seem reasonable or is my brain so fogged with thinking
about this stuff that I am missing something obvious ?

Chuck Boone



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