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On Thu, 23 Jul 1998 CHKBOONE@ao*.co* wrote:
>
> Because it gets into the deeper tissues faster, and the deeper
> tissues take longer to off-gas.
>
> ---------------------------------------------------
> 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.
>
I don't have the time to draft up a full explanation, but, if
you check back in the archives, either techdiver or rebreather,
there was a thread ("Heliox" something,I think) a couple of months ago
where I actually listed the math, with annotations, for Buhlmann's
halftime model.
The short answer is that it (Buhlmann's algo, which your dive
software is probably based on) uses an exponential half-time
decay formula, with 16 seperate compartments, each with
a different halftime. On-gassing and off-gassing rates are equal.
There's another set of 16 compartments for dealing with helium.
The two gasses aren't treated quite independantly, because
there is the issue of total tissue saturation level.
But the reason that Helium creates apparently longer
deco times in models (as for physical reality, nobody
has a clue how it actually works - although there are
endless theories) is that when you are diving, say, air
to 180 for 20 mins, the long halftime compartments
don't load up all that much, because they're *really*
long. Remember the key thing is not that a tissue
has dissolved gas, but whether the saturation is
beyond a critical level. Until it goes critical,
the tissue can be ignored, for immediate decompression purposes.
Anyway, the same dive done on heliox will result in the longer
helium compartments taking on quite a bit of gas and
getting above critical saturation, because the helium
halftimes are three times shorter than N2.
This relationship actually starts to work in your
favor after a couple hours of deep exposure, and helium
starts to produce FASTER deco times than Nitrogen,
because once those long N2 compartments start to
go critical, you are in for a VERY long hang - N2
compartment 16 is 640 minutes, under Buhlmann, vs
210 (about - this is from memory) for Helium
compartment 16.
Incidentally, that's also why Heliox is a much safer gas
for repetetive multi-day diving - it clear almost completely
out of your system overnight, unlike N2, which takes
36 hours or so.
Hope that's a little clearer.
Regards,
-Will
> 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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