Robert C. Helling helling at atdotde.de
Mon Jan 14 11:19:39 PST 2013

On Jan 14, 2013, at 4:46 PM, Lutz Vieweg wrote:

> BTW, has anyone ever seen any scientific evidence that for the
> purpose of "avoiding DCS when diving" it is valid to assume that
> the saturation of N2 in the human tissues depends only on the N2
> partial pressure, and is reasonably independent from the partial
> pressure of other gases and the absolute pressure around the body?

That is a natural consequence of the kinetic theory/diffusion theory of tissue loading. If I may plug my text once more, this is explained in fairly elementary terms in http://euve10195.vserver.de/~robert/dekotheorie.pdf . The basic idea is that the molecules randomly move in and out to/from the tissue and don't care about what else is going on. In particular they are too small (to a very good approximation) to collide and thus notice other gas molecules (i.e. N2 is independent of He). Only O2 is different since that chemically binds to haemoglobin as opposed to the other gases that just dissolve in the blood. By using the same argument you derive the ideal gas law pV=nRT, i.e. Boyle-Mayotte and friends. This is a good approximation but there are corrections for real gases (that mainly come about as the molecules are not point like but have a non-zero size or at least their interactions have a finite distance over which they act).

> And if such evidence exists, wouldn't that mean that the same
> assumption / simplification should also be used for de-saturation,
> so the decompression stops should be done at different depths when
> using EAN > 21 than with air?

Not necessarily. According to Buehlmann (which goes beyond kinetic theory) you have to compare the partial pressure of a gas in the tissue to the total ambient pressure if you want to decide if it goes out of solution and forms bubbles. To some degree there is experimental evidence for this statement. Note that the Buehlmann model consists of two (more or less independent processes): Tissue loading and off-loading on the one side and bubble formation or not on the other. The former is gas by gas separately, the condition for the second is only w.r.t. total ambient pressure. 

Ignoring bubble models you want to go up as far as possible according to process 2 in oder to maximize off-gasing according to process 1. The only exception is deco on 100% O2: There you don't gain any advantage in off-gasing time by further ascending (as ambient partial pressure of inert gases is always zero). Thus to cause the minimal conflicts with process two you stay as deep as possible (not getting into O2 toxicity), i.e. you deco at 6m.


> The literature I have seen so far makes the first assumption
> (N2 saturation depends only on the pN2) but doesn't make the second
> (decompression stop depths are independent of the pN2, only the length
> of the stops differs according to the accumulated N2 saturation).
> Without having dug deeply into the topic, this seems somewhat
> implausible to me.
> Regards,
> Lutz Vieweg
> PS: Has anyone ever been asked to do a "safety stop" at != 5m when
>    diving EAN instead of air? :-)
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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics  
                      Scientific Coordinator                                   
                      Ludwig Maximilians Universitaet Muenchen, Dept. Physik    
print "Just another   Phone: +49 89 2180-4523  Theresienstr. 39, rm. B339       
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