> On 17.10.2016, at 00:41, Rick Walsh <rickmwa...@gmail.com> wrote:
> On a related point, I'm starting to think the ongassing zone should be the % 
> inert gas saturation for whatever gas is being breathed, rather than air.  
> What do you think?  Currently, a dive with deco on 100% O2 will show 
> "ongassing" for the fastest and slowest tissues, when in actual fact they are 
> offgassing.  I think this is misleading.

I have thought about this as well. Of course what you say makes total sense 
when the question is “are we on- or off-gassing?”.

That is, however, a different question than “what is the quality of the 
decompression” that the heat-map is supposed to show: To my understanding, the 
point of the Michell talk is to look at how much gas is release to the body 
(potentially in the form of bubbles but who knows). And if the deco algorithms 
that we use make any sense at all, the rate is determined by looking at the 
difference between the inert gas pressure in the tissue and the (total) ambient 
pressure (not just the ambient inert gas pressure): It is this difference that 
goes into the calculation of M-values (including gradient factors) and also 
goes into VPM-B in the form of the time integral. I think, people believe that 
it is this number that is proportional to the rate of forming bubbles in the 

In this sense, for example you could be off-gasing while not sourcing bubbles, 
for example when breathing oxygen while the tissue pressures are below ambient 

I am not saying it has to be like this (or I am convince that this is a good 
description for what is actually going on in the body) but it seems to me this 
is at least a (tacit) assumption of decompression models.

There is another practical complication (when you want a representation like 
the heat map with one value per tissue and instant of time): There is 
potentially more than one inert gas. It it absolutely possible for example that 
He is off-gassing while N is on-gassing (but some people argue that this is bad 
for the effectiveness of decompression, they call this isobaric counter 
diffusion). What are you going to plot in this situation?


Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics
                      Scientific Coordinator
                      Ludwig Maximilians Universitaet Muenchen, Dept. Physik
                      Phone: +49 89 2180-4523  Theresienstr. 39, rm. B339

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