# User manual update: Heat map

Robert Helling helling at atdotde.de
Mon Oct 17 06:27:06 PDT 2016

```Hi,

> On 17.10.2016, at 00:41, Rick Walsh <rickmwalsh at 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 body.

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 pressure.

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?

Best
Robert

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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
http://www.atdotde.de

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