[PATCHES] Vary undersaturated portion of heatmap according inert gas

[Rick Walsh]

On 18 October 2016 at 20:55, Robert Helling <helling at atdotde.de> wrote:

> Will try. I definitely like patch 2, that’s a no-brainer.
>
> For patch 1, I see your point (the user can see if on-gasing is
> happening). I am not yet convinced, though. the trouble I am having is
> “what is the heat-map supposed to show?”. I think, it should be a measure
> of deco quality (I argued for this when answering if it depends on the deco
> model used, and no, it does not depend on VPM and not on the gradient
> factors chosen). So, and I think this is a philosophical question, should
> it depend on what the diver is breathing? Without that patch it is only
> about the state of the gases in the diver’s body (and not about the time
> derivative as in on- vs. off-gassing). It shows the potential for bubble
> formation (at least that is what it is supposed to show). If we do it
> relative to then breathing gas, then it is more about the time derivative.
>
> I am not really against it, just need a bit more convincing.
>

I understand what you are saying, so while I will try to convince you, I
won't mind if I don't, and I'll rehash the second patch to make it apply
without the first.

As you say, it really does depend on what the heat map ought to show, and
by adjusting the scale over the range with tissue inert pressure less than
the ambient pressure, it does represent the time derivative.  But I'll
present a few examples / reasons why this could be beneficial.

Compare a dive on 40% nitrox to 24m with a dive on air to 24m.  On reaching
24m, the rate of on-gassing is much faster with the air dive, but the
colours are shown the exactly same if we do not adjust the heatmap
according to the breathing gas inert gas pressure.

Now consider an air dive to 16m (equivalent air depth of a dive to 24m on
40% nitrox).  Theory says the on-gassing is exactly the same as the deeper
nitrox dive, so it makes sense to show the same colours on the heatmap.  If
we take the patch to vary colours according to the breathed inert gas
pressure (and descent and ascent take the same time), then the same heatmap
is shown, which I think is helpful.

Looking at a decompression dive, with 100% O2 as decompression gas.  If we
don't adjust for breathed inert gas pressure, during decompression the slow
tissues appear as blue or purple, which are the "on-gassing" colours, when
in fact they are off-gassing (but still below ambient total pressure),
whereas if the colour adjustment is made, they are shown in darker shades
of green to indicate they are off-gassing and below ambient total pressure.

Also (but I think this is less important), by setting the point where
tissue inert pressure equals breathed gas inert pressure to black, this
aligns with the 'inert gas equilibrium pressure' line in the instantaneous
bar graph.  So then we have exact colours to align with the GF100 line
(red), the total gas pressure line (bright green), inert gas equilibrium
pressure (black), and zero pressure (cyan).

Please consider the above, but as I said, I want be offended if I haven't
convinced you.  I do also appreciate the cleanness of keeping a more pure
colour scale.


>
> Anyway, here is an experiment that one should do (of course not oneself
> but under controlled conditions): Do an air dive with an aggressive profile
> that has a significant chance of causing deco symptoms (or bubble count for
> that matter) in the first few minutes of the 6m stop. Discard all dives
> where the problems occur before reaching 6m. Then have two groups: One
> doing the stop with air, the others with oxygen.
>
> If it is really about the difference to ambient pressure, the incidence
> rate in the first minutes of the 6m stop will not depend on the gas
> breathed. But if it is about the inert gas gradient, there should be _more_
> incidents with O2 (irrespective of the fact that for the rest of the dive
> and the aftermath the more effective desaturation with O2 will take over).
>
> I might pass on your offer of the experiment, but yes that is a good
point.  The Buehlmann and VPM models do both imply that offgassing without
exceeding the ambient total pressure will not cause any damage.


> On an unrelated note: When googling for isobaric counter diffusion I came
> across http://www.advanceddivermagazine.com/articles/icd/icd.html where
> Wienke etal describe Mark Elliot’s plan to dive to 1049 fsw. For the fun of
> it I entered the gases used in our planner, see the file attached.
>
>
>
> Turns out, with VPM-B+3 I can roughly produce that profile (that was
> apparently calculated in RGBM) but based on Buehlmann that guy should be
> dead. You need GF 150/80 or similar to get in the ball park of the runtime
> used. And of course the heat-map is white to a large degree… And I had
> always thought that you can model bubble model dives with the appropriate
> gradient factors...
>
>
> An interesting article and a crazy dive.  Not one I'd volunteer for.
Another point - he must have been pretty narked.  Yes, it is interesting
how the models compare with such an extreme bounce dive.  I've found while
you can approximate the first stop depth and total deco time of VPM-B with
(sometimes unusual) gradient factors, the shape of the profile is still
different.

Cheers,

Rick
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