Artifacts with VPM-B ceiling on level changes

Rick Walsh rickmwalsh at gmail.com
Sat Aug 29 14:20:18 PDT 2015


Good morning,

On 30 Aug 2015 7:12 am, "Robert C. Helling" <helling at atdotde.de> wrote:
>
> Gaetan,
>
>> On 28 Aug 2015, at 05:31, Gaetan Bisson <bisson at archlinux.org> wrote:
>>
>> I've noticed what looks to me like artifacts regarding the ceiling VPM
>> calculates for multilevel dives. See the attached screenshot.
>>
>> Basically, during manually entered ascent segments, the calculated
>> ceiling becomes deeper as the diver goes shallower...
>>
>> It might just be a well-known VPM "feature" but I still thought I would
>> ask here if anyone knows what is going on.
>
>
> I figured out the origin of this „feature“: The ceilings we plot are
based on the allowed gradient (tissue pressure minus ambient pressure) of
the tissue. This gradient is depth dependent (according to the „Boyle
compensation which says that (gradient + ambient pressure)/gradient^3 must
be constant during the ascent. When we plot, we use the gradient for the
current depth but that shrinks as soon as we ascend and so the ceiling
comes down upon the ascent. We should better plot a ceiling defined as the
depth where the Boyle compensated gradient is the actual gradient and then
the ceiling would no longer be depth dependent. But this requires some more
work, which I have to postpone to tomorrow.

I no longer see the feature. I think it was the sixth patch in the series
that got rid of it.

Cheers,

Rick
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