VPM-B Oddity

[Rick Walsh]

John,

On 6 April 2016 at 12:02, John Van Ostrand <john at vanostrand.com> wrote:

> I have an example where VPM-B +0 gives an earlier and longer deco
> obligation than VPM-B +1 and +2 for the same imported dive.  I found
> another dive where +0 and +1 were almost identical.
>
> That is indeed odd; +0 should not be more conservative than +1 or +2.
Similar obligations are less surprising.


> I was comparing VPM-B to Cochran's algorithm on a dive were the Cochran
> gave me a minor deco obligation. To do that I'd change to the VPM-B algo in
> the Plan dive function and it would show the obligation for a dive in the
> profile display for imported dives.
>
> I'm not familiar with the Cochran.  Do you know what the deco model and
conservatism you were using?


> I tried pulling out the XML for just the two dives in question but I was
> unable to duplicate the problem. When I included previous dives from the
> trip it was reproducible. It seems to be an issue when repetitive dives are
> included. The SIT was 18 hours.
>
> It sounds like it's somehow related to tissue loading from previous dives,
but I can't work out what.  The VPM-B algorithm tracks tissues mostly like
Buhlmann, but there are a few funny quirks/features.


> I can provide an XML file to whomever is interested in investigating. The
> algo implementation is way beyond me.
>
> If you send the file to me and let me know which dives are giving the odd
results, I'll have a look when I get a chance, but don't hold your breath.

A few questions:

What gasses were you using?
Were the gas changes (if any) picked up when downloading from the DC?
Does the ceiling look normal with the Buhlmann model with varying gradient
factors?

Rick
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