VPM-B Oddity

[John Van Ostrand]

On Tue, Apr 5, 2016 at 10:55 PM, <john at vanostrand.com> wrote:
>
>
>
> 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?
>
> It's using a modified Thalman. They've added bubble mechanics to it and
> other modifications. It's proprietary. Conservatism is 20% on one dive and
> 30% on another, Greater means more conservative.
>
> Personally I'm interested in how it compares but this bug report doesn't
> involve the Cochran deco algo.
>
>
>> 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.
>
> I'll do that in the morning.
>
> A few questions:
>
> What gasses were you using?
>
> Ean32 in the most significant case, ean31 in another.
>
> Were the gas changes (if any) picked up when downloading from the DC?
>
> Yes. Ean32 on the entire dive.
>
> Does the ceiling look normal with the Buhlmann model with varying gradient
> factors.
>
> I looked at them in the past and they looked reasonable under Bühlmann‎.
>

For those following this. This might be a display problem more than a bug
with the algo. Turning "3m incremental stops" on makes it break. So it's
the 3m ceilings that appear broken not the actual ceilings.


-- 
John Van Ostrand
At large on sabbatical
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.subsurface-divelog.org/pipermail/subsurface/attachments/20160406/3ae62223/attachment.html>