Re: VPM-B Oddity

[john]

Sent from my BlackBerry 12 neural implant.  From: Rick Walsh Sent: Tuesday, April 5, 2016 10:24 PM To: John Van Ostrand Cc: subsurface Subject: Re: VPM-B Oddity John, On 6 April 2016 at 12:02, John Van Ostrand <j...@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.

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