Deco for Divers & VPM.

[Kern]

In another forum a member made this statement, "according to Deco for Divers, a flaw in the VPM-B algorithm is that it tends to be overly conservative for NDL dives like this". I responded thus, "I think you're talking about the 1st version of VPM. The flaw, pressure gradient allowed, was fixed in the VPM-B revision, by taking Boyles Law into account." If Deco for Divers suggests that VPM-B doesn't scale well from NDL through to hypoxic decompression dives, I think the author got it wrong."

Unfortunately, I'm operating from old memories of papers I'v read, & my experience using the algorithm from 2mt to 90mt, n = 1, & the member no longer has access to Deco for Divers.

I'm hoping someone who has access to Deco for Divers, might confirm what it actually says on the subject.

 

[bygolly]

In another forum a member made this statement, "according to Deco for Divers, a flaw in the VPM-B algorithm is that it tends to be overly conservative for NDL dives like this". I responded thus, "I think you're talking about the 1st version of VPM. The flaw, pressure gradient allowed, was fixed in the VPM-B revision, by taking Boyles Law into account." If Deco for Divers suggests that VPM-B doesn't scale well from NDL through to hypoxic decompression dives, I think the author got it wrong."

Unfortunately, I'm operating from old memories of papers I'v read, & my experience using the algorithm from 2mt to 90mt, n = 1, & the member no longer has access to Deco for Divers.

I'm hoping someone who has access to Deco for Divers, might confirm what it actually says on the subject.

I believe you are right Kern. It does talk of the VPM-A model in that it performed well in the range of 50-80 meters but outside these depths the model was too conservative on the shallow end and not at deeper depths. My version of the book talks of this on page 167 top paragraph. I think a typo on the part of the other forum member.
Eric Baker in 2003 was responsible for introducing the change to the A version by introducing the effect of the expanding bubble ala Boyle.

 

[String]

Can't find that exact wording but some passages: (emphasis is mine)

The ORIGINAL VPM model (sic!) used a bubble number limit where the ascent was limited to ensure that no more than a certain number of bubbles are formed. This works well for saturation diving but results in a decompression schedule that is far too conservative for 'bounce' dives, ie non saturation dives

It refers to the "Original" VPM meaning VPM-A not the commonly used B and by bounce it doesn't mean a dive within the NDL it means dives not going into saturation.

Also:

...This flaw, however, only manifested itself in certain depth ranges. Most of the testing of the model took place in the ...50-80m range..... As the depth increased the decompression predicted by the model became much too short whereas for shallower depths the decompression was far too long

Maybe this is where the original poster got confused?

FWIW the info in this book tallies with the papers ive read and been taught elsewhere.

 

[Kern]

Good of you chaps to take the time, thanx.