VPM vs BUHLMAN

[BOTTOMTIMER]

I am a litle confused about what are these two models are prezent. What are the benefits of VPM and what are the benefits of BUHLMAN.
Sorry for my English.

 

[BarryNL]

I'm not quite sure what the 'benefits' are of the two different models. For most recreational diving they produce pretty similar no-deco limits and both have excellent track records for avoiding DCS. If you're trying to decide which you want to use for your diving then I would say either is fine.

Concerning how the models work - that's a pretty complex issue. In simple terms, the VPM model is theoretically similar, but also theorizes a bubble in each of the Buhlmann compartments and it computes the expected growth or shrinkage of these bubbles based on the expected difference between the internal and external gas tension around them. As such, VPM adds a theoretical calculated element to replace the more arbitrary M-values of Buhlmann models.

 

[BOTTOMTIMER]

What the GF has to do with.

 

[BarryNL]

GF? Gradient Factor? This allows you to tune Buhlmann models for greater conservatism by reducing how close you allow your profile to come to a compartment M-value.

There's a lot of good intro articles over on Rebreather World - Rebreathers for Scuba Diving - the next step about all this stuff.

 

[nereas]

VPM is programmed into V-Planner software.

Buhlman is programmed into NitekHE dive computers.

I have not noticed much difference between them, within the ranges of 10 meters to 75 meters. I have not tested either of them deeper than 75 meters yet.

I normally use the VPM (V-Planner set at +3) for planning dives and gas volumes, and as laminated backup print-outs to attach to my O2 bottle. And then I dive with the Nitek HE computer (which uses Buhlman) and check it against the VPM print-out during the deco phases (after all the short deep stops).

The minor variations between these two programs is academic. There really is no perfect model. Even GUE's "deco on the fly" if you want to go through the mental hassle of keeping track of it, is adequate, and compares very similarly to either of the others. You should always, of course, run pre-dive plans to make sure you are not committing a mental calculation error, however.

With a tech-diving computer, the possibility of error is mechanical. With a mental rote method like "deco on the fly," the possibility of error is mental. A good pre-printed backup sheet, laminated, negates the risk of such an error.

 

[BOTTOMTIMER]

Wich of those two (VPM, BUHLMAN) is more conservative?

 

[Rick Murchison]

Wich of those two (VPM, BUHLMAN) is more conservative?

Depends on the dive, and how you tweak the algorithm...
Remember, because of the widely differing variables from person to person and from day to day, all decompression algorithms are akin to measuring with a micrometer, marking with chalk and cutting with an ax.
Both these models (and their several variants) have excellent track records. I like custom tables cut from V-Planner, but have used both the Nitek rendition of Buhlmann (ZH-L16) and the SUUNTO version of RGBM. They all work just fine.
Unless I were contemplating some monster dive way out on the edge of the envelope, I wouldn't lose any sleep on which one I was using... just pick one and go with it.
And remember... a few extra minutes looking at the plankton in the 10F/3M range covers many sins
Rick

 

[BOTTOMTIMER]

Have you ever heard that Buhlman is more aggressive to helium? Is that truth?

 

[nereas]

Remember, because of the widely differing variables from person to person and from day to day, all decompression algorithms are akin to measuring with a micrometer, marking with chalk and cutting with an ax.

Rick

Bravo! I love that! Good rule.

 

[MXGratefulDiver]

VPM is programmed into V-Planner software.

Buhlman is programmed into NitekHE dive computers.

I have not noticed much difference between them, within the ranges of 10 meters to 75 meters. I have not tested either of them deeper than 75 meters yet.

I normally use the VPM (V-Planner set at +3) for planning dives and gas volumes, and as laminated backup print-outs to attach to my O2 bottle. And then I dive with the Nitek HE computer (which uses Buhlman) and check it against the VPM print-out during the deco phases (after all the short deep stops).

The minor variations between these two programs is academic. There really is no perfect model. Even GUE's "deco on the fly" if you want to go through the mental hassle of keeping track of it, is adequate, and compares very similarly to either of the others.

I haven't found that to be the case.

Buhlmann ... particularly as it's used with a Nitek He ... is not very helium-friendly at all. Nor does it give you credit for deep stops. Following a Buhlmann profile will have you doing almost all of your deco at 30 feet and above. When I owned a Nitek He, I bent the computer several times trying to "make" it fit deep stops into the VPM profile I was getting from V-planner ... even when padding the 20-foot stop by a few extra minutes.

Ratio deco uses deep stops ... starting at roughly 2 ATA above your deepest depth and every 10 feet thereafter up to the surface, with progressively longer stops as you get closer to the surface. In effect, ratio deco will have you deeper, for longer periods of time than Buhlmann. If you consider a bubble model, the latter will basically allow the bubble to form, and keep you at what's considered a "safe" depth until it dissipates. The former will bring you up slowly enough to prevent the bubble from forming in the first place.

VPM is somewhere in between ... when I run a VPM profile at a conservatism level of +2, it approximates ratio deco in certain depth ranges (180-220) with bottom times of 25 minutes or more. With shallower dives, or shorter run times, there seems to be more variance between the two, with VPM requiring less time on the deep stops and more time for the 30 and 20 foot stops. But overall run times are still similar.

When planning a dive, I use a modified version of ratio deco ... not because I think it's the best approach for everyone, but because it works for me. I'll look at V-Planner at +2, compare it to a standard ratio deco profile, and modify my stops a couple minutes here and there, depending on profile and conditions.

... Bob (Grateful Diver)