Are People backing away from VPM?

[rjack321]

Few OC divers exceed 20min BT below 300ft as the cost and the volume of breathing gasses required becomes a “stumbling block”. We complete may OC dives to 300ft+ on standard VPM without incident or niggles (BT<20min).

This is in part because OC gas switches reduce your EAD dramatically. Unlike the NEDU study which was air at 170ft followed by air deco with some time shifted deeper (at 70ft mostly). Coincidentally, 70ft is right where many OC divers dramatically reduce their EAD to 32ft using 50%. The gas switches "counteract" the added risk which NEDU found when using the bubble model which added extra air time at 70ft. Basically, more backgas time deep is counter-indicated. But a bubble model with gas switches reducing the EAD? We don't really know if that is or isn't riskier than a buhlmann model since that has not been tested.

---------- Post added November 21st, 2014 at 07:26 PM ----------

Not sure about the rest of the DIR world, but doesn't GUE teach that the first stop should be at 50% rather than 80%?

Ratio deco "rules" assign the first stop (pause) to 80% of ATAs. Its fine for really deep dives but anything shallower than about 250ft it ends up being waaaay too deep. I have not stopped that deep in almost a decade. But at least back in 2003-04 starting to slow down this deep was in vogue. The thought being "control bubbles deep" and avoid the "bend n mend" profile.

Bend and mend might still be the case (bubbling profusely during the dive) but nobody really knows since getting Doppler scores during a dive is basically impossible.

Trying to stay deep and avoid forming bubbles in the first place creates a situation where you need to extend your shallow stops to offgas all the slow tissues that were on-gassing in your attempt to control bubbles with extra deep stops.

 

[Michael Guerrero]

The good news is that this discussion is largely academic. The use of EAN50 and pure O2 for decompression, along with less rich mixes for deeper diving, largely eliminates the dangers of DCS outside of other, individual-specific physiological circumstances.

On top of that, it looks like the NOAA O2 limits, which plenty on this board have pilloried, are not as significant a constraint as we've been taught through our training agencies. Thus, use O2 in various, appropriate concentrations to washout inert gas and you're probably going to be OK across a broad spectrum of profiles.

It's likely we'll never know what's optimal given the tools we have at our disposal to mitigate all the sub-optimal permutations.

 

[Kevrumbo]

Look at the Bottom Mix Gas used in the NEDU Study (essentially Deep Air):

(Abstract, p.i)). . .Divers wearing swimsuits and tshirts, breathing surface-supplied air via MK 20 UBA, and immersed in 86 °F water were compressed at 57 fsw/min to 170 fsw for a 30 minute bottom time during which they performed 130 watt cycle ergometer work. . . Results indicate that slower tissue gas washout or continued gas uptake offsets the benefits of reduced bubble growth at deep stops.

(Conclusion p.18) The practical conclusion of this study is that controlling bubble formation in fast compartments with deep stops is unwarranted for air decompression dives.




This is the simple main point IMO/IME, to take away from the study:

Of course you're going to have significant residual inert Nitrogen and potentially on-gas N2 at your deep stop & even at intermediate deco stops on Eanx50 which may encroach on critical slow tissue M-values --if you were using a working bottom mix with a high fractional N2 content to begin with like Air. Plan accordingly, use a computer to track your inert tissue loading (i.g. Shearwater Petrel) and be prepared to extend your 6m depth 100% Oxygen deco profile along with a stand-by In-Water-Recompression Table as a DCS treatment contingency if necessary should you choose to use Deep Air on mandatory decompression dives, especially on multiple dives per day over a week or more.

 

[shoredivr]

After reading all 100 pages and the spin off threads, I am impressed with the scientists participating on the thread who made the information accessible in the face of continuous disinformation.

The thread was also an object lesson in how not to react to new science on an Internet forum if you have business interests in a decompression model.

 

[Dr. Lecter]

The thread was also an object lesson in how not to react to new science on an Internet forum if you have business interests in a decompression model.

Oh yeah.