Are People backing away from VPM?

[boulderjohn]

In a recent thread, some of the participants talked about problems with VPM. (See posts 126-132.) Here are a couple of the quotes:

and AG doesn't take into account the growing number of folks getting bent on VPM...

based on findings from friends who are decompression researchers. none of them use VPM

ask around friends who have taken hits and find out if they were using VPM or a schedule approximating it

Can confirm anecdotes and deco researcher friends' recommendation to use Buhlmann.

I had not heard anything about this before that thread. I thought I would start a new discussion here instead of buried on the back pages of an another unrelated thread. Does anyone have any information related to this?

 

[Dr. Lecter]

This is a good place to start: Deep stops debate (split from ascent rate thread) - Rebreather World

 

[kafkaland]

Is there a good place to read up on the algorithms? I mean, where they present the differential equations underlying the algorithms, and the justifications for each and every term in them? I can find only non-mathematical discussions, or papers that go into particular details, but no good quantitative review.

 

[PfcAJ]

The NEDU deep stops paper is a good one.

 

[boulderjohn]

This is a good place to start: Deep stops debate (split from ascent rate thread) - Rebreather World

The discussion here is fascinating, but it is huge. Enormous. I can't even guess when I will get though it all. I will guess that most people reading in this thread don't have that many hours at their disposal. I am also going to guess that a lot of people reading this thread are going to be interested in the results.

Do you think you could provide your summary of what you saw in it?

 

[Dr. Lecter]

I have a 10 hour flight tonight, so I'll try and review and summarize. Basically it lead me to switch from VPM+3 with a GF surfacing limitation of 70, to 50/80 or 50/85 for both OC and CCR dives with significant decompression...and I've felt better for it. No more shoulder niggles. Dives involved are in the 80m-100m range, with BT between 20 and 60 minutes on 10/50 or 10/60 trimix dil, though it has also worked well on the occasional 200' air dive on OC that I may do. Guys with whom I do these dives report similar results, though because we tend to plan and execute our own dives even while diving "together," it may be hard to really compare our findings.

My take away is that deep stops, even with relatively rich trimix, have been overemphasized to the point that they require more shallow stop time than the dive otherwise demands. I'm not as sold on it as the guys I know diving 70/75 or 80/85, but I am likely to try 60/80 on a shallower, 200' range dive and see how that goes. As much as I like feeling better with fewer deep stops, the risk seems to be that going too high on the GF lo could trigger a nasty type II hit due to the helium...but how correct that conventional wisdom is clearly seems questionable.

 

[rjack321]

The discussion here is fascinating, but it is huge. Enormous. I can't even guess when I will get though it all. I will guess that most people reading in this thread don't have that many hours at their disposal. I am also going to guess that a lot of people reading this thread are going to be interested in the results.

Do you think you could provide your summary of what you saw in it?

You should read it yourself. But in a nutshell...

VPM +2 for some reason has been considered the "default" for VPM users for years.
GFs have changed over the years. 30/85 in years past, 30/70 is currently in vogue based on some relatively recent microbubble data.
The DIR peoples stop waaaay too deep (80% of ATAs is nuts). The overall profiles sometimes end up being like 10/125 though (too abbreviated on the shallow end according to buhlmann)

The NEDU study basically says for that for any given deco time, reassigning time deeper in the profile does not lower DCS risk it increases it. It leads to excess slow tissue on-gassing which if you don't extend the shallow stops makes for higher DCS risk not lower. GI3 was full of cra*p saying that if you got the deep stops right you could blow off shallow time. Bollocks.

VPM as a model is not "invalid", people are slowly accepting that VPM+2 is pretty damn aggressive and the higher levels of conservatism are needed to negate the aggressiveness of the deeper portion of the +2 profiles. The popularity of the shearwater's which only run VPM with an additional $$ unlock code has shifted some people to the "new" GF settings of 30/70 which are somewhere around VPM+4 or 5 in overall shape and time.

There's more but you wanted the cliff notes version.

 

[Dr. Lecter]

Rjack pretty much nailed it, but the following (edited to remove personalities) exchange early on in the thread sums it up IMO. Deep stops have previously been overemphasized, but Pyle wasn't wrong either, and helium's properties make nailing an optimal amount of time spent deep on deco a somewhat chancey gamble.

Side A: Deep stops out of fashion? Really? Are we all heading for a 85/85 profiles with a fast 10m/min ascent to match. Side B: Who knows where we are headed? Where we are right now is faced with an emerging realization that the religion around deep stops in the early to mid 2000s probably over-emphasized their efficacy in safely shortening decompression times. The weight of evidence available at this moment in time suggests that over-emphasizing deep stops in decompression will probably do more harm than good. The difficulty is knowing what "over-emphasizing deep stops in decompression" means. It probably does include profiles generated by some bubble models, especially on low conservatism. The tangible effect of this knowledge is that many people who are using gradient factors are increasing their gradient factor low (and sometimes decreasing their gradient factor high). I believe this is a reasonable response to the evidence. How far should we go? I don't know the answer to that, and the situation will gradually evolve. It could even end up with a tongue-in-cheek 85/85. Who knows?

So far, we're (those of us diving with me locally near or past 100m on relatively rich helium blends) finding that traditional GF LO numbers have been counterproductively small, leading to too much time deep and either less successful deco or longer shallow stops. At what point we find we've gotten too aggressive with out first stop is still pretty much just throwing darts blind...though I don't know anyone diving 100/100.

 

[Michael Guerrero]

Here's another thread:

Deep Stops (rebreather dive charts) - Rebreather World

The basic summary with respect to deep stops (those called for by bubble models) is that trying to protect the fast compartments by starting your stops deep has the negative effect of causing greater on-gassing of the slow compartments. Thus, if you start stops deeper, you must make up that time in extended shallow stops. This notion is backed up by two man-tested studies (NEDU and the Italian Navy). Further, greater instances of type 2 DCS and higher bubble scores were associated with deep stop algorithms that try to accommodate bubble mechanics.

I don't think this invalidates the Pyle stop, and I do this kind of stop, but only 1, not a series of them starting from a GF10 or whatever. That's where the model breaks down. By employing GFs, you alter the entire curve, rather than inserting a short stop mid-ascent on the way to your first (Buhlmann-esque) stop.

UHMS did a Deep Stops workshop on this in 2008, and DAN covered some of it in it's 2008 Tech Diving Workshop. The DAN videos are available online. Several papers are also available on Rubicon.

EDIT: you can also look at the RF 3.0 videos online, that discussed these issues.

 

[ajduplessis]

IMO most divers moving away from standard VPM model are CCR divers. Most of these divers I know switched to VPM-B/E without any niggles, feeling great after the dives. The original VPM model causes some niggles where the BT exceed 30min below the 300ft mark.

Few OC divers exceed 20min BT below 300ft as the cost and the volume of breathing gasses required becomes a &#8220;stumbling block&#8221;. We complete may OC dives to 300ft+ on standard VPM without incident or niggles (BT<20min).