Evolving Thoughts on Deep Decompression Stops

[Storker]

Is what I'm learning similar to what others are learning?

What I like about that simple sentence is the difference in attitude compared to my first foray into tech.

What I like about that simple sentence is that is shows a scientific attitude to the world. Not "how can I argue that I'm right", but "what is right and how can I get closer to that"?

 

[dmaziuk]

Then suddenly everything changed, and radically different approaches were developed, all with little to no actual research.

"Then suddenly" it wasn't: Yount, Thalmann, DCIEM, and ZH-L12 came out within a couple of years of each other. The realization that staying under M-values does not eliminate bubbles, like Workman promised, is a bit older than those. E.g. this: A fundamental approach to the prevention of decompression sickness. was published in 1978.

Also ZH-L16 M-values were apparently back-calculated from "physical properties of gases" originally, so 100%-actual-empirical-research they ain't. According to Powell anyway, I couldn't understand Tauchmedizin myself even if I tried reading it.

 

[rjack321]

Brian Hills paper was in the reading list for my very first tech class (2004ish) as part of "why we don't do bend and mend". I love the language in the abstract - "economical on time." Over thirty years later, innumerable pages have been spent arguing on that exact concept so concisely (and apparently non-controversially) articulated by Hills so long ago.

The pendulum has swungeth

 

[tursiops]

The pendulum has swungeth

Also, if the pendulum is actually moving, it spends the least amount of time in the middle.

 

[rjack321]

Also, if the pendulum is actually moving, it spends the least amount of time in the middle.

The middle of what? To my eye, my own selection of 50/75 isn't 100/100 nor is it VPM +2, looking at different shapes and stops for the dives I actually do in the 100-250ft range its between those two modelled extremes.

This is one reason why I feel that advocates for 100/70 and other such profiles are playing with fire. These are as untested and extrapolated from the central pattern of the last 10+yrs of experience (and evidence) as VPM+2 was a departure from the 100/100 era of Pyle.

 

[tursiops]

The middle of what? To my eye, my own selection of 50/75 isn't 100/100 nor is it VPM +2, looking at different shapes and stops for the dives I actually do in the 100-250ft range its between those two modelled extremes.

This is one reason why I feel that advocates for 100/70 and other such profiles are playing with fire. These are as untested and extrapolated from the central pattern of the last 10+yrs of experience (and evidence) as VPM+2 was a departure from the 100/100 era of Pyle.

It's a metaphor, not a description of deco strategies!
The pendulum swings back and forth (as you said in post #32).
I'm only philosophizing that of course it does. The pendulum does not stop in the middle, and it spends less time there than anywhere else. In fact, it spend the maximum amount time at its two extremes.

 

[dmaziuk]

This is one reason why I feel that advocates for 100/70 and other such profiles are playing with fire. These are as untested and extrapolated from the central pattern of the last 10+yrs of experience (and evidence) as VPM+2 was a departure from the 100/100 era of Pyle.

The fundamental problem is the basic premise of the dissolved gas model: if you stay under M-value line, you run acceptably low risk of clinical DCS. It follows that any combination under 100/100 runs acceptably low risk of clinical DCS and 100/70 must be "no less safe" and nor is 10/100. Conversely if 100/70, or 10/90, is "riskier" than 100/100, then the model is built on a faulty premise and is therefore wrong.

 

[boulderjohn]

The most important point of the article was that after more than a century of searching, we still don't have the best ascent process nailed down. We are still searching. As was said above, we have the attitude of searching, which means you have to be able to accept new information that contradicts what you believed before.

A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines. With consistency a great soul has simply nothing to do. He may as well concern himself with his shadow on the wall. Speak what you think now in hard words, and to-morrow speak what to-morrow thinks in hard words again, though it contradict every thing you said to-day.

--Ralph Waldo Emerson​

 

[ChuckP]

Is what I'm learning similar to what others are learning?

I too like that and it really shows how much isn't known about individual decompression strategies it seems.

Just completed deco related classes so very little experience in actual deco and realistically little experience even in rec diving.

For maybe 60 of 100 dives using a Scuba Pro G2 recreationally, I've tried to pay attention to how I felt after diving days. Switching to a Peridix, add another 75 dives flopping GF all around 30-55/50-85, no real rhyme or reason which really told me nothing - I never felt bad after dives at any of those numbers.

Deco class with our instructor really drilled on DCI is caused by not enough deco, everything is deserved, you didn't do enough deco. Most of the arguments I see related to dive plans and models is how fast you can get out of the water verses did the plan give you enough deco - this at times can be confusing for someone trying to understand things.

In class (class was deco and trimix, dives 130'-200', 25 minute bottom times) we dove 30/70 w/air to force more deco and earlier stops - more practice on ascents from 130-140' range. On other dives we switched to 40/70..... I felt fine after all dives, not tired at all.

My real questions are how much does GF settings play into the type of dive you do? I happen to live in Cozumel, dive with Aldora for the next few months so it's HP120's, longer, deepisher diving daily, gradual ascents to safety stop. With 40/70, I'm feeling good, I don't think I short myself on any dives never really more than 5 minutes to NDL. How does that compare to diving the Lowrence at 200' for 25 minutes, then a direct ascent to first deco stop at say 80'?

The above dives are different, but are they? I'm in the water for 75-80 minutes for both, one has an average depth of 80', the other 50' - the deeper dive has 45 minutes of deco, the shallower dive has none.

I wonder if diving in a chamber watching theoretical tissue loading graphs on a big screen TV would help us understand more.....

 

[rjack321]

The fundamental problem is the basic premise of the dissolved gas model: if you stay under M-value line, you run acceptably low risk of clinical DCS. It follows that any combination under 100/100 runs acceptably low risk of clinical DCS and 100/70 must be "no less safe" and nor is 10/100. Conversely if 100/70, or 10/90, is "riskier" than 100/100, then the model is built on a faulty premise and is therefore wrong.

where did this idea come from?

I work in risk assessment, we use thresholds to decide what is/isn't acceptable all the time. There all sorts of risk assessments where the results are considered societally acceptable like 3 in 100,000 incidence of XYZ. And under a different exposure scenario that incidence can be reduced to 1 in 100,000 which in my entire career I've never seen anyone argue that isn't "better". Some might argue that its within the margin of error and statistically identical (and it usually is if you take the time to develop the data to do a probabilistic risk assessment) but on its face using the linear models we use, its by definition "more safe" or causing less harm.