Bubble model vs. Gradient Factors redux

[ChuckP]

Isn't this where Spisni study went, kind of? They had longer ascent times for ratio deco profiles, or am I mis-remembering?

Yes - here's the study. I make no claim to fame, just a dumb diver that never wants to ride in a helicopter.

I had thought VGM type planning stopped sooner and outed you sooner...... These ratio deco dives for this study had longer run times....

 

[rsingler]

I had thought VGM type planning stopped sooner and outed you sooner...... These ratio deco dives for this study had longer run times....

That was the most striking thing in Spisni for me - the longer total deco time for Ratio Deco. For early proponents of deep stops, it wasn't just that the bubble models didn't need all that shallow time, but that they were better because of the deep stop. Then VPM-B +2, +3 or even +4 were added as the cost of the ongassing at depth was finally acknowledged by the bubble model community.
Yet here in Spisni, despite RD having both deeper stops than VPM-B and a longer run time than VPM-B +5, the results were still not favorable.
Now Spisni was done with GF 30/85 (the apparent loser in the upcoming DAN study?). How much more damning would this be for deep stops if Spisni were repeated with not only RD and 30/85, but 50/75 as well?

 

[ChuckP]

Well, if you want to compare models, you can only change one parameter at the time. Otherwise, you can't know where the difference is. That's research 101.

So, if I were to design an experiment, I'd keep the bottom time and runtime constant and only change the ascent schedule. So I'd choose a VPN conservatism level and a GF combo which got the subjects out of the water at the same time. And I'd choose a GF combo which gave a significant difference in deep/shallow stop distribution compared to the VPN schedule. Otherwise, the difference in schedule would be too small to give any meaningful difference in the results.

And then add the ethical issues of deliberately bending a bunch of divers (which, generally, only NEDU is allowed to, and even they, only to a very limited degree), the statistical problems if your cohorts are too small and the issue of financing the study... Well, damn, I'm pretty happy I'm not a hyperbaric scientist trying to find answers to those questions.

You say your plan is better than my plan. - (Your boat is faster than mine)

I say prove it, let's have Dan test it. - (Not a chance)

Dan makes some changes to one plan or even both and comes back with the results. - (Dan adds a bunch of weight to one boat to make it ride better)

We didn't really compare the plans because Dan made some changes, we know the differences between the changed plans but nothing else. - (Well heck, I didn't really care about the ride quality, I just want to go fast, that test isn't fair!!)

LOL - The words in () are my simple mind way of thinking about it. I have a fairly good scientific background and spent many days in the lab (plastics). If I wanted to compare UV properties of two different polymers in their natural state with best packages, I didn't have to use the same UV package in both polymers, I'd use the best one designed for each polymer and then compared the results.

I think when we changed GFLo and compare results we are getting an understanding of differing GFLo's in that model but I fear you can't effectively extrapolate that out to a differing model - maybe you can - two models produce different paths to the surface, allow them to follow their path, no short cuts and test the results.

 

[scubadada]

Hi @rsingler

As was discussed in the video, there's not much money to support decompression research these days, not the Navy, not the oil industry. I'm glad to have whatever new data we can get.

Being constrained by run time controls that factor, but limits your ability to adjust deep stops, GF low, and surfacing GF, GF high. Controlling GF low or GF high makes it impossible to control run time.

Take the NEDU profile as example, 170 ft for 30 min on air. Suppose you want to compare a change in GF low from 20 to 50 or a change in GF high from 70 to 85. I ran these on MultiDeco with a stop time of 1 min:

View attachment 583095

Comparing group 1 to group 2 or group 3 to group 4 would tell you what the effect of changing the GF low was. I'm not smart enough to know if they would give exactly the same result. Comparing group 1 with group 3 or group 2 with group 4 would tell you the effect of changing the GF high was, same caveat. I believe that if you require that the run time be the same, you will have to adjust both GF low and GF high and make the analysis more complex. At least in the example I chose, the change in GF low changed the deco time much less time than the change in GF high, I had not noticed that.

I think that you and I are in the same chapter if not on the same page, we would just like to know how this works

To continue my example. To match run time of 20/70 with 50/X one would have to choose 50/65. To match 20/85, one would have to choose 50/80. As mentioned before, a relatively small change in GF high has a relatively large effect on run time, larger than a change in GF low.

 

[dmaziuk]

That was the most striking thing in Spisni for me - the longer total deco time for Ratio Deco.

That's why I'm not buying "more conservative" = "safer". Yes, the exact profile matters, but in the first approximation longer ascents mean less delta-pressure to drive off-gassing and considering the mess that real tissues and real gasses are, I'm not convinced less delta-P is "safer". Common sense says it should be, but "common sense" is an oxymoron.

 

[rsingler]

No matter what we do, we have to change two variables.
- GF Hi and Lo for a given Run Time
- GF Lo and Run Time for a given GF Hi
- GF Hi and Run Time for a given GF Lo
We don't need one great study, we need eight, lol! And it still won't be good enough, because "we just tweaked Buhlmann to try to emulate a true bubble model."

 

[J-Vo]

I would have rather seen a direct GF vs VPM comparison, but for results to be valid the run times must be equal. A more extreme exposure in order to set them apart would be good as well. How about comparing VPMb to flat GF, and to something like Doolette's 83% rule?

When putting this together I couldn't help but notice how much VPM diverges from Buhlmann as the dive's get more extreme. At the exposure levels of most of these studies, there almost isn't any reason to worry about it, but when you get to dives like the second one below, things get interesting. I even switched from VPMb+3 to +4 in order to keep the GF High below 90 for the deeper profile. For comparisons sake, I also included the GFs that most closely mimicked the VPM numbers.

As I'm not a CCR diver this is all for OC.

170' for 30 minutes:
21/35
50%
99%

300' for 30 minutes:
12/65
21/35
50%
99%

Edit: cleaned up formatting.

 

[rsingler]

I would have rather seen a direct GF vs VPM comparison

If you can sit through to the twenty minute mark of the Powell interview, there is a striking theoretical heat map comparing VPM on/offgassing with GF's.
It's not a study, but the heat map concept has been borne out by the bubble data, IMO.

And this is GF 40/74 compared with VPM-B+4 !! Imagine if it had been only +2...
If you can't read the text, the top graph is VPM-B. On the left it points out "More on-gassing in slow tissues [early]", and on the right, "More supersaturation in slow tissues late".
For the bottom GF 40/74 graph, it points out, "More offgassing in fast tissues early", and "Less supersaturation in slow tissues late".

@jvogt , here's a 270' comparison for ya!
For the lurkers, you can generate these heat maps yourself in Subsurface. For free!
Here: Subsurface

 

[dmaziuk]

For comparisons sake, I also included the GFs that most closely mimicked the VPM numbers.

At GF 108 Herr Dr Buhlmann says you should get bent. This kind of experiment would be unethical unless you're NEDU.

 

[Umuntu]

I would have rather seen a direct GF vs VPM comparison, but for results to be valid the run times must be equal.

While I can see the argument for the same run times from one scientific perspective, are identical run times a worthwhile comparison from a diving perspective?

The run times are one of the outcomes of the particular algorithm. A valid diving comparison is one which compares different algorithms for the identical diving mission: i.e. Each diver dives to X depth for Y minutes and performs Z work. The 1. run times** taken to decompress, 2. the resulting DCS, 3. residual nitrogen, 4. blood/tissue micro-bubbles, and 5. anything else -- are all outcomes of each algorithm.

To eliminate individual subjective physiological factors, each diver should repeat the same set of dives with each algorithm, with sufficiently long surface intervals to completely recover between sets of dives. Eg performing the dives a week apart.

EDIT: ** By run times I mean to encompass the entire decompression stops depths/times ladder.