I'm having trouble understanding/deriving Bühlmann coefficients "a" and "b".

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So much admirable diligence and expertise here 🙏🏼🙇‍♂️🧮

More lay general questions for the experts/future models:
Do we use a 16-compartment model because it is simply not a problem for modern computers, or is it because of actual weird boundary or precision effects with only 8?

Is there value in revisiting the treatment of helium vs. nitrogen? Is it true that they are particularly bad in combination (trimix), even if helium in isolation (heliox?) might not necessarily(?) warrant a penalty?

Will computers sense diving conditions (temp gradients, ascent rates, exertion, etc) to adjust diffusion parameters, rates, deco? I think my old G2 offered some kind of optional HRM or breathing rate adjustments? Currently we arbitrarily choose 'conservatism levels,' extend stops, or 'personal GFs' based on diving circumstances, with no actual data input

Could someone go to a clinic that measures meaningful personal parameters in a way that prescribes them an adjusted accurate 'iso-risk' personalized decompression model? What relevant measures could be done? Is there a safe clinical procedure to push someone to the threshold of DCS for the sake of discovering such personalized parameters?
 
My guess is because Haldane started with 2:1 M-values and "half-times", powers of two seemed a natural fit. And then the binary computers came along.

With modern computers and Buhlmann's formula for M-values, you could make up any number of tissue compartments. But since the point is to get the diver out of the water not bent, the question is: would e.g. 111 get the diver out any more "not-bent" than 16?

For no-stop diving DSAT's 14 compartments don't seem to do any worse than 16. Or ScubaPro's 8, or Cressi RGBM's 9, for that matter.
 
Could someone go to a clinic that measures meaningful personal parameters in a way that prescribes them an adjusted accurate 'iso-risk' personalized decompression model? What relevant measures could be done? Is there a safe clinical procedure to push someone to the threshold of DCS for the sake of discovering such personalized parameters?
Due to the significant variability -- even for the same diver on the same profile (discussed in RB4) -- I'd say it's highly doubtful. The models have to focus on the population, and it will probably always be a personal call from there.
 
Could someone go to a clinic that measures meaningful personal parameters in a way that prescribes them an adjusted accurate 'iso-risk' personalized decompression model? What relevant measures could be done? Is there a safe clinical procedure to push someone to the threshold of DCS for the sake of discovering such personalized parameters?

Get skinny: Can I dive if OVERWEIGHT? | DDRC Healthcare
 
Currently we arbitrarily choose 'conservatism levels,' extend stops, or 'personal GFs' based on diving circumstances, with no actual data input
No, it's not arbitrary, at least for many of us. This recently came up in a different thread, and @tursiops gave a pretty good explanation. Give it a search. We do have data: our own dives, our circle of friends, industry researchers, etc.
 
Get skinny
I was thinking actual clinically measurable parameters directly related to gas science, like:
  • tissue/blood nitrogen instantaneous values + accumulation/clearance rates under rest & exertion
  • gas composition of exhaled breath
  • 15N stable isotope tracing?
  • doppler
  • conditions under which first (reversible) sensations or symptoms of DCS actually occur
Given the level of fine precision that is followed in these models, it seems weird to not want to measure and model much bigger differences due to the effects of environment or physiology.
 
Due to the significant variability -- even for the same diver on the same profile (discussed in RB4) -- I'd say it's highly doubtful. The models have to focus on the population, and it will probably always be a personal call from there.
Yes exactly, but why not try to measure this variability? Certainly it is much more than the 0.0001 difference in model params being discussed. Probably more like ~5-10x differences in DCS risk according to personal differences, diving behaviors and circumstances?

We believe that every person and every dive is different, and that this affects deco and DCS risk. Is there nothing measurable about that? What if we could measure something that improves the guesswork?
 
I was thinking actual clinically measurable parameters directly related to gas science, like:
  • tissue/blood nitrogen instantaneous values + accumulation/clearance rates under rest & exertion
  • gas composition of exhaled breath
  • 15N stable isotope tracing?
  • doppler
  • conditions under which first (reversible) sensations or symptoms of DCS actually occur
Given the level of fine precision that is followed in these models, it seems weird to not want to measure and model much bigger differences due to the effects of environment or physiology.

Good enough is tough to overcome. You'd have to demonstrate convincingly that you found something better and that requires expensive and potentially dangerous testing on real divers.

Think about how you'd devise an experiment to check if any of your parameters are a more useful measure of decompression stress than a straight tissue compartment model. And that's the easy part. Assuming you find something, you would then have to build and test your alternate algorithm.

It's extremely difficult to get academic institutions to approve experiments where a negative outcome is a bent diver. Militaries will occasionally do it and those studies are valuable. However they tend to be focused on specific dive profiles that aren't a great match for what rec and tech divers typically do. Also the participants are more homogenous (and generally younger and fitter) than the average civilian diver so you are less likely to find significant differences for variables related to physical characteristics.

Dive computer manufacturer have dabbled with alternate algorithms and adjustments to their algorithms for external conditions, but they have never released any data to back up their approaches. So we are left with the supposition that it's primarily a marketing feature.

The recent trend is the reverse. It seems like everyone is adopting a version of Bühlmann. I suspect the insurers are behind this. It's much easier to respond to a lawsuit with "we use the industry standard" than to defend why you deviated from it with proprietary tweaks.

I am actually surprised that manufacturers were willing to sell dive computers based on RGBM algorithms when there was no experimental confirmation that they were superior. But it appears the lure of marketing was too great for some and they felt the hypothetical arguments along with early Doppler results was a sufficient liability shield. The 2011 NEDU study marked a practical end to that approach and is why I believe that new computers are overwhelmingly Bühlmann.

 

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