Question Questions about the Buhlman decompression algorithm

[Chaehwasu]

I'm afraid AI has led you down the wrong path. The total tissue tension is a critical value in deco theory.

This is my understanding. The Buhlman algorithm calculates the partial pressures of N2 and He separately. The sum of the partial pressures of the noble gases is not considered. It simply generates a linear m-value line based on the ratio of N2 to He. Is my understanding incorrect?

 

[inquis]

The Buhlman algorithm calculates the partial pressures of N2 and He separately.

This is correct, and then uses those to calculate a weighted average of the N2 & He maximum tolerated supersaturation values to which the total inert tension is compared.

The sum of the partial pressures of the noble gases is not considered

This is incorrect. Also, the term is "inert". A "noble" gas is something different.

 

[Chaehwasu]

This is correct, and then uses those to calculate a weighted average of the N2 & He maximum tolerated supersaturation values to which the total inert tension is compared.


This is incorrect. Also, the term is "inert". A "noble" gas is something different.

Yes i know the concept of inert and noble. Sorry for using inaccurate word caused by translator
Let me ask you one more question. So, for example, let's say the partial pressure of nitrogen in a tissue is 0.5 and the partial pressure of helium is 1.5. If the m-value of nitrogen in that tissue is 1.5 and the m-value of helium is 2.5, does this mean that the complaint algorithm compares the modified m-value (let's say roughly 2.25) according to the ratio of helium to nitrogen and the PHe+PN2 value to make a judgment? Like in the example above, if the modified m-value is 2.25 and PHe+PN2=2, then it is possible to ascent, but if PHe+PN2=3, then it is not possible to ascent.

 

[inquis]

Conceptually, that is correct. In practice, however, it is the a & b coefficients that are averaged, not the individual m-values. That describes the modified M-value line directly. Eric Baker's paper (attached) explains it in greater depth.

 

[dmaziuk]

Although there has been outstanding studies by NEDU and others around the world the last 20 or so years, Bühlmann is still mathematically derived and not based on actual humans.

For the nitpicky among us, ZH-L12 was, if I'm not mistaken, based on actual humans (as well as prior art involving exploding goats), and if you compare the coefficients for matching compartments with ZH-L16, you'll find they're identical or pretty close. At least on the nitrogen side.

 

[noprowiththebow]

Interesting stuff! Now, I am going on a rabbit trail to educate myself!

 

[huwporter]

Another factor I didn't see mentioned that plays a small part for short dives;

All tissues start with a 0.78 bar of nitrogen due to being in equilibrium with surface air. In Buhlmann theory, during a heliox dive, this leaves the tissues more slowly than helium is entering. Maximum supersaturation is dependent on the sum of N2+He, so, for short dives on heliox where the nitrogen has not fully desaturated from the tissues, this sum will be higher than the quantity of N2 after the same dive on nitrox, hence longer decompression required.

 

[inquis]

Maximum supersaturation is dependent on the sum of N2+He, so, for short dives on heliox where the nitrogen has not fully desaturated from the tissues, this sum will be higher than the quantity of N2 after the same dive on nitrox

The limit on that sum, however, is also proportional to the amount of each gas, and He has a higher tolerated pressure than N2. Depending on what you mean by "short", there may be instances where a heliox dive might have less deco.

Then there's the practical matter that the reason heliox is used is because you're going deep, so nitrox is simply not an option.

 

[huwporter]

Then there's the practical matter that the reason heliox is used is because you're going deep, so nitrox is simply not an option.

Yeah naturally - but the OP was asking why planning the same 40m/20 min profile in Buhlmann generated more deco on 32% heliox than 32% nitrox. We aren't talking about reality here, just Buhlmann theory.

 

[leadduck]

Allow me to share a few thoughts, which are totally useless to the decompression theory obsessed.

Although there has been outstanding studies by NEDU and others around the world the last 20 or so years, Bühlmann is still mathematically derived and not based on actual humans.

No, Bühlmann did a ton of experiments. He was neither a diver nor a mathematician, but the medical director of the hyperbaric chamber laboratory, had his own institute with hyperbaric chamber. The a/b values are fine-tuned from experimental results. Bühlmann's valuable contribution was not in the mathematics (the model is very simple), but in his rich experimental fine-tuning of the models. See

https://watchangels-live-171a32ea5b4446588cdf25-2d56b1d.divio-media.com/filer_public/0c/7f/0c7f3b42-d4e3-4142-9495-2ccf12915ad0/bul_timeline_01ld.pdf

All decompression models are approximations to an infinitely more complicated reality; models are just formulas with coefficients that must be set somehow to minimize the approximation error, either by experiments like Bühlmann did for ZHL or just guessing like what happened with VPM. I'd say a simple model with strong experimental validation is much more reliable than a complicated formula with no tested connection to reality.
The big effort and value of making a decompression model is not about inventing a formula but in the experimental calibration and validation.

Most of the studies I have read have been done using young healthy men as subjects. I have not seen one that was based only on women or old fat guys like me. .

The PADI RDP (DSAT) was experimentally validated with average divers

https://www.angelofarina.it/Public/Scubapro/NEDU-reports/DSAT_1994_unsecured_opt.pdf

They focused on repetitive NDL dives whereas Bühlmann's experiments were more focused on single industrial deep dives with longer decompression times.