Question Questions about the Buhlman decompression algorithm

[boulderjohn]

A small digression....

Over the past few years, I have toyed with various scuba-themed social media. In almost all others, a good question like this would have been followed by a host of replies ranging from very good to pure nonsense, with no real way to know the good stuff from the nonsense. Here the question was met by thoughtful replies from knowledgeable divers. I am often very critical of ScubaBoard, which is why I did search those other media. I think this thread shows the value of the site.

 

[Chaehwasu]

One factor in the longer deco time for Trimix is that the first stop is deeper. (This because He diffuses faster.) However, N2 is still being absorbed at that stop (and some of the successive ones), leading to an overall larger N2 load compared to the N2-only ascent schedule (with the shallower first stop).

ETA: this is also why the stop times are not very different until the end. In those middle stops, the controlling tissue was near saturation in either case. However, toward the last stop, the controlling tissue is one of the slower N2 tissues. (Again, that one has a larger tension than it would have with the N2-only ascent.)

That's right. That's the principle behind the commonly referred to helium penalty. However, in the scenario I assumed, deco stop times at depths deeper than 6 meters are virtually identical. They only increase by one minute at 12 meters and by one minute at 9 meters. It's difficult to believe that this difference accounts for the 11-minute difference at 6 meters. Furthermore, decompression times at 6 meters are longer for Heliox than for Nitrox, which has the same oxygen content. The helium penalty is difficult to explain.

 

[Chaehwasu]

Because for a non-saturation duration dive the slower tissues have accumulated more He than they would with N2. For saturation length dives, the total deco time would be the reverse (longer) when N2 is the inert.

Because for a non-saturation duration dive the slower tissues have accumulated more He than they would with N2. - This is right.
However, if you calculte, you can easily see that in non-saturated tissues, if decompressed for the same amount of time, the accumulated helium will be lost more quickly due to a 2.65-fold difference in diffusion rate. Of course, there are tissues with higher helium partial pressures, but the nitrogen partial pressure in those tissues is correspondingly higher. Ultimately, the highest nitrogen partial pressure in the entire tissue is always very similar to the highest helium partial pressure. The difference is barely noticeable. In fact, the initial PN2 is 0.79. Considering this, it is unlikely that the PHe will exceed PN2 in a dive that we can perform in OC (while preserving MG). Below is a simple MATLAB code I wrote with the help of LLM. You can verify this mathematically.

 

[inquis]

They only increase by one minute at 12 meters and by one minute at 9 meters. It's difficult to believe that this difference accounts for the 11-minute difference at 6 meters.

As I said, it all comes down to the controlling tissue compartment. Use your matlab code to inspect those, and you'll see little difference in the mid-ascent, but the controlling compartment for the final stop will have a larger difference and therefore a correspondingly longer required time.

 

[ofg-1]

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. The compartments are derived and do not represent any actual tissues. M values are derived. Somewhere in the magic of your dive computer, calculations are done to smooth the different compartment times between N2 and He, different O2 percentages, while depth, time and temperature are varying, and come up with something that the computer manufacturer believes will avoid cacking their customers.

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. In my mind, this is where gradient factors come in. They are basically an expression of the amount of personal risk you are willing to take, and the benefits you get from taking it, expressed in decompression time. In my opinion, when you get to a point where you are quibbling over less decompression time than it takes you for your morning dump, you may want to rethink your priorities.


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[Chaehwasu]

As I said, it all comes down to the controlling tissue compartment. Use your matlab code to inspect those, and you'll see little difference in the mid-ascent, but the controlling compartment for the final stop will have a larger difference and therefore a correspondingly longer required time.

I understand what you're saying, but if I were to simply set up a 30-meter dive for 30 minutes, followed by 15-meter dives for 1 minute, 12-meter dives for 1 minute, and 9-meter dives for 1 minute, according to that Matlab code, if I dive with Tx21/35, the partial pressure of each gas in all tissues would be below 1.5 as soon as I reached 6 meters. However, if I set up the dive with Air, it would take 48 minutes at 6 meters for the nitrogen partial pressure in the highest tissue to drop below 1.5.

 

[inquis]

if I dive with Tx21/35, the partial pressure of each gas in all tissues would be below 1.5 as soon as I reached 6 meters.

It's the total of both inert gases that matters.