Question Questions about the Buhlman decompression algorithm

[Chaehwasu]

Please ignore recent research results.
In the ZH-L16 algorithm, helium has a half-life 2.65 times shorter than nitrogen.
Helium and nitrogen do not interact with each other.
And the m value is calculated based on the half-life of each gas.
Therefore, in the same tissue, helium has a higher m value than nitrogen.

Now, let's assume:
EAN32 and Heliox 32 (32% oxygen) have the same fO2 value, and the rest are noble gases. Intuitively, the Heliox 32, which contains helium, should feel like it has a shorter decompression time.

In fact, if you apply a very long dive time (enough to saturate a large area of tissue), such as 40m and 1,000 minutes, and run the results through planners (subsurface and decoplanner) that apply the Buhlmann algorithm, you can confirm this.

However, when we planned a 40m, 20min dive using EAN32 and Heliox 32 (32% oxygen), the decompression time for EAN32 was significantly shorter. Similarly, most planners, when applying the same dive profile to Tx21/35 and Air, found that Tx21/35 had a longer decompression time at 6m.
In fact, Subsurface plan for dives conducted with Tx20/40 and Nx20 show that Tx20/40 is only 1 minute more at 12 meters and 1 minute more at 9 meters than Nx20, but 11 minutes more at 6 meters.

Due to the rapid saturation of helium, the first deco stop must be made deeper when using a higher helium mix, which might have affected the overall decompression. However, the remaining sections had similar stop depths and times, with only the 6m decompression time showing a longer time.

I simply cannot understand this. I'd like to know more about the reason. Would love to hear your thoughts.

 

[Mobulai]

Having not compared heliox vs nitrox or air deco profiles, I can't say exactly, but you made an important point

However, the remaining sections had similar stop depths and times, with only the 6m decompression time showing a longer time.

When comparing runtime/deco time you need to consider the entierty of the profile not just the TTS/deco time

The integral of time spent at various depth/pressure differentials should be eventually the same, so if you start your deco at a earlier deeper point, it shaves only a tiny bit vs the greatest diffrential at 6m

To compare how stops are mamanged by an algorithm you need to have the same total runtime (1:03:00 in linked video)

Without playing with it (and also the GF Hi/Lo values) I cant say more, but I believe the NDEU study on deep stops did an intresting control on conparing runtimes that might help you get the gist of what I'm refering (tho they were comparing algorithms not gases); and @Dr Simon Mitchell made a nice lecture about it thhat you can find on youtube

I look forward to what other users (that have better understanding and explain it better) have to say about this

 

[harvalen]

Here is a good blog post that talks about how helium and nitrogen interacts in the model and why it gives you longer decompression times with trimix:

N2 vs. He, what’s the difference? – The Theoretical Diver

thetheoreticaldiver.org

 

[elmo]

Please ignore recent research results.
In the ZH-L16 algorithm, helium has a half-life 2.65 times shorter than nitrogen.
Helium and nitrogen do not interact with each other.
And the m value is calculated based on the half-life of each gas.
Therefore, in the same tissue, helium has a higher m value than nitrogen.

Now, let's assume:
EAN32 and Heliox 32 (32% oxygen) have the same fO2 value, and the rest are noble gases. Intuitively, the Heliox 32, which contains helium, should feel like it has a shorter decompression time.

In fact, if you apply a very long dive time (enough to saturate a large area of tissue), such as 40m and 1,000 minutes, and run the results through planners (subsurface and decoplanner) that apply the Buhlmann algorithm, you can confirm this.

However, when we planned a 40m, 20min dive using EAN32 and Heliox 32 (32% oxygen), the decompression time for EAN32 was significantly shorter. Similarly, most planners, when applying the same dive profile to Tx21/35 and Air, found that Tx21/35 had a longer decompression time at 6m.
In fact, Subsurface plan for dives conducted with Tx20/40 and Nx20 show that Tx20/40 is only 1 minute more at 12 meters and 1 minute more at 9 meters than Nx20, but 11 minutes more at 6 meters.

Due to the rapid saturation of helium, the first deco stop must be made deeper when using a higher helium mix, which might have affected the overall decompression. However, the remaining sections had similar stop depths and times, with only the 6m decompression time showing a longer time.

I simply cannot understand this. I'd like to know more about the reason. Would love to hear your thoughts.

From a purely conceptual perspective of the equation, as opposed to empirical evidence and noting the Buhlmann’s and earlier testing was with air and extrapolated to include helium, helium is absorbed into the tissues faster. So for a relatively short heliox dive, there is enough helium to require a longer decompression than if the gas were nitrox. For a relatively long dive both gasses approach saturation so the faster offgassing is more important and the decompression is faster with heliox.
You can try to beat the system by switching to nitrox during decompression, in which case the nitrogen is ongassing (from a low base) while helium is offgassing. It works in the algorithm but has no basis in testing.

 

[Chaehwasu]

From a purely conceptual perspective of the equation, as opposed to empirical evidence and noting the Buhlmann’s and earlier testing was with air and extrapolated to include helium, helium is absorbed into the tissues faster. So for a relatively short heliox dive, there is enough helium to require a longer decompression than if the gas were nitrox. For a relatively long dive both gasses approach saturation so the faster offgassing is more important and the decompression is faster with heliox.
You can try to beat the system by switching to nitrox during decompression, in which case the nitrogen is ongassing (from a low base) while helium is offgassing. It works in the algorithm but has no basis in testing.

Let's assume you breathe only backgas. Because helium diffuses quickly, it saturates your tissues more easily, but it also evaporates just as easily. If you pressurize for the same amount of time, you'll become more saturated with helium, and at this situation, if you decompress for the same amount of time, less helium will remain. Calculations will tell you this. However, the planner shows a different result.

 

[Chaehwasu]

Here is a good blog post that talks about how helium and nitrogen interacts in the model and why it gives you longer decompression times with trimix:

N2 vs. He, what’s the difference? – The Theoretical Diver

thetheoreticaldiver.org

I share this understanding. In fact, the m-value lines for nitrogen in tissue 11 and helium in tissue 15 are almost identical. I also understand that the Buhlman model is not rigorous in its calculations when nitrogen and helium are mixed. However, if the noble gas in the mixture consists solely of pure nitrogen or pure helium, Buhlman's non-rigorous calculations do not apply. However, even in such cases, the decompression time for the mixture containing helium is longer.

 

[inquis]

Because helium diffuses quickly, it saturates your tissues more easily, but it also evaporates just as easily.

You're neglecting the greater solubility of He. Not only is it faster, but tissues can hold more of it. Edit: sorry, totally backwards, please disregard.

 

[Chaehwasu]

You're neglecting the greater solubility of He. Not only is it faster, but tissues can hold more of it.

This is not true. Helium has a shorter half-life, but it has less solubility. If a tissue contains the same partial pressure of helium and nitrogen, then it has fewer molecules of each gas in it. Above all, the Buhlman algorithm does not take into account absolute solubility.

 

[inquis]

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.)

 

[rjack321]

I share this understanding. In fact, the m-value lines for nitrogen in tissue 11 and helium in tissue 15 are almost identical. I also understand that the Buhlman model is not rigorous in its calculations when nitrogen and helium are mixed. However, if the noble gas in the mixture consists solely of pure nitrogen or pure helium, Buhlman's non-rigorous calculations do not apply. However, even in such cases, the decompression time for the mixture containing helium is longer.

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.