Decompression Theory

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I have done courses with Mark and I don’t think he pushed deep stops. Quickly skimming Deco for Divers just now I need to be reminded where he says deep stops are better.
In the chapter about deep stops and his conclusion that they will become as standard as safety stops. I don't have the book in front of me right now so I can't quote chapter and verse.
 
With multiple compartments whatever compartment is closest to its m-value (maximum allowable compartment partial pressure) controls the decompression. You might think that the fast compartments will control decompression since they will reach saturation first. But the maximum ascent rate to the surface allows time for the fast compartments to partially off gas and also they have higher m-values. So generally the middle compartments will drive.

Take a look at “Understanding M-values” by Erik C. Baker. Also you should be able to find a C or FORTRAN Buhlmann program by Baker out there if you want to get into the details of the calculations. You may also be able to find Buhlmann’s 1984 book on Decompression Sickness which is lucid and worth a read.
 
Edit: The ss will show you the actual tissue compartment pressures and which TC is controlling each phase of the dive. You'll see that for most profiles if you maintain a slow ascent of 30 fpm (10 mpm) the fast tissues (TC's 1-4) clear on ascent. While the slowest TC's (13-16) are ongassing their half-times are too long to pose a problem. In general, It's the medium TC's that require deco stops to prevent an unsafe critical supersaturation on surfacing.

After running some NDL profiles looking at the change in SurfGF based on varying ascent speeds I noticed that for some dives the faster TC's were in control all the way to the surface.
 
I learned a lot about theoretical compartments by diving one of these.
Imagine the compartments coming down to the curved line until touching being your NDL. On the first dive of the day the fast tissues always controlled the NDL. But start doing a lot of dives in a day and the slower tissues would start to take over.
It would seem at 100' or better your decompression ceiling could be quite deep but cleared as you ascended. By dive 4 on any ceiling translates into actual deco and it would be a slower compartment limiting ascent. After a few thousand dives you can see the shape very clearly and how the shape was affected by even a minute or two.
Going deeper and the effect was profound.
In 150' you could have a ceiling of 60' clear on the way up on dive 1 and by the end of the day 10 feet ceiling meant definitely hanging for a while.
20200605_214018.jpg

The credit for SI increased rapidly from 1 hour to two and then leveled off.
 
In the chapter about deep stops and his conclusion that they will become as standard as safety stops. I don't have the book in front of me right now so I can't quote chapter and verse.
Deep deco stops were already standard, in fact they still are as GFs like 50/80 are popular. His particular point was that deep “safety” stops at 15m and 6m would become more popular following the DAN work on those. That happened via the inclusion of optional deep stops by various dive computer manufacturers and by the inclusion in course materials.

Reading the whole chapter is very educational about why deep stops seems so attractive, it is also revealing about the simplicity of the dissolved gas models and so quite interesting in terms of how much faith to put in a computer that relies exclusively on those.

So, to the OP, do read Deco For Divers, but also read Introduction to Technical Diving (first probably) and in particular the chapter on the internet.
 
I get lost when I try to understand how the saturation of each compartment is used to determine deco stops and how gradient factors affect this.

To put it simply, the gradient factor sets the limit of how close you can go to the maximum saturation level, for any individual compartment. And a stop will be set so that no individual compartment exceeds that reduced limit. So a GF low of 60 says you can only go to 60% of the maximum limit. Which particular compartment doesn't matter, none of them are allowed to exceed that limit. If the GF was 50 instead of 60, the stop will be set differently so that no individual compartment will ever exceed 50% of the maximum limit. Slow compartments can still be ongassing while fast ones are offgassing yes. However, longer stops will just be set as you get shallower, so those slow ones won't ever exceed 50 or 60 either.
 
I found this interview by Dirty Dozen with Mark Powell from a few weeks ago very interesting. He talks gradient factors, preliminary results from a gradient factor study with DAN and other topics.

The big problem with decompression theory research is no one cares about it or pays for it other than tec divers. That’s a very small pool. (To paraphrase Mark)

 
I think I understand what's happening for a single tissue compartment, but not how the different ones interact.

It's very profile-dependent: you may at some point have some off-gassing, some on-gassing, and some: saturated (i.e. neither). Only the off-gassing ones are important, really, and off-gassing is normally happening during ascent.

The 2nd part of the equation is M-value (google Erik Baker Understanding M-values), it's the "allowed" overpressure in tissue compartment. Each compartment has its own one and it determines how far you can ascend before that compartment is at risk of clinical DCS. You just go through all of them and see which one is the closest to its M-value, that's the one that controls your ascent at this point.

Then you stop to let it off-gas a bit, and then go through them again to see which one's controlling now.

Rinse, lather, repeat.
 
Hello,

I'm starting to look at decompression diving and I can't seem to wrap my head around some of the theory. I've taken the online DAN classes and I understand that the modern Buhlmann algorithm uses 16 tissue compartments that each have their own half-time for saturation. I get lost when I try to understand how the saturation of each compartment is used to determine deco stops and how gradient factors affect this.

I am getting late into this conversation but ....
I will not repeat the suggestion given about book readings.

I want just to highlight that you correctly say the algorithm uses 16 compartments but these are mathematical abstractions using a dissolved gas (exponential) solution law. Gas enters and exit from one compartment according to time and differential pressure.

If there are 16 compartments, it seems like there must be a system of determining whether it's more important for fast tissues to be off-gassing at a particular speed (determined by pressure ratio?) but on those stops, won't the slowest tissues continue to on-gas? I think I understand what's happening for a single tissue compartment, but not how the different ones interact.

I read somewhere that the Buhlmann algorithm was open source and went looking for the code or mathematical explanation but I either couldn't find it or it went over my head.

Thanks for your help!

But then when you ask how compartment interact between them you give the impression you believe that there is a correspondence between a tissue and one compartment. I.e. blood is compartment 1 with an half time of 5 minutes. This is is believed not to be correct. Think about it. If it is sufficient to take a breath or two of He (no oxygen) to render unconscious a person, probably blood if faster than 5 minutes ...
The ENTIRE ZHL-16 model is representative of the whole body. If you wish to understand relationship between tissues on gassing and off gassing you need to study physiology. Perfusion and diffusion of bodily fluids and mainly blood and lymphatic circulation are the main culprit for interactions between tissues.
Buhlmann, as far as I know (ready to stand corrected), did not postulate interaction between compartments. I am out of my depth here (pun fully intended) maybe more qualified reader ( @Dr Simon Mitchell for example) might come to the rescue.

Now not to open a can of worms but, the debate about deep stop or not deep stops concerns decompression efficiency. I.e. after a given dive, would be better (more efficient) to distribute x number of minutes deeper or shallower? The NEDU study proved that is better to go shallower, but if you feel doing a deep stop and then consider it an extension of dive time rather than decompression time and therefore increase you shallow decompression time you will do more decompression. This would off gas the additional inert you have on gassed staying deep longer, you might have reduced the gradient on you fast tissues early on, and compensated for the need of off gassing longer the slower tissues after the further on gassing imposed by the early stop. Obviously it would have increased the total deco time and decreased decompression efficiency. We recreational diver enjoy being in the water and a few minutes more do not matter, but people doing it for work, in difficult conditions (sea state, temperature), costing money, want out of the water safely as fast as possible, after the working phase of the dive is over.

Finally and once again the use of tissue and compartment is not interchangeable, this was my key message.

Hope this helps and not confuse.
Critical reading of the available material might prove helpful (meaning weighting source and reference not only number of posts :cool:). Some vocal supporters of the deep stop theory, loudly advocating against scientific evidence, lost their posting privileges here thus reducing the background noise.
 
Buhlmann, as far as I know (ready to stand corrected), did not postulate interaction between compartments.

DCIEM is a serial model where gas exchange is between the compartments. Scroll down to the "serial and parallel models" for the explanation.

In practice it doesn't matter: neither model is the real thing, it just needs to approximate the real thing close enough to get the diver out of the water not bent. Whether you model that as six of one, of as half a dozen of the other, does not make any difference.
 

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