Decompression Theory

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acreichman

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Location
Los Angeles, CA
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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.

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!
 
Highly recommend you read Deco for Divers by Mark Powell, if you haven't already.
 
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.

The Buhlmann model tracks all compartments and mandates a stop when necessary to keep any of the compartments from exceeding the M-value for that compartment (as that value may be adjusted by whatever GF you pick). That is the "limiting" tissue. Think of it as 16 simultaneous and wholly independent calculations going on, with the deco stops being determined by whichever of those calculations happens to mandate a stop. The compartment calculations are not "interacting" in the way you suggest.

So, in answer to your question about determining the "importance" of fast vs slow and continued on gassing of slow tissues: (a) that issue is a real live debate in decompression science (see the million threads on emphasizing deep stops or not) and (b) not something that that the Buhlmann model, itself, does. The model simply keeps all compartments below the M-value (as adjusted by GF). The model is not "looking forward" to subsequent stops to somehow influence your initial stops. It simply tracks any additional ongassing and generates appropriate stops to account for it.

The determination/judgment about whether and to what extent to protect fast tissues at the expense of ongassing the slow tissues (i.e., setting a lower GFLo to generate deeper initial stops) is one the diver makes when they set the GF based on their judgment.

And, yes, start with Deco for Divers.....
 
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.

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!


Congrats on getting into decompression diving and studying deco theory. I will also state that you should read Deco for Divers.

As for what I put in bold, this is the standard argument many tech divers love to discuss ad nauseam and not one we will have a clear-cut answer to anytime soon. Feel free to search on here for "deep stops" and make plenty of time for reading because there is A LOT!
 
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.

There are two parameters per compartment which determine the over pressure limit at different depths (imagine a line y = mx+c, y being the limit at depth x). The slower tissues have these set to produce smaller over pressure limits. This means the will be relatively important sooner than otherwise and so become the limiting tissue. We rely on the numbers picked by Buhlmann managing the relative importance. The GF scheme modifies how that works but it sounds like you are still getting your head round the simple case for now. That dates back to 1907.
 
Never lose sight of the fact that all decompression models are mathematical approaches to a theoretical understanding of what is happening.

We DON'T understand what is happening. There are mathematical approaches that mimic what APPEARS to be happening.
John Scott Haldane I believe was the first who proposed 'mathematical tissue compartments' to attempt to model what was happening to the body in an attempt to develop safe diving tables.

Please also remember, every dive you have done to date has involved decompression. The difference is that your dives to date have not required staged decompression. At the end of every dive you are still decompressing for a period after the dive (generally it is assumed after a maximum of 24hours you have fully decompressed).
The ascent phase of your dives is a decompression profile. This is why it is critical that you don't exceed the ascent rate specified in the table (computer algorithm) you are using.

Another vote for Deco for Divers
 
I recommend this video from the author of Deco for Divers:
It has a really nice introduction explaining the theoretical model of tissues and gradient factors.
 
This isn't exactly source code, but this excellent spreadsheet by @EFX implements Buhlmann and exposes a lot of stuff. If you really want to dig, you can look at the formulas in all the cells, but the easier first step is just to plug in some profiles and explore the results.

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+1 more for Deco for Divers.

https://www.scubaboard.com/community/posts/8759696/

Thanks DavidFL for mentioning my ss. I created it for educational and training purposes for whatever merit users may get from it. The ss does contain a few formulas but there is also a wealth of code underneath the "hood". I wrote it in Visual Basic because VBA (Visual Basic for Applications) is included with Microsoft's Excel if your version supports it. The code in the ss is source code. To look at it open the ss and press Alt-F8, then select calc_press in the left pane, and select edit in the right pane. Be careful. The code is not locked and can be edited and saved. If you tinker with the code I would advise you to make a copy first before looking at it. The ss has a dive_help page that explains the workings of the ss and the VB code among other things.

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.
 
https://www.shearwater.com/products/perdix-ai/

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