Understanding deco theory

chrismc

All models are wrong, some are useful.

Tissue based models approximate the way your tissue behaves, however it is an approximation. The theory is based on real results, and they all have the same format. x number of compartments, that hold y inert gas nominal and z max. Then based on ambiant inert gas they try and equalise to this level. At a rate specifc to each compartment. For an actual human you would have probably millions of tissue compartments, and the values would be dependant on each person, and a multitude of other factors.
The entire basis for these tables is at what point do we think measurable damage willl occur and then add amargin to that.

Bubble based models are theorteically better as they are modelling the fromation of the source of the problem rather than the sympton. However these models are based upon theory on bubble formation only.

Freef

Try "The Decompression Matrix" by Bob Cole, and PADI's book on decompression and dive computers for the basics.

__________________
David.

Currently owner of two differently sized ankles.

Rebreatherlab

Hey Abbo. Check out Mark Ellyatt's new version of Deco Chek. It is a very easy and fast to use dive planner with ICD warnings and lots of goodies. I just received a beta of the new "Pelagian" version that has a new GUI and lots of CCR functions like set point deco / CNS offsets and the best mix functions I have ever seen. The algo is AB2 from Comex, is tested on people and then tweaked by Mark to better suit typical techie deep bounce dives.

The profiles give more deep stops than a Buhlmann, but a whole lot less than VPM which I believe is a good thing. The recreational profiles are overly conservative though, but that don’t matter as you don’t really need fancy software to plan your no deco dives.

You can down load a fully functional version foc that is limited to 40m, play with it, email Mark and shoot some questions and then pay to get rid of the 40 m limit if you are happy with it. Mark has a lot of opinions on deco algo's and have tried a lot of what is out there paying the price of getting bent enough times to finally sit down and develop this program. I have used it for 6 months with zero conservatism and so far not got hit. www.inspired-training.com/decochek.htm

Shoot me an email an I can send you the Pelagian / java beta version.

Cheers,

Andy

__________________

www.rebreatherlab.com

silent running

Hi Mark, whatever your reasons for being scared of RGBM, it shouldn't be because of a lack of testing. Manned mixed gas testing has been done at Los Alamos National Lab, the USN-over 3000 dives-and Naui Technical dive operations, all with no cases of DCS reported. There has also been animal testing done by a lab in Europe to validate the algo.

I think the best way to pick a deco model is to try and understand the theory behind each and use the one that makes the most sense to you. That's about all anybody can reasonably do.

RGBM goes into a high level of detail in it's modeling and tracking of bubble growth and bubble composition. And since the relationships between free phase bubbles and disolved gas are complicated and poorly understood, the more we can quantify the formation and behavior of bubbles and especially the very tiny bubble seeds that they grow out of, the more accurate the model will be. I can't say I undersatnd all of it, but the level of detail RGBM goes into impressed me enough for now.

fin

Models are models. Your choices of personal use deco model is impossible to be evaluated in long term anyway so it is more like a politics matter.
There are hundreds variations of 5-10 simple ideas practiced with different approach. Non scientific books are so easy that web reading is ok and I would stick to reading the original pieces of Hahn, Buhlman , Yount and Baker. If you really want to descent into deco therory waters, I must warn, you that you will subconsciously start to ascent from acual diving waters. After all my personal long time exhausting deco descent I am sure that no deco model variation have or will manage to keep neither determinism nor statistics out of its foundation.
The outcome as years go by, will always remind Heisenberg's Uncertainty principle. Seemingly, the closer we come to reality the more annoying the error will be to us.
My estimation is that until physical characteristics analysis of human body after new technology examinations will affect decompression physiology (like in sports medicine), we should also be assisted by prayers.

Mark Chase

Thats not the point, point is I look at the profile it kicks out and think "blimey thats 30mins less time in the water than i would normally do" and i get twitchy. I know from experience of running 10/100+GFs (that give me similar in water times) that I feel niggly after the dive. I run 10/95 on dives under 2 hours clear at 6 and spend 5 getting up and it seems OK but when I do 3hours+ I am switching to 10/80

I dont take the depth into consideration at all its the planned in water time that decides it for me. If i plan to do 25mins at 70 id run the 10/90 but if i plan 45mins at 70 I switch to 10/80. When i do a comparison with VPMB/L2 or RGBM the shallow stop times look a little aggressive QED it scares me a bit.

The second issue is i feel confident manipulating GF deco. I think (thow i might be kidding my self) that i can alter the ascent in an emergency (blowing off deep stops to conserve bailout gas for example) and manage the final stage of the deco to compensate. With RGBM or VPM I cant relay say i feel confident doing this at all.

Again it all comes back to experience of how good / bad you feel after a dive.


I didn't know about the manned testing so thanks for that info.

ATB

Mark Chase

__________________
Is it supposed to make that noise ?

I took my unit to the dive shop and demanded they bolt on every thing that would fit.

Join my elite diving teem and get a Tshirt

"Doing It Chasey"Hammerhead Eccr Advanced Diving System

Simon Mitchell

Hi,

Do you have a reference for this data?? I would be most interested in seeing it.

Warm regards,

Simon M.

ScubaDadMiami

I have read some of this in Wienke's writings. Much of this can be found in Sources, which is the quarterly publication that NAUI members receive.

For example, in RGBM for the Layperson (Sources, Fourth Quarter 2004), Weinke lists a six year trial consisting of 32,000 open circuit mixed gas dives down to as deep as 350 fsw without incident; over 300,000 dives using decometers without incident (though this would be no-stop and multilevel dives), and others.

You can probably obtain the information at RGBMDiving.com. However, it is a fee site, and I have never had a chance to look at the contents since I do not subscribe.

__________________
Howard Packer
IANTD CCR Instructor
Miami Beach, Florida
CCRDiveTraining.com

silent running

Hi Mark, I understand your reservations when you see such a discrepancy btw the run times of one alg and another and your own emperical observtions.

I think the basic concept of using pressure to keep bubble seeds small
(if you accept the idea of bubble seeds) is a sound one. And if you were to use RGBM the way I have (follow the deep stops and add padding at 6M, usually to stay with your VR3 buddy during his very loooong 6M stop) you can't loose. In other words, follow the deep stops and pad the shallow stops. And if you are in an emergency and have to get out of the water, at least you know you'll probably be OK.

silent running

Hi Simon, the reference is my tmix insrtuctor Tim O'Leary, who helped write the Naui tmix CCR course and worked closely with Bruce Weinke. I can certainly try and find reference materials to document and confirm what I was taught. I will try to get some links to the data. -Andy