Re: Re: Algorithm question: why I asked

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Posted by Ross Overstreet on September 26, 2006 at 22:35:06:

In Reply to: Re: Algorithm question: why I asked posted by FARMHAND on September 26, 2006 at 21:12:25:


I'm not sniping here, I'm just trying to catch you up a bit on terminology.

A dive computer doesn't MEASURE anything on or in you - it only computes. For Buhlmann's ZHL-16, these computations involve time, depth, and a bunch of constants. The computations may or may not have anything to do with what is actually going on inside your body. For example, it's now understood that the theory behind Haldane and Buhlmann's methods is largely incorrect. Interestingly enough, the dive times that the models generate generally work but through mechanisms other than what the scientists originally thought. I don't have the specifics in front of me, but I believe the M-values of Buhlmann's model were arrived at experimentally. This is a neat way of closing the loop so the model, the test subjects, and the physics all seem to be in agreement.

The concept of "tissues" having half-times has been largely dismissed. The term COMPARTMENT is used these days as scientists believe that it is incorrect to assume that a kidney has a half time of 5 mins, the bowels have a half time of 20 mins, fat has a half time of 640 mins, etc. However, if you're referring to Buhlmann's work, I guess it is historically accurate to use the word "tissue" since that is what he believed.

You specifically asked about Buhlmann ZH-L16 model. The "ZH" stands for Zurich, the "L" stands for limits, and the 16 is the number of M-value sets. So BY DEFINITION, ZH-L16 has 16 compartments.

Your MOD on Nitrox and ZH-L16 have NOTHING whatsoever to do with each other. Your MOD is arrived at by choosing a partial pressure of oxygen that you're willing to dive. Choose too high and you may have a seizure - not a good thing for a solo wreck diver hiding in a silt cloud.

It's widely accepted these days that you shouldn't exceed a P02 of 1.4 during the working part of the dive or a P02 of 1.6 during deco. P02 of 1.4 with EAN36 gives you a MOD of 95.3fsw (You DO know how to come up with this, don't you? If not, go smack your Nitrox instructor and then make him review it with you!). If you choose 100fsw on 36%, then your P02 = 1.45. Is this a big deal? Probably not. Hyperbaric chambers frequently run a P02 of 2.8. That said, it's no biggie if you have a seizure in the chamber. The same isn't true in the water. Plus, I think I remember reading some research showing that people are more susceptible to OxTox in the water...

The bottom line is that you should probably draw the line somewhere, and 1.4 is good reasonable number that has a fair amount of real world use behind it.

Again, not breaking your balls here. I'm just happy to see you still diving at 74. Genetics will probably dictate otherwise, but I pray that I'll still be able to dive 41 years from now.


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