Confused about M-values...

[korikancha]

Hi! I have been trying to learn about decompression theory, reading all the material I got my hands on (I have two of Dr. Wienke's books but, as another poster said, they are not that easy to read...).

I read the material from http://www.deepocean.net/deepocean/index.php (pretty good, I think) and I think I learned quite a bit. However, I am still confused when it comes to M-values (aka critical gradients)... In Buehlmann's they are linear functions of pressure. Now, here is the confusion: WHAT pressure? Is it the alveolar pressure (ambient pressure corrected for water vapor, etc), or is it the partial pressure of the inert gases in the tissue in question? Similarly, in Workman's model the M-values are functions of depth. WHAT depth? Is it the depth that is EQUIVALENT to the alveolar pressure, or to the partial pressure of the inert gases? (I know it is NOT the real under water depth...)

Thanks in advance for all the help! (Just as background, I am a newbie diver but also trained, many, many years ago, as a physicist...)

 

[miketsp]

See this thread:
http://www.scubaboard.com/showthread.php?t=40102

and

I'm sure this has been posted many times on SB.

 

[lamont]

i believe it is ambient pressure (or pressure depth) vs. tissue compartment tension.

 

[Charlie99]

In Buehlmann's they are linear functions of pressure. Now, here is the confusion: WHAT pressure? Is it the alveolar pressure (ambient pressure corrected for water vapor, etc), or is it the partial pressure of the inert gases in the tissue in question? Similarly, in Workman's model the M-values are functions of depth. WHAT depth? Is it the depth that is EQUIVALENT to the alveolar pressure, or to the partial pressure of the inert gases? (I know it is NOT the real under water depth...)

Buhlmann didn't use M-values, but instead a and b coefficients. Both systems calculate the maximum allowed tissue/compartment pressure as a function of ambient pressure, and it's easy to do the algebraic conversion between them, so the Buhlmann coefficients are often shown in M-value form. The "a" is the value at zero ambient pressure, and "1/b" is the slope. The M-value equivalents are M0 for the value at sea level, and delta-M as a slope.

 

[Dr Deco]

Hello korikancha:

Pressure in all decompression calculations is the ambient pressure on the diver. This is depth in the water plus the atmospheric pressure [which is altitude dependent].

The other pressure [Ptissue] in the calculation is the tissue dissolved gas partial pressure that is dependent on the blood flow [half time], duration at depth, actual depth, and gas mixture.

Dr Deco :doctor: