M-value?
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The short version? Maximum value...for a better answer, read this:
http://www.gap-software.com/docs/mvalues.pdf
Stone
Contributor
The M-value is the maximum tissue pressure allowed in the compartment when the diver surfaces, so as to prevent exceeding the maximum acceptable gradient. If the diver exceeds the M-value in any compartment, theres an unacceptable risk of decompression sickness. The faster the compartment, the higher its M-value, with slower compartments having lower M-values
It has to do with modeling a maximum supersaturation level that a dissolved gas can reach in a theoretical tissue compartment before bubbles start to form. It can be used by computer programs, for example, in determining deco ceilings and stop times.
R..
Dr Deco
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Dear Readers:
This question arises anew from time to time.
M-values
They are variously referred to as matrix values or maximum values. The method was instigated by Dr Robert Workman of the US Navy to assist in the calculation of decompression tables. This was in the 1950s.
The initial concept of John Scot Haldane was to allow the pressure to be reduced by ½ (2:1 ratio) in all of the tissue compartments. [This was actually 1.58:1 if allowance was made for the fact that much of the air was actually oxygen and not pure nitrogen.] It was found that this small ratio was only necessary for saturation diving. This ratio for pressure reduction was modified when it was noted from experiments that fast compartments (those with short halftimes) could tolerate a much higher supersaturation than slow compartments (large halftimes).
Start with NDLs
If one started with the no-decompression limits (NDLs) determined experimentally for a group of test subjects (age dependent, lean/fat dependent, physical fitness dependent, work load dependent, hydration dependent, etc., etc), it could be found that deep dives are short and are limited by small halftimes (5-10 minutes). Intermediate depths (60 fsw) are limited by medium halftimes, and shallow dives (35-40 fsw) are long half time dependent. The pressure in the limiting tissue is the surfacing pressure and its ratio to absolute pressure is the limiting ratio. This value (in fsw) is the pressure in a given half time compartment for ascent to the surface and is the Mo-value.
Deeper depths
As one moves from the surface to the ten-foot depth, the allowed tissue gas ratio becomes a bit smaller. The ATTACHMENT (from my Decompression Physiology class) shows the allow tissue pressure (M-value) for ascents to the ten-foot stop and deeper stops. There are ten-foot increments in the Mo values that allow the calculation of tables down to hundreds of feet.
Problems
These M-values do not always work as the tables go deeper and decompression becomes extended. This deco is conducted in chambers for commercial and military divers. I believe that the major failure in decompression is the result of tissue nucleation as the divers walk about in the chambers over the hours to days of decompression.
Safe decompression
It is clear for recreational SCUBA that you can add to the safety of decompression by
One advantage of tables is that it is easy to conservatively read and apply the tables. Computers always give numbers that are the table limits.
Dr Deco :doctor:
This question arises anew from time to time.
M-values
They are variously referred to as matrix values or maximum values. The method was instigated by Dr Robert Workman of the US Navy to assist in the calculation of decompression tables. This was in the 1950s.
The initial concept of John Scot Haldane was to allow the pressure to be reduced by ½ (2:1 ratio) in all of the tissue compartments. [This was actually 1.58:1 if allowance was made for the fact that much of the air was actually oxygen and not pure nitrogen.] It was found that this small ratio was only necessary for saturation diving. This ratio for pressure reduction was modified when it was noted from experiments that fast compartments (those with short halftimes) could tolerate a much higher supersaturation than slow compartments (large halftimes).
Start with NDLs
If one started with the no-decompression limits (NDLs) determined experimentally for a group of test subjects (age dependent, lean/fat dependent, physical fitness dependent, work load dependent, hydration dependent, etc., etc), it could be found that deep dives are short and are limited by small halftimes (5-10 minutes). Intermediate depths (60 fsw) are limited by medium halftimes, and shallow dives (35-40 fsw) are long half time dependent. The pressure in the limiting tissue is the surfacing pressure and its ratio to absolute pressure is the limiting ratio. This value (in fsw) is the pressure in a given half time compartment for ascent to the surface and is the Mo-value.
Deeper depths
As one moves from the surface to the ten-foot depth, the allowed tissue gas ratio becomes a bit smaller. The ATTACHMENT (from my Decompression Physiology class) shows the allow tissue pressure (M-value) for ascents to the ten-foot stop and deeper stops. There are ten-foot increments in the Mo values that allow the calculation of tables down to hundreds of feet.
Problems
These M-values do not always work as the tables go deeper and decompression becomes extended. This deco is conducted in chambers for commercial and military divers. I believe that the major failure in decompression is the result of tissue nucleation as the divers walk about in the chambers over the hours to days of decompression.
Safe decompression
It is clear for recreational SCUBA that you can add to the safety of decompression by
- simply reducing bottom time,
- slowing ascent, and waiting a bit at the surface (the hidden stop) before boarding the boat, and
- reducing strenuous physical activity on the surface.
One advantage of tables is that it is easy to conservatively read and apply the tables. Computers always give numbers that are the table limits.
Dr Deco :doctor:
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