Decompression – M-Values

Units


As a diver we like to use bar’s as pressure unit, since we are familiar with the bar. The pressure in this section, however, are presented in msw (meter sea water) to be consistent with literature. 1 msw = 0.1 bar. M-values in bar can be derived by deviding the M-value in msw by 10.

(1)   \begin{equation*}   M = M_0 + \Delta M d \end{equation*}

What M-values describe


The maximum allowed (absolute) partial pressure M of the Nitrogen in a hypothetical tissue compartment, characterized by half-time τ, is defined as:

M
Partial pressure limit, for each tissue compartment (half-time)
M0
The partial pressure limit at sea-level (zero depth) (msw)
ΔM
Increase of M per meter depth (msw/m)
d
Depth (m)

Next sections present for various models the M-values.

The Workman M-values (1965) for Nitrogen

No. τ (min) M0 (msw) ΔM
1 5 31.7 1.80
2 10 26.8 1.60
3 20 21.9 1.50
4 40 17.0 1.40
5 80 16.4 1.30
6 120 15.8 1.20
7 160 15.5 1.15
8 200 15.5 1.10
9 240 15.2 1.10

The Bühlmann ZH-L12 model (1983) for Nitrogen

No. τ (min) M0 (msw) ΔM
1 2.65 34.2 1.2195
2 7.94 27.2 1.2195
3 12.2 22.9 1.2121
4 18.5 21.0 1.1976
5 26.5 19.3 1.1834
6 37 17.4 1.1628
7 53 16.2 1.1494
8 79 15.8 1.1236
9 114 15.8 1.1236
10 146 15.3 1.0707
11 185 15.3 1.0707
12 238 14.4 1.0593
13 304 12.9 1.0395
14 397 12.9 1.0395
15 503 12.9 1.0395
16 635 12.9 1.0395

The a and b parameter Bühlmann originally used in his models can be derived:

(2)   \begin{equation*}   b=\frac {1}{\Delta M} \end{equation*}

(3)   \begin{equation*}   a =  M_0 - \frac {P_{amb\_sea\_level}}{b} \end{equation*}

In this equation Pamb_sea_level is the ambient pressure at sea level expressed in msw. Remember the Bühlmann equation is now:

(4)   \begin{equation*}   P_{t.tol}ig = M = \frac {P_{amb}}{b} + a \end{equation*}

The Bühlmann ZH-L16 model (1990) for Nitrogen

No. τ (min) M0 A (msw) M0 B (msw) M0 C (msw) ΔM
1 4.0 32.4 32.4 32.4 1.9802
1a 5.0 29.6 29.6 29.6 1.7928
2 8.0 25.4 25.4 25.4 1.5352
3 12.5 22.5 22.5 22.5 1.3847
4 18.5 20.3 20.3 20.3 1.2780
5 27.0 19.0 19.0 18.5 1.2306
6 38.3 17.8 17.5 16.9 1.1857
7 54.3 16.8 16.5 15.9 1.1504
8 77.0 15.9 15.7 15.2 1.1223
9 109 15.2 15.2 14.7 1.0999
10 146 14.6 14.6 14.3 1.0844
11 187 14.2 14.2 14.0 1.0731
12 239 13.9 13.9 13.7 1.0635
13 305 13.5 13.4 13.4 1.0552
14 390 13.2 13.2 13.1 1.0478
15 498 12.9 12.9 12.9 1.0414
16 635 12.7 12.7 12.7 1.0359

The DCAP MM11F6 model (1988) for Nitrogen

No. τ (min) M0 (msw) ΔM
1 5 31.90 1.30
2 10 24.65 1.05
3 25 19.04 1.08
4 55 14.78 1.06
5 95 13.92 1.04
6 145 13.66 1.02
7 200 13.53 1.01
8 285 13.50 1.00
9 385 13.50 1.00
10 520 13.40 1.00
11 670 13.30 1.00

The DSAT Recreational Dive Planner (PADI) model (1987)

No. τ (min) M0 (msw)
1 5 30.42
2 10 25.37
3 20 20.54
4 30 18.34
5 40 17.11
6 60 15.79
7 80 15.11
8 100 14.69
9 120 14.41
10 160 14.06
11 200 13.84
12 240 13.69
13 360 13.45
14 480 13.33