Diving Science

The 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.

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  =  M 0 + ΔM d
(1)
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
1 531.71.80
2 1026.81.60
3 2021.91.50
4 4017.01.40
5 8016.41.30
612015.81.20
716015.51.15
820015.51.10
924015.21.10

The Bühlmann ZH-L12 model (1983) for Nitrogen

No. τ (min) M0 (msw) ΔM
12.6534.21.2195
27.9427.21.2195
312.222.91.2121
418.521.01.1976
526.519.31.1834
637 17.41.1628
753 16.21.1494
879 15.81.1236
9114 15.81.1236
10146 15.31.0707
11185 15.31.0707
12238 14.41.0593
13304 12.91.0395
14397 12.91.0395
15503 12.91.0395
16635 12.91.0395

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

 
b  =  
1
ΔM
(2a)
 
a  =  M 0 - 
amb_sea_level
b
(2b)

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

 
t.tol. ig  =  M  =  
amb
b
 + a
(3)

The Bühlmann ZH-L16 model (1990) for Nitrogen

No. τ (min) M0 A (msw) M0 B (msw) M0 C (msw) ΔM
14.0 32.432.432.41.9802
1a5.0 29.629.629.61.7928
28.0 25.425.425.41.5352
312.522.522.522.51.3847
418.520.320.320.31.2780
527.019.019.018.51.2306
638.317.817.516.91.1857
754.316.816.515.91.1504
877.015.915.715.21.1223
9109 15.215.214.71.0999
10146 14.614.614.31.0844
11187 14.214.214.01.0731
12239 13.913.913.71.0635
13305 13.513.413.41.0552
14390 13.213.213.11.0478
15498 12.912.912.91.0414
16635 12.712.712.71.0359

The DCAP MM11F6 model (1988) for Nitrogen

No. τ (min) M0 (msw) ΔM
15 31.901.30
210 24.651.05
325 19.041.08
455 14.781.06
595 13.921.04
6145 13.661.02
7200 13.531.01
8285 13.501.00
9385 13.501.00
10520 13.401.00
11670 13.301.00

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

No. τ (min) M0 (msw)
15 30.42
210 25.37
320 20.54
430 18.34
540 17.11
660 15.79
780 15.11
8100 14.69
9120 14.41
10160 14.06
11200 13.84
12240 13.69
13360 13.45
14480 13.33