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
- 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:
| | | b = |  |
| (2a) |
| | | a = M 0 - | |
| (2b) |
In this equation Pamb_sea_level is the ambient pressure at sea
level expressed in msw. Remember the Bühlmann equation is now:
| | | P t.tol. ig = M = | | + a |
| (3) |
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 |
Last modified on January 31 2006 20:30:10.Copyright 1999-2008 Deep Ocean
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