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NAUI Master Scuba Diver 154 Decompression and Recompression 10 tion. The third modification was the inclusion of longer half-time tissues in the model (figure 5-6). By 1965, the U.S. Navy decompression model had evolved through research and use. While the total pressure surfacing ratio is based on total tissue pressure (figure 5-5), the nitrogen pressure surfacing ratio is only concerned with the tissue nitrogen pressure. When breathing air, the nitrogen ratio is 0.79 times the total ratio, the gas fraction of nitrogen. It is easy to calculate the maximum allowable tissue nitrogen pressure by simply multiplying the nitrogen ratio by the 10 mswa (33 fswa) surface pressure. The M-value system was developed in the model to address the concept of decreasing supersaturation ratios. The M-value is the maximum allowable nitrogen pressure in a specific tissue of the model (table 5-1). The nitrogen pressure surfacing ratio when breathing air is 0.79 times the total ratio, as used by Haldane. The surfacing M-value is determined by multiplying the nitrogen pressure surfacing ratio by the pressure at the surface, 10 mswa (33 fswa). Pre-Dive Surface Nitrogen 60 Minute NDL Dive Time (Minutes) 7.9 22.4 5 5 10 20 40 80 120 0 25 50 75 100 125 150 175 200 9 14 18 19 28 Nitrogen Pressure (MSWA) Depth (MSW) M-Value M-Value M-Value M-Value Dive Nitrogen 10 Minute M-Value Dive Profile FIGURE 5-4. TIME TO REACH M-VALUE ON A DIVE TO 18 MSW (60 FSW) ABSOLUTE NITROGEN PRESSURE = 22.4 MSWA (73.5 FSWA) Dive 1 Depth Meters (Feet) 10 5 Time (Minutes) 0 10 20 0 30(100) 10 20 30 40 50 60 FIGURE 5-5. TISSUE PRESSURES DIAGRAM


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