Chapter 5- Decompression and Recompression Decompression and Recompression 155 SUMMARY OF THE 1965 WORKMAN - U.S. NAVY DECOMPRESSION MODEL TISSUE TOTAL PRESSURE NITROGEN PRESSURE SURFACING HALF-TIME SURFACING RATIO SURFACING RATIO M-VALUE 5 minutes 4:1 3.15:1 32 MSWA 10 minutes 3.4:1 2.67:1 27 MSWA 20 minutes 2.75:1 2.18:1 22 MSWA 40 minutes 2.22:1 1.76:1 17.7 MSWA 80 minutes 2:1 1.58:1 15.8 MSWA 120 minutes 1.96:1 1.55:1 15.5 MSWA TABLE 5-1. SUMMARY OF 1965 WORKMAN - U.S. NAVY DECOMPRESSION MODEL Therefore, when diving to 18 m (60 ft), where the nitrogen pressure is 22 mswa (73.5 fswa), the 5, 10, and 20 minute tissues cannot exceed their maximums, regardless of how long you stay down. The 40 minute tissue will reach its limit after 64.5 minutes. Rounding this value to the nearest 5 or 10 minutes produces the 60 minutes at 18 m (60 ft) USN No-Decompression Limit (NDL). The 40 minute tissue is said to “control” this dive to 18 m (60 ft), since that tissue forced the end of the dive. The 40 minute tissue controls single dives using the U.S. Navy tables for depths between 13 and 22 m (43 and 74 ft) (figure 5-4). The U.S. Navy made a major addition to the Haldane model with the concept of repetitive diving. Working surface supply divers stay on the work site until the job is 75 finished. The decompression tables are their only limits, as their air supply is virtually unlimited. With the invention of scuba in 1943, the scuba diver, who bounced up and down to change tanks, become important. The Haldane tables gave a diver no credit for time spent on the surface. In the U.S. Navy model, the diver is assigned to a letter group based upon the pressure on surfacing in the 120 minute tissue, 0.6 msw (2 fsw) per letter. So, an “A” diver has between 10 and 10.7 mswa (33 and 35 fswa) total pressure, or 8 to 8.5 mswa (26-27.6 fswa) nitrogen pressure, in his 120 minute tissue. While the diver remains on the surface, the nitrogen leaves the body, producing a reduced letter group. On returning to the water, the new letter group is used to determine the residual nitrogen time (RNT) at the dive depth. RNT is based on the amount of nitrogen in the 120 minute tissue, and is measured in minutes. Residual nitrogen time is considered as time already spent at the repetitive dive depth. British Theory The Royal Navy theory is also a descendant of the early Haldane-Royal Navy tables. The R.N. tables use more conservative surfacing ratios than their U.S. coun- Longer Half-times Body Haldane U.S. Navy 5 10 20 40 5 10 20 40 80 120 FIGURE 5-6. U.S. NAVY MODIFICATIONS TO ROYAL NAVY THEORY SUMMARY OF THE HALDANE-ROYAL NAVY DECOMPRESSION MODEL TISSUE SURFACING RATIO HALF-TIME 5 Minutes 2:1 10 Minutes 2:1 20 Minutes 2:1 40 Minutes 2:1 75 Minutes 2:1 TABLE 5-2. SUMMARY OF HALDANE-ROYAL NAVY DECOMPRESSION MODEL
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