NAUI Scuba Diver 134 Decompression, Dive Tables, and Dive Computers 4. At the bottom of the first profile (next to ADT), record your ADT. Remember to subtract your three minute precautionary decompression stop, which is considered neutral time. For example, your total dive time was 35 minutes – subtracting the three minute precautionary stop gives you an ADT of 32 minutes. 5. Add your ADT to the RNT to get your TNT. For example, because this was your first dive of a trip, your RNT is 0 (zero), so your TNT is 32 minutes plus 0 minutes for a total of 32 minutes. 6. On Table 1, find the row corresponding to your deepest depth and follow the row across to the block containing the time corresponding to your TNT. For example, your depth was 20 meters (66 feet), which rounds up to 21 meters (70 feet) and you follow the row over to 32 minutes, which rounds up to the 35 minute block. 7. Trace the column containing the appropriate time down until you reach a letter. This is your Letter Group. For example, for a dive to 21 meters (70 feet) for 35 minutes, your Letter Group is G. 8. Record the Letter Group on the line above the time you completed your dive. This completes the dive profile for your first dive. Recording Your Surface Interval and Finding Your New Letter Group Once you have completed your first dive, you must spend at least 10 minutes on the surface to avoid counting two dives as one. The length of your Surface Interval Time (SIT) will determine what your new Letter Group will be for planning your next dive (figure 5-14). Use the following steps to find your new Letter Group after you have completed your SIT: 1. Record your SIT in the time slot to the right of your Letter Group. For example, your SIT is 2 hours. 2. Find your Letter Group in the row of letters between Table 1 and Table 2. For example, you had a Letter Group of G. G 9:45 10:20 : MDT = 45 MDT = 35 RNT = 0 ADT = 32 TNT = 32 21(70) 24(80) 3 20(66) FIGURE 5-13. THIS PROFILE IS COMPLETE FOR A DIVE TO 20 METERS (66 FEET) FOR 32 MINUTES. 2:00 G D 10:20 : = 45 = 35 T = 0 T = 32 T = 32 AMDT = AMDT = RNT = ADT = TNT = 3 20(66) FIGURE 5-14. YOU RECORD YOUR SURFACE INTERVAL TIME BETWEEN YOUR LETTER GROUP FROM YOUR PREVIOUS DIVE AND YOUR NEW LETTER GROUP. 3. Look down the column until you find the block into which your SIT fits. For example, you are on the surface for 2 hours. The block into which 2 hours fits is the one that ranges from 2:00 to 2:58.
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