Chapter 5- Decompression and Recompression Decompression and Recompression 161 Solution: 33 m (110 ft) for 30 min = Group J. Ascent time = 7 mins. decom. + 220 sec (approx. 4 min) ascending = 11 mins. Note that this is a decompression dive and is shown for illustration purposes only. 3. Your first dive is to 21 m (70 ft) for 28 minutes, followed by a surface interval of 1 hr 30 min. Your second dive is to a depth of 15 m (50 ft) for 35 minutes, followed by a surface interval of 9 minutes. Your third dive is to a depth of 13 m (45 ft) for 35 minutes. What is your repetitive group after the third dive (figure 5-15)? Solution: 21 m (70 ft) for 28 min = Group F. SIT of 1:30 = Group D. TNT for 15 m (50 ft) = 29 min RNT + 70 min ADT (combine second and third dives because SIT was less than 10 minutes) = 99 min = 15 m (50 ft) for 100 min = Group L. Note that this is a decompression dive and is shown for illustration purposes only. 1:30 :09 4. As a Group D diver, you plan a dive to 23 m (75 ft). According to the procedures presented in this section, how many minutes may you spend on the bottom (not just underwater) during this dive? For this question, use a descent rate of 23 m/min (75 ft/min) when determining the maximum time you can spend on the bottom without exceeding the AMDT (figure 5-16). Solution: Your descent should take 60 sec (descent rate = 23 m/min 75 ft/min) and your ascent should take 150 sec (maximum ascent rate 9 m/min 30 ft/ min). The precautionary decompression stop time is considered “neutral” time and is not counted. The AMDT for Group D at 24 m (80 ft) is 17 min. Subtracting the descent and ascent times leaves a maximum of 13:30 min that may be spent on the bottom during the dive. This is hardly worth the effort, so a longer surface interval or a shallower dive should be considered. This example illustrates the need to plan your dives. If you actually spent 17 min on the bottom (including descent time), a decompression stop would be required rather than precautionary. D Flying After Diving Reducing ambient pressure below that of sea level by ascending in an aircraft after diving can produce decompression sickness. The following procedures are to be followed when you plan to travel by aircraft after diving. • The altitude in an unpressurized aircraft in which you are flying should not exceed 2,500 m (8,000 ft), which is equivalent to the cabin pressure of a commercial airliner. • Wait at least 12 hours before flying following a single no-decompression dive (safety stops are not considered decompression stops for this procedure). • Wait at least 18 hours before flying following multiple 33m(110') :20 :10 15m(50') FIGURE 5-14. QUESTION #2 PROFILE 21m(70') 15m 13m (50') (45') :28 :35 :35 FIGURE 5-15. QUESTION #3 PROFILE 23m(75') ? FIGURE 5-16. QUESTION #4 PROFILE
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