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Where When How May/June 2017 : Page 97

deep-first rule is a relative newcomer to the diving liturgy, traceable only back to the 1970s. The story is also an interesting lesson in the history of diving. A popular assumption is that the recreational diving community simply adopted the deepest-first rule from the U.S. Navy. Such an assumption seems en-tirely reasonable given that the USN Ta-bles were the first standard for recreational diving. The problem is, it’s not true. The U.S. Navy does not now, nor did it ever, have any prohibition against reverse profile diving. In fact, one of the example dive table problems in an older version of the U.S. Navy Diving Manual involved a reverse profile. Like-wise, there is no such prohibition in commercial diving. So where did the rule come from? Like many rules, “deep dive first” probably came about out of necessity, or at least based on some practical advan-tage. Imagine, for example, that you are planning to do the following: You wish to make two dives, one to 100 feet (30 m) for 20 minutes and a 40-footer (12 m) for 60 minutes. The surface interval, re-gardless of the order, will be one hour. Using the U.S. Navy tables, you’ll see that the order in which you dive does make a very big difference. By diving deep, then shallow, the dive sequence is easily within the no-decompression limits. However, reverse the order — shallow to deep — and the second dive requires two decompression stops totaling 26 minutes. (The reversed series also be-comes a decompression dive using other tables such as PADI’s [Professional Asso-ciation of Diving Instructors] Recre-ational Dive Planner.) Thus, the initial recommendation of making the deepest dive first was a practical, common-sense suggestion to maximize bottom time and avoid decompression. Still, the question remains, how could what was once merely a suggestion become an absolute hard-and-fast rule? And more impor-tantly, was there any physiological basis for the rule that might decrease the risk of decompression sickness? The first suggestion to make the deepest dive first appears to have been offered in a relatively obscure reference — and only as a suggestion — by a re-Diving as it should be Diving • Snorkelling • Instruction Tel: 649.946.5040 Visit the Turks & Caicos Islands at MAY/JUNE 2017 • • • • • 97

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