Where When How May/June 2017 : Page 95
Deepest Dive First? Editor’s note: The bulk of this article originally appeared in the February 2000 issue of Dive Training . We’re revisiting the topic here, for its historical value to our readers, and also as a reminder that “hard and fast” diving practices change over time, with the advancement of research and technology. Visit the Turks & Caicos Islands at www.WhereWhenHow.com MAY/JUNE 2017 • • • • • 95
Deepest Dive First? How We Plan Our Dives
The His tory and Science Behind How We Plan Our Dives
One aspect of my personality that used to drive my mother absolutely nuts is that I don’t follow rules very well. It’s just something about being told that I “have to” do something that brings out the delinquent in me. And there’s no other phrase in the English language that I find more deserving of a challenge than, “just because I said so.” Needless to say, my resistance to rules, in an otherwise bright kid, forever labeled me by my teachers and other authority figures as someone destined never to “live up to his potential.” Then, like today, I just hate being told what to do
Reprinted with permission from Dive Training
Editor’s note: The bulk of this article originally appeared in the February 2000 issue of Dive Training. We’re revisiting the topic here, for its historical value to our readers, and also as a reminder that “hard and fast” diving practices change over time, with the advancement of research and technology.
The reason that I mention this is because it’s such a contrast to my approach to diving. Oddly enough, as a diver I find it easy to abide by the rules. The reason for my dutiful obedience as a diver is because of my training. I learned to dive at a very young and impressionable age, and was taught by an incorrigible ex-Marine Corps drill instructor and commercial diver who had turned the idea of not following rules into an art form. Suffice it to say that the Hell’s Angels would have felt comfortable in my scuba course (and I wouldn’t have been that surprised to see them in it). The instructor who trained me wasn’t the kind of guy you said no to easily, so when he said, “Hey kid, follow the rules or you’ll die!” I listened and never questioned his rationale.
It seems that rules and diving go together like spaghetti and meatballs. Never hold your breath, never dive alone, always do this and never do that make up a major part of scuba instruction. In fact, as I escaped military service due to my draft status as a student, the only endeavor where I think I had to learn more rules than in diving was when I took my driver’s license exam.
I am, of course, not saying that rules aren’t necessary. But it is interesting to watch, in an organized activity where extensive training is necessary — like scuba diving — how common-sense suggestions can evolve into rules as binding as those Moses brought down from Mount Sinai. A case in point is a rule so ingrained in divers that few consider violating it — openly, at least. The dictum I’m referring to has been part of a canon of scuba diving almost as long as organized training has been around: Always make your deepest dive first. Not surprisingly, when dive computers came on the scene, this repetitive dive rule was expanded to cover multilevel diving. And today, “stair-stepping” — moving from the deepest to the shallowest part of a dive — remains as much an SOP as the buddy system or wearing an octopus regulator.
So, it’s within this context of these rules and regulations that this article is part history lesson, and part reminder, that even in scuba diving, things change.
Before any of you old-timer divers start asking why such a sacrosanct rule could be called into question, let’s first examine how we came to regard what have been termed “reverse profiles” as taboo. And since many of you may be very new to the diving scene, let’s start even further back with some definitions. The issue of reverse profiles is sometimes confusing because the term actually can have two meanings. First, a reverse profile can refer to a series of repetitive dives in which the deepest is not the first in the series. On the other hand, it can also describe a single multilevel dive that doesn’t follow the “stair-stepping” described earlier; and the diver finds him or herself in the deeper phase of a dive after completing a shallower segment. Both procedures violate the deepfirst rule.
Now that we’re all on the same page, it may surprise you to learn that the 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 entirely reasonable given that the USN Tables 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. Likewise, 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 advantage. 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, regardless 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 becomes a decompression dive using other tables such as PADI’s [Professional Association of Diving Instructors] Recreational 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 importantly, 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-searcher named Dennis Walder in 1968. His rationale was that by making the deeper dive first, one might crush “silent bubbles” — assumed progenitors of decompression illness — making the development of bends less likely on that and subsequent repetitive dives. But this was based primarily on theory and informed speculation, not empirical evidence. Moreover, no reference to any deep-first guideline appeared in any recreational diving literature until 1972, when the following statement was published in PADI’s Basic Scuba Course Manual (a curriculum segment of the then-current PADI Instructor Manual): “One very important rule—WE ALWAYS MAKE OUR DEEPEST DIVE FIRST when using the dive tables.” No rationale was provided, though examples were often included during training showing the aforementioned advantage of avoiding decompression stops.
Other sources picked up on the deep-first issue, such as Dr. Chris Dueker. In his 1978 publication, Scuba Diving Safety — a book that received wide distribution within the recreational diving community — he recommended the deep-first guideline. Dueker was also one of the few to provide any rationale for the recommendation, when he wrote, “Generally it saves time to take the deeper of the two dives first.” Once again, his suggestion was based on pragmatism, not decompression safety concerns.
In 1979, a symposium entitled “Decompression in Depth” was sponsored by PADI and brought together many of the leading experts in recreational diving safety. Interestingly, that program included no discussion of the deep-first issue. However, all example problems contained in the symposium proceedings followed the rule, except for one reverse profile, which was labeled a “contingency.” This was a clear implication that reverse profile dives were not to be planned.
By the 1980’s “deep dive first” was growing beyond a recommendation, and various convoluted attempts were made to provide a physiological rationale for what had been a purely practical recommendation. Furthermore, by 1984 the recommendation had been recast as a “rule,” and it appeared in both the PADI Open Water and Advanced Open Water Diver manuals.
By the 1990s, the mantra of deepfirst was firmly ensconced in diver training materials as well as in the psyche of divers; and no prudent diver even considered violating the warning. But by this time dive computers had become standard equipment, and whether by accident or intention, divers were making — and getting away with — repetitive and multilevel reverse profile dives.
Tossing the Gauntlet
In all probability, the deep-first rule might never have been challenged had it not been for the widespread use of dive computers. Although the rules say otherwise, no dive computer in existence explodes, calls the police or ceases to function if the user engages in a reverse profile dive. This dichotomy of rule versus practice began to make divers think and — horror of horrors — question the rule. And it was against this backdrop that many in the scientific diving community, led by former Smithsonian Institution diving officer, Dr. Michael Lang, decided to explore the issue more fully.
Putting various diving practices under a microscope is nothing new for Lang nor the scientific diving community. The American Academy of Underwater Sciences (AAUS), the organization that sets standards for scientific diving operations, has long been a leader in examining cutting-edge issues. In conjunction with other interested parties such as military and commercial divers, recreational diver training organizations, equipment manufacturers and dive industry publishers, AAUS has regularly co-sponsored important symposia exploring some of diving’s most fundamental practices. In 1988, they hosted what was probably the first formal scientific discussion of dive computers. That was followed a year later with a workshop on the biomechanics of safe ascents; and in 1991 they convened a symposium on safety concerns involving repetitive diving.
In October 1999, the gauntlet was tossed once again, and in the impressive setting of the Capitol Mall, the Smithsonian Institute, AAUS and DAN — along with additional support by DEMA (Diving Equipment and Marketing Association) and this magazine — sponsored a workshop examining reverse dive profiles. I was fortunate to attend. Invited speakers comprised a Who’s Who of diving experts. The group’s findings, listed in the sidebar, indicate that no significant evidence was found to warrant a rule against reverse profile diving.
For those who’ve been diving for decades, this represents a substantial edit to the rulebook. One final point shouldn’t be forgotten when rewriting the rules, however. Regardless of how decompression may or may not be affected by reverse profile diving, let’s not forget an important safety implication that has an even more immediate consequence than getting the bends: When making a multilevel dive — as are most recreational dive profiles — common sense still dictates that it’s always best to be in the shallower portion of a dive when your air supply is low. So, the question still remains whether such a seemingly fundamental change in a rule will really make any practical difference in the way we dive. Only time will tell.
Findings and Conclusion of the Smithsonian Institution, Reverse Dive Profiles Workshop, Washington, D.C., October 29-30, 1999
Historically, neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.
Reverse dive profiles are being performed in recreational, scientific, commercial and military diving.
The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates an increased risk of decompression sickness (DCS).
No convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.
We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 meters sea water (130 fsw) and depth differentials less than 12 meters sea water (40 fsw).
Overview of Reverse Profile Workshop
The reverse profile workshop brought together a formidable group of diving physiologists, physicians, physicists and mathematicians along with some of diving’s most knowledgeable educators, industry experts and diver training organization representatives. Also present were Naval personnel from the United States and other world navies, and diving safety specialists from the commercial diving industry. You can download a copy of the proceedings at archive. rubicon-foundation.org/xmlui/ handle/123456789/4244.
Read the full article at http://onlineissues.wherewhenhow.com/article/Deepest+Dive+First%3F+How+We+Plan+Our+Dives/2782783/407630/article.html.
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