The US Navy Dive Tables 5% Failure Rate Myth

by

Larry "Harris" Taylor, Ph.D.

This is an expanded version of a rec.scuba newsgroup post in response to the supposition that the US Navy dive tables were unsafe and that a conspiracy to hide the "actual numbers" was in place to conceal this failure. This material is copyrighted and the author retains all rights. This editorial is made available as a service to the diving community by the author and may be distributed for any non-commercial or Not-For-Profit use.  

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Like many recreational divers, I have often heard it said that the US Navy dive tables operate with a failure rate of 5% at recreational diving depths. (Sometimes a 10% failure for deco dives is cited.)  This alleged failure is sometimes given as a reason for using no-decompression obligation procedures other than those promulgated by the US Navy. I have been unable to validate this 5 % number. As far as I know, the US Navy  Dive Tables are the most-used, most documented set of dive tables in existence. No one that I know in the hyperbaric medical community can trace the origin of this oft-quoted 5 % figure. It seems like an unacceptably high number to me. Let's examine the magnitude of this number in light of some published statistics.

 

The US Naval Safety Center report on diving illness and safety (as described in SPUMS, Sept 1997, p. 179) stated that for the period 1990-1995, there were 648, 488 logged dives. with about I fatality per 100,000 dives and 382 reported cases of decompression sickness. (This SPUMS summary did not break down the reported cases into no-deco or planned deco diving) I submit that 382 cases of DCS / 648,488 Dives (0.06 % cases of DCS per dive ) is a number FAR BELOW the mythical 5% DCS hit rate that is often heard in recreational training programs.

 

Thus, the documented operational DCS rate for US Navy divers using the US Navy tables is about 0.06 %.

 

Now, for the sake of discussion about hiding a conspiracy, let's look at some numbers and see what a 5% rate would imply.

 

Chamber treatments require at least two persons on the outside (operator and recorder) with the possible addition of supervisor, medical officer and inside tender, etc.  All chamber operations require documentation (inventory of gas used, medical supplies used, medical procedures performed, compressor hours used, time/depth profile of treatment protocol, persons present, etc.) ... the body of documentation associated with any treatment makes a "conspiracy" (ala X-Files) to hide numbers of treatments most unlikely. There would simply be too much of a paper trail, in too many different disciplines, to totally dispose of, or conspire to conceal, all records.

 

Taking the 5 % number to absurdity:

 

If 5% of Navy dives lead to DCS, then in the 6 year reporting period above, there would be:

 

5 % of 648,488 = 32,424 cases of the bends for a mean of 5,404 cases per year.

 

A chamber treatment for pain only bends requires (US Navy Treatment Table 5) 2 hours and 15 minutes

A chamber treatment for serious bends (US Navy Treatment Table 6) requires 4 hours and 45 minutes

 

In both cases, the medical officer has the option to extend time of treatment, depending on patient response, but in this absurdity, I will assume minimum time for treatment using the pain-only scenario (table 5) 

 

If only 50 % of the REPORTED CASES require treatment (and given the training/experience of US Navy divers, I would ASSUME that the ACTUAL number of chamber treatment cases is higher than 50% of the number the Navy reported as DCS incidents In other words, I assume a high correlation between reported cases and actual treatment in a chamber. Thus, I suggest a 50% estimate will be below actual.), then, there would be a minimum mean of 2, 702 treatments per year. This would involve more than 6,000 hours of chamber time per year. With a patient and three people involved in treatment, this would require a conspiracy to suppress about 24,000 man-hours (or more) of time/people commitment, plus an accounting of all resources consumed for each chamber operation.   

 

Finally, chamber treatments consume enormous quantities of gas. The Brooks AIr Force Base  chamber air spreadsheet calculator predicts that for a chamber of 170 ft3 (default value) with 2 people inside (patient and attendant)  a single Table 5 treatment  consumes  approximately 5,283 ft3 of air. So, the estimated mean of 2,702 treatments would require an annual consumption of 14,815,066 ft3 of compressed air. If treatments requiring more time, depth, or additional medical attendants than a standard non-extended Table 5 were used, then the amount of gas consumed would be larger. Obviously, more treatments than the estimated 50% of total would also increase the amount of compressed air consumed. ( If 100% of the cases in this scenario  were a table 5, then the estimated gas consumption would be 29,630,132 ft3 .)  The largest commonly available cylinder in the US of compressed air is 311ft3. This suggests an estimated annual consumption of compressed air would be equivalent to at least 47,637 of these 311 ft3 cylinders. (More, if smaller volume cylinders were used.) Oxygen and other gases used in any treatment protocol would be in addition to this. AND, if more than 50% of the reported cases lead to treatment (highly likely), then the actual amount of consumed gas would be much greater (somewhere between 14 and 30 million cubic feet) I rather doubt that this amount of gas could be consumed (with no paper trail for its manufacture, transport, use and maintenance) in total secrecy.

 

Bottom line: I don't believe the claim for a 5% failure rate for the US Navy Dive Tables is defensible. Nor do I believe that, given the scope of a 5 % failure, any US government bureau could sustain, over decades, a conspiracy of this magnitude.  

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About The Author: 

Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 100 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information In North America.

  Copyright 2001-2022 by Larry "Harris" Taylor

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