giovedì 6 novembre 2008

The U.S. Navy Decompression Computer

Article by:

CAPT.  Frank K. Butler, M.D.

Director of Biomedical Research

Naval Special Warfare Command

   Most civilian SCUBA divers have long since added decompression computers (DCs) to their dive bag. Interestingly enough, the U.S. Navy has never approved a DC for its divers to use - until now. This article will review the development and approval of the U.S. Navy DC.

  In 1977, the Navy SEAL community formally requested that the U.S. Navy develop a decompression computer. The SEAL community has played a key role in the advancement of Navy diving techniques in the past. One of the first Americans to use Jacque Cousteau’s new Aqualung in 1948 was Commander Francis Fane, a member of the Navy Underwater Demolition Teams, the forerunner of today’s SEAL's.

Text Box:   Preparing an SDV for launchIn the late 1970s,  SEAL's introduced two innovations to Navy diving. The first was a new closed circuit mixed gas SCUBA that used a microprocessor to control the partial pressure of oxygen. This SCUBA rebreather maintained the oxygen partial pressure at a constant 0.7 ATA, regardless of depth. The other diving innovation was the Dry Deck Shelter - an underwater garage that fits onto the deck of a nuclear submarine to house a small underwater vehicle called an SDV (SEAL delivery vehicle). SEAL's operating  SDV's from a Dry Deck Shelter perform very long (over 8 hours) dives at a variety of depths. Use of the Standard Navy Air Decompression Tables to calculate decompression for this type of diving results in decompression times that are unnecessarily long. As with recreational divers who commonly do multilevel dives, a decompression computer is a far better way to calculate decompression for these dives. In addition, because of the new UBA with its varying nitrogen fraction depending on depth, new tables had to be developed by the Navy to use in the DC.

  The Navy Experimental Diving Unit (NEDU) with its unique pressure chambers began the  effort to develop the Navy’s decompression computer in 1978.   Initial studies were aimed at developing a computer algorithm that reflected, as closely as possible, the known science of gas kinetics.  Once the algorithm was established, the Navy set out to test it with a series of dives to be certain that the profiles were indeed safe.  The primary investigator for the development of the new constant oxygen partial pressure tables was Captain Ed Thalmann, the Senior Medical Officer at NEDU. By 1981, CAPT Thalmann had supervised hundreds of experimental dives and completed the development of the new tables. The tables were approved for Navy use and the mathematical model that had produced them was ready to be put into the Navy DC. Prototype computers built in a Navy lab, however, failed because of repeated flooding.  Negotiations were then begun to contract with a commercial DC manufacturer to have the Navy algorithm programmed into a commercial DC, but this effort also failed when the manufacturer’s plant was destroyed in a fire. Another delay occurred when the SEALs decided that their operations would require the ability to breathe both air and mixed-gas on the same dives.  CAPT Thalmann and his colleagues at NEDU then performed a series of experimental dives designed to retest selected schedules from the Standard Navy Air Decompression Tables prior to modifying the nitrox decompression algorithm. The deeper air No-Decompression limits were found to be safe, but dives with very long bottom times were found to have an unacceptably high (up to 30-40%) incidence of decompression sickness.

  After CAPT Thalmann left NEDU, the Navy decompression research effort was continued over the next few years at the Naval Medical Research Institute (NMRI). The NMRI team developed an innovative new approach to decompression modeling called the probabilistic model. Whereas the older Haldanian approach used by CAPT Thalmann provides for one single No-D limit or one single safe decompression time for a decompression dive, the NMRI probabilistic model used a statistical approach to calculate a probability of decompression sickness for any no-decompression limit or decompression profile that a diver might choose. The tables chosen could than be tailored to whatever level of risk was acceptable to the diver. This approach showed that the incidence of DCS rises gradually with increasing decompression stress, not suddenly as a single arbitrary threshold is passed. The DC research effort had slowed to a crawl by 1990, when it was energized again by the establishment of the Naval Special Warfare Biomedical Research Program.  The NMRI probabilistic model needed some additional experimental diving to be ready for Navy approval and funding for this effort was obtained from the new SEAL research program.  By 1993, the required diving had been completed and acceptable probabilities of decompression sickness had been agreed upon. The new decompression tables generated by the NMRI probabilistic model were considerably more conservative than the standard Navy air tables in many areas.

  Implementation of the new tables into Navy diving practice was delayed when the ship’s husbandry divers, who maintain and repair Navy ships while they are in their berths, complained that the proposed new tables were too conservative. They noted that there was a marked reduction in the 40-foot No-D limits despite the fact that this limit had been used safely by ship’s husbandry divers for many years. Because of the negative impact that the new tables would have on the ship’s husbandry divers, implementation of the new Navy air tables was suspended indefinitely.

  As a result of this decision, attention was then re-directed by the SEAL community to CAPT Thalmann’s model, which had been used to generate the mixed-gas rebreather tables approved and used by the Navy. This model has the ability to compute decompression for air as well as for a constant partial pressure of oxygen of 0.7 ATA in a nitrox mix. Tables produced by this model result in no-decompression limits that are somewhat more conservative than the current Navy No-D limits in the shallow range, similar in the 60-80 foot range, and less conservative at deeper depths. Like the NMRI probabilistic model, this model becomes much more conservative than the current Navy air tables as total decompression time increases. Very long bottom time profiles may require decompression times 3 or 4 times as long as those found in the Standard Navy Air Tables. 

  The decision was subsequently made by the Navy that the Thalmann decompression algorithm (VVAL18) was the best choice of decompression software to incorporate into a commercial DC. A competitive bid was won by Cochran Consulting Company and the Thalmann algorithm was programmed into the commercially successful Cochran Commander. The first units of the Cochran NAVY decompression computer arrived at NEDU for testing in November of 1996. NEDU testing, now led by CAPT Dave Southerland,  revealed some deficiencies that were corrected, and in January 1998, NEDU declared the Cochran NAVY ready for field testing by the SDV teams.

  SEAL divers in the two SDV teams carried out field-testing in 1998 and 1999. This testing revealed additional items of concern that were corrected. One of the most significant changes was that the DC’s programmable options are now preset at the factory rather than programmed by the individual diver.  This change both made the DC simpler to use and ensured that all DCs were programmed in an identical manner. In addition, the Thalmann decompression algorithm was programmed to assume that the diver is breathing air at 78 FSW and shallower and nitrox  with a constant oxygen partial pressure of 0.7 ATA at 79 feet and deeper. This allows SEAL divers to breathe from either an open-circuit air source (higher decompression stress shallower than 78 feet) or from the mixed gas rebreather (higher decompression stress deeper than 78 feet) and still be assured that he will be safely decompressed. An improved diver training course was also developed and all SEAL divers are tested on their knowledge of the computer prior to use of the Cochran NAVY.

Text Box:   The Cochran NAVY  On 20 October 2000, NEDU recommended approval of the Cochran Navy for SEAL use. On 25 January 2001, the Supervisor of Diving and Salvage for the U.S. Navy authorized the use of this DC by selected SEAL units. The Navy’s first decompression computer dive was conducted by Bravo Platoon of SDV Team One on 31 January 2001 in the waters off of Barber’s Point in Hawaii.

Is the Cochran NAVY suitable for use by sport divers? Since most recreational divers do not routinely make decompression dives, the extra safety incorporated into those areas of the Thalmann tables will not benefit them. The air No-D limits found in the Thalmann model are less conservative than those in most, if not all, other dive computers. Navy divers have, however, used less conservative shallow No-D limits for many years with a very low incidence of decompression sickness. As outlined in CAPT Thalmann’s NEDU Report 8-85, additional testing of the deeper No-D limits in his model resulted in no DCS cases in the 107 experimental dives performed. These trials were performed under worst-case conditions with divers immersed in cold water and exercising strenuously on the bottom. The 3-5 minute safety stop that has become common in recreational diving practice would add a significant measure of safety to these limits. Still, recreational divers should know that the Cochran NAVY is probably the most aggressive dive computer currently in use on No-D profiles. Two other factors lower the decompression risk of the Cochran NAVY as it will be used by SEAL teams. Since the computer assumes that the diver is breathing the gas mix with the highest possible partial pressure of nitrogen for the depth sensed, in many cases, the decompression calculations provided will be much more conservative than those required had the diver’s breathing mix been recorded precisely. In addition, since SEAL diving operations entail multiple divers, all divers decompressing as a group will be decompressed on the DC that displays the longest decompression time, providing an extra measure of safety for the other divers on the profile.

  Approval of the Cochran NAVY heralds the dawn of an exciting new era in Navy diving. Use of the  computer offers the opportunity to accurately capture research-grade data about dive profiles. This data will be collected by NEDU and archived there. It will then be available to the country’s leading decompression researchers (both military and civilian). If and when episodes of decompression sickness occur, the profiles that caused the episodes will have been recorded precisely, rather than having to rely on possibly inaccurate data supplied by the diver.  Clusters of bends cases on similar profiles can then be addressed by revision of the Thalmann algorithm in the targeted areas. NEDU has established a standing oversight panel on decompression computer diving to oversee these efforts and to recommend needed changes to the decompression algorithm or the DC hardware.

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