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.

In 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.

  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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Scientists recover North Pole mooring from 2½ miles deep in ocean

Scientists returned last week from the North Pole after recovering 3,500 pounds of instruments and equipment from a mooring anchored to the seafloor for a full year, eight times longer than the only previous mooring at the pole.

The recovery – which involved hauling miles of cable and instruments out of a 4-foot-wide hole in the ice, with three divers in special dry suits standing by in case the mooring became snarled under the ice – was part of this year's North Pole Environmental Observatory camp April 18-28. Led by oceanographer James Morison of the UW's Applied Physics Laboratory, the North Pole Environmental Observatory program is a 5-year, $3.9 million project funded by the National Science Foundation to take the year-round pulse of the Arctic Ocean and learn how the world's northernmost sea helps regulate global climate.

Scientists hope data from instruments on the mooring will help them understand, among other things, changes in the top layer of cold water (28 degrees Fahrenheit) that acts as a barricade against a deeper, but warmer, layer of water capable of causing melting whenever it reaches the underside of the polar ice cap.

That upper, very cold layer grew thinner and warmer in the last decade. That trend is now reverting toward conditions prior to 1990, according to survey work done during the last two years of the North Pole Environmental Observatory program, while the warming is slowly spreading to deeper parts of the Arctic Ocean, Morison says.

In addition to recovering the mooring during this year's camp, polar scientists and engineers installed a new mooring for the coming year. And, as in the past two years, they conducted surveys of water conditions across hundreds of miles and deployed a fleet of sophisticated drifting buoys on the ice. This year one of the buoys carries a camera linked to the Internet so scientists can relate conditions on the ice to readings received via satellite from their instruments. View the images at http://psc.apl.washington.edu/northpole/ or at the NOAA site http://www.arctic.noaa.gov/gallery_np.html. Images are usually updated every six hours although the camera can be used more frequently if needed and can be zoomed.

The North Pole Environmental Observatory program involves researchers and engineers from the University of Washington, NOAA's Pacific Marine Environmental Laboratory in Seattle, the Army's Cold Regions Research and Engineering Laboratory in Hanover, N.H., Japanese Marine Science and Technology Center in Yokosuka City, Oregon State University and the Naval Postgraduate School in Monterey, Calif.

Fourteen researchers and engineers traveled to the ice. The worst weather, with winds of 30 to 35 miles per hour causing poor visibility, came at the start of the operation and delayed flights to the ice for two days. Most days temperatures were minus 13 to minus 30 F. A few days were sunny, without wind and a balmy minus 5.

The observatory program was staged this year from a privately operated camp, dubbed Borneo, that is established each April near the pole for tourist and commercial enterprises from France, Russia, Canada and Norway. While tourists cross-country skied to the pole and rode hot-air balloons, observatory researchers used the station as the starting point for their various projects. The place where scientists returned to retrieve the mooring, for example, was roughly 60 miles north of Borneo. A smaller base camp, with two 8- by 12-foot tents, was installed there for the work.

As in past years, staging and logistics were possible with the generous cooperation and support from the Canadian Forces Station Alert, part of the Canadian Department of National Defence, as well as the Defence Research Establishment Atlantic.

Click here for supplemental information, images and contacts for the North Pole Environmental Observatory.

The North Pole Environmental Observatory Web site.

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The Search for the Invincible

The Search for the Invincible    National Underwater and Marine Association (NUMA), The Texas Navy Association,  and Cochran Undersea Technology partner to find the long lost Flagship of the Republic of Texas Navy.

Engraving of the Texas Navy flagship "Invincible" from Republic of Texas bond.

 In the fall of 1835, with the Mexican Navy blockading Texas ports, the provisional government of Texas responded by issuing Letters of Marque creating privateers to defend Texas waters and the colonist's vital maritime trade.

They also created the first Texas Navy, consisting of four ships, the Invincible, Liberty, Independence and the Brutus. These two fleets made the victory at San Jacinto possible; and they, along with the third fleet, (Second Texas Navy 1839-1845) maintained Texas independence for the next 10 years by controlling the Gulf of Mexico. 

The flagship of the First Texas Navy, the Invincible, was built in Baltimore and arrived in Galveston for duty on January 1, 1836. The Invincible, a Baltimore clipper similar to the US Revenue Cutters, was said to be sharp built and, of course, fast. The Baltimore Tonnage Certificate list the ship as being 83 feet eight inches in length, 21 feet 8 inches in breath, 8 feet 8 inches in depth and weighing almost 7 tons.  Upon her arrival, the Invincible was immediately outfitted with 9 cannon, ranging from four- to six-pounders, to an eighteen-pounder at midship. She then joined her sister Texas Navy ships and the privateers to protect Texas shipping by breaking the blockade and driving the Mexican Navy from Texas waters. But the Invincible, the Navy's finest, was given an additional order - find and destroy the Montezuma, the Mexican Navy's newest and most formidable warship. 

In February 1836, the Invincible was delivering volunteers to Copana Bay for Colonel Fannin's command and continuing the patrolling the Texas coast to engage Mexican ships. On March 6th, the day the Alamo fell, the Invinciblewas in Velasco, having just returned from New Orleans where her weaponry was augmented with two nine-pounders and an additional eighteen-pounder. Within days after the fall of the Alamo, with Mexican eagles and serpents marching in three separate armies across Texas, Captain Jeremiah Brown finally found the Montezuma blockading her own port at the mouth of Rio Grande to keep news of the impeding Mexican invasion from breaking out. TheMontezuma was readying for a 2000 man division invasion of Texas to reinforce Santa Anna's troops when the Invincible engaged her and ran her aground, thwarting the invasion. 

The Invincible then captured the Pockett and took her war supplies to Sam Houston, just as her sister ship, the Liberty, had done a few days before when the Liberty captured the Pelicano.  The Pelicano was laden with barrels of gunpowder hidden in larger barrels of flour. Those supplies, all intended for Santa Anna, and the Twin Sisters delivered by the privateers, bolstered the morale of the army of volunteers so much that they forced Sam Houston to engage the enemy at San Jacinto. Sam Houston had other plans for his army (retreating to the Louisiana border) and but for the victories of the Navy at sea and the assistance of the privateers, the battle probably would not have been fought, and if fought, lost. 

For a time after the battle, Santa Anna was kept prisoner aboard the Invincible for his own safety, and the Texans enjoyed a brief peace. But the war didn't cease (both sides renounced the treaty) and within two months the Mexican Navy sailed into Capano Bay with three ships full of supplies for the Mexican armies that weren't at San Jacinto. All three ships were captured by the Texas Mounted Riflemen, who became known as the Texas Horse Marines. The Texans had bought themselves still more time. 

By the summer of 1836, the plight of the Texans worsened, as more Mexican warships resumed the blockade of Texas ports. The Texas Navy and the privateers continued to battle the Mexican Navy, but they were battling a far superior force. In April 1837 the Independence gallantly fought two larger, more powerful Mexican Navy ships.  She was eventually captured in full sight of many Texans, including the Secretary of Navy, S. Rhodes Fisher, who watched the battle from shore. The Liberty had earlier been captured by New Orleans creditors, leaving the Texas Navy with only two remaining ships, the Invincible and the Brutus. 

Fearing invasion by sea, Sam Houston (now president of the Republic) ordered his Navy to stay in Galveston to protect the city. Navy Secretary S. Rhodes Fisher and his two new captains, Henry L. Thompson (Invincible) and James D. Doylan (Brutus), knew that Houston's order amounted to a strategic disaster, so they decided to defy Houston's orders and take to sea to engage and divert the enemy from Texas waters. The entire fleet, two ships, then began a daring 77 day raid of Mexican port towns and villages. They captured dozens of parogues, at least six Mexican merchant ships, and generally raised enormous havoc and grief in Mexico. Eluding and diverting the larger Mexican Navy, the Invincible and Brutus even went to Isle Megeres and Cozumel for supplies, and R&R, and claimed them for the Republic of Texas. The offensive successfully diverted the Mexican Navy for two and half months by forcing the superior Mexican navy to stay at home to protect their own ports and shipping. 

The Texas Navy returned to Galveston triumphantly on August 26, 1837 with several prize ships. The Brutus towed a prize ship crossing the bar into Galveston Bay, while the larger, heavily laden (with booty) Invincible anchored outside the bay, only to see two Mexican brigs, the Lubardo and the Vencedor del Alamo chasing a Texan supply ship headed for Galveston. That ship, the Sam Houston, successfully made it into the Bay while the Invincible, set her sails, exchanged signals with the distant Brutus in the Navy yard, hoisted her colors and stood out to engage the two larger Mexican warships. In her haste to join the battle, the Brutus slipped her rudder and ran onto the shoal leaving the Invincible to carry on the daylong battle alone. Captain Thompson's own account of the battle says he inflicted great damage on the enemy and fired his guns until they were too hot to fire anymore and then tried to lure the enemy onto the bar only to slip his rudder as he crossed the bar causing the Invincible to run into the shoals. Both darkness and a storm were approaching and the two damaged Mexican brigs set sails for home. The ensuing storm broke up the Invincible and over the next 48 hours she sunk below the water and ultimately below the sand where her nine cannon (possibly more) and a storehouse of other historical artifacts lie today. 

The National Underwater and Marine Association (NUMA) began searching for the Invincible in 1985. That 20 year search has eliminated a large area and resulted in redefining the search area or high probability zone. In 2004 the Texas Navy Association joined NUMA in a joint venture to locate the Invincible. Last year the joint venture completed a Marine Magnetics (Canadian company) magnetometer survey of the new high-probability area which resulted in the discovery of several promising targets. These targets will be surveyed by Innovatum Inc., a Houston-based corporation, which specializes in locating pipelines for oil companies. When completed, the Innovatum survey will enable us to know which of the targets is likely to be the Invincible. Test excavations of the promising target(s) will be conducted using Cochran Undersea Technology DDRs (Dive Data Recorders) and EMC-20H dive computers.    Dive profiles will be recorded using Cochran Analyst software, and posted here as they occur.

Recovery of  artifacts from the flagship of the first Navy of the Republic of Texas is hopefully near at hand. Curation of the Invincible's artifacts will ultimately be at the Texas Navy display at the Texas Sea Port Museum. 

Author Wayne Gronquist is an Admiral in the Texas Navy, a member of the Texas Navy Association Board of Directors, and NUMA Texas Projects Director.

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