Frank K. Butler, M.D.
Director of Biomedical
Naval Special Warfare
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 Cousteaus new Aqualung in 1948
was Commander Francis Fane, a member of the Navy Underwater Demolition
Teams, the forerunner of todays 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 Navys 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 manufacturers 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 ships 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 ships husbandry divers for many years. Because of the
negative impact that the new tables would have on the ships husbandry
divers, implementation of the new Navy air tables was suspended indefinitely.
a result of this decision, attention was then re-directed by the SEAL
community to CAPT Thalmanns 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.
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
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 DCs 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
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 Navys
first decompression computer dive was conducted by Bravo Platoon of SDV
Team One on 31 January 2001 in the waters off of Barbers Point in
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 Thalmanns
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 divers 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 countrys 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.