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Operational Considerations for Recirculating SCUBA
When and where should you use a rebreather?
J. Kellon
A. DEBUNKING THE MISCONCEPTIONS
1. Some people have been touting one of the constant flow semi-closed
(active addition) units as a way to obtain longer
no-decompression dives. This claim is, in a word, hogwash. The inspired
oxygen fractions from the counterlung in a semi-closed rebreather will
always be lower than the oxygen fraction in the supply gas. This means that
no semi-closed rebreather will allow as long a no-decompression dive as
open circuit equipment using the same supply gas.
2. All the laws and restrictions that were learned in your nitrox or
trimix course concerning oxygen toxicity limits and equivalent air depth
apply to rebreathers, with the added problem that it is often difficult to
determine what fraction or partial pressure of oxygen you are actually
breathing. Physical laws and physiological restrictions do not suspend
themselves for rebreathers. There is no such thing as free lunch.
3. Some manufacturers are touting nonredundant (three oxygen sensors
do not qualify as redundancy because they are part and parcel of a single
averaging circuit) electronically controlled rebreathers as being safer than
nonelectronic units. Again, hogwash. I'm sure that each and every person
reading this has had a radio, computer, electric clock and or an
electronically controlled appliance fail on him. When it happens in a dry
environment, it's merely an inconvenience. When it happens underwater, it
can very easily become life threatening.
Any electronic or electrical failure, including sensor or circuit
readouts, is more likely to occur underwater because of the number
of bulkhead penetrations required to make the transition from a wet
environment to the dry one required for the electronics and/or displays
(see attached drawing 50-004). A good example of what happens when this is
necessary is an underwater camera case. Anyone who has used one for any
period of time knows that the case will sooner or later leak through a
strobe or flash wire penetrator, a control shaft penetrator or past the case
lid seal. Electronic control circuitry and displays don't work well when
wet, and the requirement for heat dissipation prevents potting many of the
components. Batteries don't
do well when wet either. Someone on the Internet recently suggested that
the use of electronics would make mechanically controlled units even safer.
I would love to see a real life example of this. In my opinion, the only
time that electronics in a rebreather are justfied is when the high
efficiency of oxygen usage attained by these units is absolutely required by
the intended mission, and then ONLY if full electronic redundancy is
provided, including dual sensor arrays (three each), dual averaging and
control circuitry, dual batteries and dual displays, all in separate
housings.
4. Some manufacturers and users are saying that electrical displays
and manual gas bypasses allow the diver to safely "fly" rebreathers when the
electronics fail. There have been too many cases of divers ignoring their
displays during task overloads until they have passed out for this to be a
reasonable assumption. In addition, a diver should NOT be operating manual
bypasses when the breathing loop may contain a mixture that affects his
ability to reason. If a diver has ANY reason to suspect that all is not well
in the breathing loop (including a "gut feeling"), he should immediately
switch to an open circuit bailout. If he can then rationally decide that he
can safely
continue on the breathing loop, then so be it.
B. INTENDED USE
1. General:
Rebreathers will never be as safe to use as open circuit SCUBA,
particularly in the sport diving community where operational support
normally present for the military and scientific user is virtually
nonexistent. Very often the task overload created by a single person
attempting to perform maintenance, calibration, dive planning, mission
performance and life support system monitoring during the dive creates
opportunities for disastrous results. In addition, the user must contend
with equipment complexities that increase the possibility of unit failure
and with conditions in the breathing loop that can cause rampant bacterial
activity, hypoxia, hypercapnia and/or hyperoxia. The
rebreather used for any given dive should be as simple as possible while
still meeting safety and dive objective requirements. No single type or
brand of rebreather demonstrates superiority for all types of diving. More
difficult mission requirements require more complex units, but complexity
comes at the price of higher costs, higher training requirements and more
components that are subject to failure. Only after a risk/benefit analysis
has been made in favor of the rebreather should it be used. Benefits include
a genuine operational need for exhaust bubble reduction, extended duration
requirements that cannot be met rationally with open circuit equipment
because of the amount of supply gas required on the diver, or the inability
to pack cylinders or compressors into a remote location. Being the niftiest
diver in your group because you're using a rebreather doesn't qualify as a
rational benefit.
2. Depth:
a. Fixed flow (active addition) semi-closed units.
Unless there is a genuine requirement for extreme silence or
low size and weight, these units usually fall short in the risk/benefit
analysis.
The units currently available (January, 1996) do not offer
duration benefits over existing open circuit SCUBA, and are further tainted
by reduced no-decompression limits and the inability to calculate oxygen or
nitrogen loads with any degree of accuracy for table use. This also makes
repetitive dives difficult to calculate unless one assumes a worst-case
scenario. Because the flow is set prior to the dive based on guestimates
and does not change during the dive regardless of the diver's activity
level, the actual partial pressure of oxygen in the inspired gas can vary
wildly. There are only two things that can be absolutely counted on with
these units for purposes of OTU and decompression calculations:
1. Oxygen partial pressures will never exceed those of the
supply gas times the depth in ATA's.
(EAN 36 X 99 FSW/4ATA = 1.44)
2. Assuming the diver didn't go hypoxic, oxygen partial
pressures were always above .16 ATA.
These two factors constitute the worst-case scenario. Should
a prudent diver assume otherwise when calculating OTU's and
decompression schedules?
In my opinion, the most responsible approach that I
have seen to date for this type of unit appears to be Grand Bleu with the
"Fieno". This is a limited depth and duration unit that is very light and
compact, has an innovative mouthpiece that shuts off automatically if
dropped from the diver's mouth and has a prepacked 45 minute scrubber insert
that is very easy to load. In addition, the limited duration supply gas
cylinder must be exchanged for another precharged and tested cylinder, thus
reducing the possibility of incorrect mixtures. They have attempted to
reduce the possibility of hypoxia inherent to this type of rig through training.
Units of this type should be restricted to 5 ATA or less.
b. RMV regulated (passive addition) semi-closed units.
The only unit of this type currently available to the
civilian market is the RBC "Odyssey". This type of rebreather is less likely
to cause hypoxia than the active addition units because, should the
addition fail for any reason, the diver is immediately warned by shorter
and shorter inhalations. In fact, ANY failure on this unit will result in
an immediate and recognizable change in breathing characteristics. These
changes are designed to be the warning system, and do not require the diver
to monitor anything.
The unit is designed to be used to a depth of 13 ATA, and is
more efficient relative to SCUBA the deeper is goes. Beyond this depth, the
proportioning device on the compound counterlung loses its linearity,
although on the safe side.
Because the counterlung is located within the back mounted
case, a 16 pound weight is used to minimize hydrostatic differentials in the
breathing loop caused by diver attitude changes. The weight artificially
shifts the counterlung centroid to more closely correspond to the lung
centroid. This unit is larger, heavier and more costly to purchase than most
active addition systems, but is more suitable for deeper diving ranges.
c. Electronically controlled (active addition) closed-circuit units.
I think I have already said enough on this subject to cover
most of the territory here. The diver should be aware that, although these
units aren't constantly venting, they can still easily be breathed down to
a hypoxic condition if the oxygen addition fails for any reason. They
present the added hazard of being able to cause oxygen toxicity as well.
These units cost more to purchase and far more to operate than other types
of rebreathers. I have personal knowledge of this type of unit being used to
depths greater than 17 ATA.
It looks like Bill Stone, Richard Nordstrom, Kevin Gurr and
cohorts are the pace setters here. They appear to have the best handle on
the redundancy issue and are reported to be working on units for depths
greater than 17 ATA. Use an adequate gas supply on your open circuit
bailout, guys.
d. Pure oxygen (passive addition) closed circuit units.
Although not normally used in the civilian community, these
units are quite safe when proper counterlung purging procedures and maximum
depth limitations are observed. There are very inexpensive surplus military
units available. I would advise against buying them because of what father
time does to elastomeric components.
Dan Volker
SOUTH FLORIDA DIVE JOURNAL
"The Internet magazine for Underwater Photography and mpeg Video"
http://www.florida.net/scuba/dive
407-683-3592
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