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spring 2012
By Jeffrey Bozanic, PH.D.
“Rebreathers are the greatest thing since sliced bread!”
“Rebreathers are boxes of death!”
It’s safe to say rebreathers often elicit strong emotions
in divers, but what is the true story? Among the anecdotes,
opinions and conjecture, what are the real benefits and
hazards of diving on rebreathers?
Function
There are many different types of rebreathers, but they
all perform the same basic functions: They retain a diver’s
exhaled breath, remove metabolic carbon dioxide (CO
2
),
provide safe oxygen levels and maintain an adequate
breathing volume. Different designs do these things in
different ways, but mixed-gas closed-circuit rebreathers
(CCRs) are the most common type used recreationally today.
When you exhale while diving a rebreather, instead of
venting gas as bubbles, your exhaled breath fills a breathing
bag, or counterlung. This gas then flows through a chemical
absorbent that removes CO
2
. Two cylinders supply oxygen
and a diluent gas such as air, mixing them to provide an
optimal nitrox mixture for your depth. Sensors measure the
partial pressure of oxygen (PO
2
) so the diver or electronics
can prevent oxygen levels from getting too high or too low.
Mechanical or electrical malfunctions are rare, but a failure
in any one of the tasks the rebreather is supposed to do can
lead to a serious incident, or even death, while diving. Flooding
of a rebreather can lead to a caustic cocktail that can cause
chemical burns (see sidebar). Too much oxygen may result in
convulsions and unconsciousness, and too little will result in
unconsciousness from hypoxia. Either usually results in the
diver’s death by drowning. Carrying an open-circuit “bailout
bottle” is standard practice in rebreather diving.
Hazards
About 200 rebreather divers have died since 1960, most of
them in the last 20 years. Approximately 100 open-circuit
scuba divers die in the United States each year, but it is
difficult to compare the two numbers as there are a great
many more open-circuit dives done than closed-circuit.
Some of the rebreather divers who died were exceptionally
experienced. Among them were people who had been diving
for 10, 20 or even more than 30 years. They had been to very
deep depths, explored the far recesses of complex cave systems
and visited little-known wrecks in remote locations, all without
incident using open-circuit scuba. What happened?
We have limited data on rebreather fatalities, but several
trends are apparent. One is that rebreather divers with little
experience tend to have more difficulties than those with
many dives. This is similar to data seen in open-circuit scuba:
New divers are more prone to trouble than experienced
divers. An experience level of 20 to 25 dives seems to be a
turning point at which the incident rates begin to decline.
Open-circuit dive gear is very robust; rebreathers are less so.
Traditional scuba equipment has been evolving, improving and
becoming more reliable through extensive use and development
over the past 50 years. Rebreathers are also progressing down
this developmental road, but at a slower pace.
It takes more time to prepare a rebreather for diving than
it does open-circuit equipment. It also takes more time to
clean a rebreather after a dive. Some people, by their innate
temperament, are simply unable to consistently exercise
the discipline necessary to properly maintain a rebreather.
These are the same people who do not rinse their gear after
diving, continue to dive their equipment despite inadequate
maintenance or poor function and basically fail to exercise
due care for their equipment. This approach is never a good
idea, but it’s generally possible to get away with this attitude
in open-circuit diving more consistently than it is while using
a rebreather.
Two major factors contribute to the deaths of very
experienced divers using rebreathers. The first is that the
type of diving they do (caves, wreck penetration, exceeding
recreational depth limits) involves more risk. Because of
the increased risk we expect to see higher incident rates
regardless of the type of equipment used. The second issue
is that because of their vast experience with open-circuit
gear, these divers may quickly feel at ease on a rebreather.
They are tempted to pursue the same types of diving using
their rebreather they did while using a scuba cylinder. The
problem is they have not yet built the muscle memory and
patterning they need before extending their diving into
more advanced environments. Events that would be simple
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