Will your next dive be 3 hours long?
With PADI endorsing the launch of recreational
rebreathers with the introduction of their own rebreather courses,
there may be a revolution coming.
How do they work?
At shallow depths, a diver using open-circuit
breathing apparatus typically only uses about a quarter of the
oxygen in the air that is breathed in, which is about 4 to 5% of the
The remaining oxygen is exhaled along with nitrogen
and carbon dioxide - about 95% of the volume. As the diver goes
deeper, much the same mass of oxygen is used, which represents an
increasingly smaller fraction of the inhaled gas.
Since only a small part of the oxygen, and virtually
none of the inert gas is consumed, every exhaled breath from an
open-circuit scuba set represents at least 95% wasted potentially
useful gas volume, which has to be replaced from the breathing gas
A rebreather recirculates the exhaled gas for re-use
and does not discharge it immediately to the surroundings. The inert
gas and unused oxygen is kept for reuse, and the rebreather adds gas
to replace the oxygen that was consumed and removes the carbon
Thus, the gas in the rebreather’s circuit remains
breathable and supports life and the diver needs only carry a
fraction of the gas that would be needed for an open-circuit system.
The saving is proportional to the ambient pressure, so is greater
for deeper dives, and is particularly significant when expensive
mixtures containing helium are used as the inert gas diluent. The
rebreather also adds gas to compensate for compression when depth
increases, and vents gas to prevent overexpansion when depth
Around 1620: In England, Cornelius Drebbel made
an early oar-powered submarine. To re-oxygenate the air inside
it, he likely generated oxygen by heating saltpetre (potassium
nitrate) in a metal pan to emit oxygen. Heating turns the
saltpetre into potassium oxide or hydroxide, which absorbs
carbon dioxide from the air. That may explain why Drebbel’s men
were not affected by carbon dioxide build-up as much as would be
expected. If so, he accidentally made a crude rebreather more
than two centuries before Saint Simon Sicard’s patent. (!)
1808: The oldest known rebreather based on
carbon dioxide absorption was patented in France by Sieur
Touboulic from Brest, a mechanic in Napoleon’s Imperial Navy.
This early rebreather design worked with an oxygen reservoir,
the oxygen being delivered progressively by the diver and
circulating in a closed circuit through a sponge soaked in
Touboulic called his invention Ichtioandre (Greek for
‘fish-man’). There is no evidence of a prototype having been
1849: A patent for the oldest known rebreather
for which a prototype was built, also using an oxygen reservoir,
was granted to Pierre Aimable De Saint Simon Sicard.
1853: Professor T. Schwann designed a rebreather
in Belgium; he exhibited it in Paris in 1878. It had a large
back mounted oxygen tank with working pressure of about 13.3
bar, and two scrubbers containing sponges soaked in caustic
1878: Henry Fleuss invented a rebreather using
stored oxygen and absorption of carbon dioxide by rope yarn
soaked in caustic potash solution, to rescue mine workers who
were trapped by water.
About 1900: The Davis Submerged Escape Apparatus
was designed in Britain for escape from sunken submarines. It
was the first rebreather which was practical for use and
produced in quantity. Various industrial oxygen rebreathers such
as the Siebe Gorman Salvus and the Siebe Gorman Proto, both
invented in the early 1900s, were derived from it.
1903 to 1907: Professor Georges Jaubert invented
Oxylithe, which is a form of sodium peroxide (Na2O2) or sodium
dioxide (NaO2). As it absorbs carbon dioxide in a rebreather’s
scubber it emits oxygen.
1907: Oxylithe was used in the first filming of
Twenty Thousand Leagues Under the Sea.
1907: Dräger rebreather used for mines rescue.
1909: Captain S.S. Hall, R.N., and Dr. O. Rees,
R.N., developed a submarine escape apparatus using Oxylithe; the
Royal Navy accepted it. It was used for shallow water diving but
never in a submarine escape;
1912: The first recorded mass production of
rebreathers started with the Dräger rebreathers, invented some
years earlier by Hermann Stelzner, an engineer at the Dräger
company. The Dräger rebreathers, especially the DM20 and DM40
model series, were those used by the German helmet divers and
German frogmen during World War II.
1930’s: Italian sport spearfishers used
rebreathers systematically. This practice came to the attention
of the Italian Navy, which developed its frogman unit Decima
Flottiglia MAS, which was used effectively in World War II.
War II: Captured Italian frogmen’s rebreathers influenced design
of British rebreathers. Many British frogmen’s breathing sets’
used aircrew breathing oxygen cylinders salvaged from shot-down
German Luftwaffe aircraft. The earliest of these breathing sets
may have been modified Davis Submerged Escape Apparatus; their
full-face masks were the type intended for the Siebe Gorman
Salvus, but in later operations different designs were used,
leading to a full-face mask with one big face window, at first
oval and later rectangular (mostly flat, but the sides curved
back to allow better vision sideways).
Early British frogman’s rebreathers had rectangular counterlungs
on the chest like Italian frogman’s rebreathers, but later
British frogman’s rebreathers had a square recess in the top of
the counterlung so it could extend further up toward the
shoulders. In front they had a rubber collar that was clamped
around the absorbent canister. Some British armed forces
divers used bulky thick diving suits called Sladen suits; one
version of it had a flip-up single faceplate for both eyes to
let the user get binoculars to his eyes when on the surface.
Early 1940s: US Navy rebreathers were developed
by Dr. Christian J. Lambertsen for underwater warfare and he is
considered by the US Navy as "the father of the
frogmen". Lambertsen held the first closed-circuit
oxygen rebreather course in the United States for the Office of
Strategic Services maritime unit at the Naval Academy on 17 May
c.1960 to c.1990: In this period in Britain
there was very little rebreather use by civilians, and no easy
way for the general public to obtain rebreathers, and the BSAC
prohibited rebreather use by its members. The Italian firms
Pirelli and Cressi-Sub at first each sold a model of sport
diving rebreather, but after a while discontinued those models.
Some home made rebreathers were used by cave divers to penetrate
1989: The Communist Bloc collapsed and the Cold
War ended, and with it the perceived risk of attack by Communist
Bloc forces, including by their combat divers. After that, the
world’s armed forces had less reason to requisition civilian
rebreather patents, and automatic and semi-automatic
recreational diving rebreathers started to appear.
Long dive times
Perhaps the most significant advantage is greatly
increased gas efficiency. Usually, a diver only uses a small
fraction of the oxygen of each inhaled breath and most of the oxygen
leaves the lungs unused when the diver exhales. This means that
oxygen and other gases in the exhaled gas is wasted when the diver
exhales - and as you know, this gets worse with greater depths
because of the increased pressure.
Rebreathers retain most or all of the exhaled
breath, processes it, and returns it to the diver as breathable air.
Because there are almost no exhaled bubbles at all, there is no
change in gas usage efficiency at greater depths. This makes
rebreathers more beneficial as the depth increases.
For example, a standard scuba cylinder contains
enough gas to sustain an average resting person for about an hour
and a half at the surface. The same cylinder will last only 45
minutes at 10 meters and less than 10 minutes at a depth of 90
meters - but if that same cylinder were filled with oxygen and used
to supply a closed-circuit rebreather, the diver could theoretically
stay underwater for two days - whatever the depth!
This advantage only applies to closed-circuit
rebreathers, not oxygen or semi-closed rebreathers. Oxygen
rebreathers are limited to depths where decompression is not an
The reason it applies only to closed-circuit
rebreathers and not semi-closed rebreathers has to do with
differences in the breathing gas dynamics of these two types of
rebreathers. Semi-closed rebreathers maintain a more-or-less
constant fraction of oxygen in the breathing gas, whereas
closed-circuit rebreathers maintain a constant partial pressure of
oxygen in the breathing gas.
his means that the non-oxygen portion of the
breathing gas which the part that determines decompression
obligations is kept at a minimum. This allows the diver to stay
longer at depth without requiring decompression and also to speed up
the decompression process whenever there is a decomression
Breathe warm, moist gas
The breathed gas is warm rather than cold, as in
open circuit because of the pressure reduction of substantial
amounts of gas just prior to inhalation. So, less gas used in
rebreathers means less cool gas is introduced.
Our lungs contain a dense vascular system and much
heat can be lost through exhalation. The rebreather allows the
retention of body heat because temperature is maintained, in varying
degrees, within the loop. In addition, rebreather gases are also
warmed by the chemical reaction during the carbon dioxide scrubbing
The Sound of Silence
Conventional scuba releases a large burst of noisy
bubbles at each breath. This affects the behavior of marine life and
fish are often frightened and will not allow close proximity.
Semi-closed rebreathers reduce the volume of exhaled
bubbles, and closed-circuit rebreathers essentially eliminate
bubbles entirely. This allows divers are able to approach marine
life much more closely without disturbing behavioral patterns -
great news for specimen collection and marine photographers.
Compared with open circuit scuba, rebreathers have
some disadvantages, including expense, complexity of operation and
maintenance, and more critical paths to failure.
A malfunctioning rebreather can supply a gas
mixture which contains too little oxygen to sustain life, or it
may allow carbon dioxide to build up to dangerous levels. Some
rebreather designers try to solve these problems by monitoring
the system with electronics, sensors and alarm systems. These
are expensive and susceptible to failure, improper configuration
Oxygen rebreathers (simple closed circuit) are
limited to a shallow depth range of approximately 6 m, beyond
which the risk of acute oxygen toxicity rises to unacceptable
levels very rapidly.
Semi-closed circuit rebreathers are less
efficient than closed circuit, and are more mechanically complex
than open circuit or closed dircuit oxygen rebreathers.
Closed circuit rebreathers are yet more
mechanically complex, and generally rely on electronic
instruments and control systems to monitor and maintain a safe
breathing gas mixture. This makes them more expensive to
produce, more complex to maintain and test, and sensitive to
getting their circuitry wet.
Depending on the complexity of the rebreather,
there are more failure modes than for open circuit scuba, and
several of these failure modes are not easily recognised by the
diver without technological intervention.
A major disadvantage of a rebreather is that, due to
a failure, gas may continue to be available for breathing, but the
mixture provided may not support life, and this may not be
apparent to the user.
With open circuit, this type of failure can only
occur if the diver selects an unsuitable gas, and the most common
type of open circuit failure, the lack of gas supply, is immediately
obvious, and corrective steps like changing to an alternative supply
would be taken immediately.
The bailout requirement of rebreather diving can
sometimes also require a rebreather diver to carry almost as much
bulk of cylinders as an open-circuit diver so the diver can complete
the necessary decompression stops if the rebreather fails
completely.Some rebreather divers prefer not to carry enough bailout
for a safe ascent breathing open circuit, but instead rely on the
rebreather, believing that an irrecoverable rebreather failure is
very unlikely. This practice is known as alpinism or alpinist diving
and is generally maligned due to the perceived extremely high risk
of death if
4 Golden Rules
The last decade of statistics show that equipment
brand has not been a factor in accidents. Ergo, the most important
decision you can make is not which make of rebreather you will buy,
but rather the commitment to use your equipment safely.
Four factors will prevent 90 percent of the
Use a checklist every time you dive.
Do a proper, relaxed and complete pre-breathe
for five minutes with your nose blocked.
Don’t jump in the water unless all systems are
Replace your oxygen cells proactively every
Factors when buying a rebreather
Beyond that, several factors may influence a
3rd Party tests. Check for third-party tests
for oxygen tracking, canister duration and other factors., as
well as for details on their CE or equivalent testing.
Weight and size. Is it portable enough for
your needs - will it conform to airline baggage weight
Training. Check the availability of
instruction from a trusted, experienced and current instructor
Support. Make sure there is local support
and other divers using the same kit near you. Also check the of
support and service, turnaround times, availability of oxygen
cells and other parts. Check online for comments and complaints
Manufacturing company. Check the history and
stability of the manufacturing company.
Alarm systems. Make sure the unit provides
visual, and or audio and tactile alarm systems.
Other features . Check for features such as
"auto on" electronics, redundancy. battery type and sharing or
Cost. You should not buy a cheap parachute,
so buy the best rebreathing equipment that you can afford.
Self service. Check what you can reliably do
yourself regarding needs
Consumables. Check the availability and cost
of consumables like the scrubber and batteries.
Make a pledge to yourself and your family that you
will abide by the four Golden Rules.
Unlike normal gear, you just drop the mouthpiece
into the water - because water would fill the breathing loop and the
scrubber canister. You will have to close the valve on the
mouthpiece before you take it out of your mouth.
You can’t adjust buoyancy by inhaling or exhaling,
because the same amount of air just goes back and forth between your
lungs and the rebreather. and never changes volume or buoyancy.
As depth increases, the increasing pressure will
collapse the counterlungs just as it collapses BCs and dry suits.
Some rebreathers automatically add more gas to the breathing loop
while other models require you to add gas manually.
Using the gas
You will need to be frugal when inflating your BC or
clearing your mask. This is because gas used for those actions is
depleting a much smaller total supply. Also, you need to watch your
gauges closely and buddies need to check each other‘s kit for air
If you are exerting yourself - like pulling
something or swimming against a current - your body will take oxygen
out of the breathing loop faster than normal.
Closed-circuit systems and passive
semiclosed-circuit systems will sense this and add extra oxygen.
However, active semiclosed systems will not. This
means that you will have to remember to purge the breathing loop.
This is done by exhaling the oxygen-poor gas through your nose so
the rebreather can replace it with richer gas.
When commencing an ascent on a semi-closed-circuit
rebreather the loop must be purged to enrich your breathing mixture,
or the drop in pressure may cause the partial pressure of oxygen may
become too low. Closed- circuit systems add oxygen automatically.
You will also notice that during the ascent the
rebreather will vent gas as the counterlungs expand. This is the
only time the rebreather dumps a lot of gas, the reason being that
"sawtooth" profiles are especially wasteful.
You may find that you are becoming increasingly
buoyant during and ascent, caused by the rebreather not venting gas
fast enough. You may have to manually dump from your BC, your dry
suit or from the rebreather to maintaind acceptable buoyancy.
The bottom line
It is not only about the equipment - it is
especially about you. Don’t even consider a rebreather is
you are a fly-by-the-seat-of-your-pants person. You may die
without even realising it. With a rebreather, you will have
to take full responsibility for your own safety.
Rebreathers are more complex than
open-circuit gear - and you will have to be able to
assemble, clean, maintain and repair the kit. The chances of
your local African dive shop having a specialist are slim.
The simplest rebreather has all the parts of your
open-circuit setup plus a lot more - and all those parts,
the connections between them and the 50+ O-rings need
Like a pilot, you will need to be
disciplined and religiously follow procedures and
checklists. Filling scrubber canister and assembling the
breathing loop involve steps that must be followed precisely
- and tests that can’t be skipped. During the dive you have
to watch gauges more closely than on open circuit.
All said, the rebreather frontier offers great
advantages - and great dangers.
Be honest with yourself and make your decision with your head -
not with your heart.