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| Whether
you enjoy stalking fish for the ultimate photo
or exploring the haunted gangways of a ship
wreck, the silent Dolphin is ready to take you
there. |
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Rebreather diving is a unique experience. The
onboard counter lungs offset annoying buoyancy
changes. Warm moist gas is breathed by the
diver, so 'cotton mouth' is a thing of the past.
Thermal balance is maintained loger too. The
most noticeable feature is the absence of
noisy exhaust bubbles. |
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To understand what a rebreather is and how it
works,
it is useful to understand how conventional scuba
works
Nearly all diving
apparatus presently available to the public falls into
a class known as open-circuit scuba. This type of
system was first introduced to recreational divers by
Cousteau and employs a compressed gas supply and a
demand regulator from which the diver breathes. The
exhaust gas is discarded in the form of bubbles with
each breath, hence the term "open-circuit".
Open-circuit scuba is inherently inefficient: because
only a small fraction of each inhaled breath is
actually used by the diver for metabolism, there is a
tremendous waste of useable oxygen (O2) with each
breath. Furthermore, the quantity of O2 lost in this
manner increases with increasing depth.
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A
rebreather is a fundamentally different kind of diving
apparatus. Its design start with a breathing loop
equipped with a mouthpiece, through which a diver
breathes. If the entire breathing loop is of rigid
construction, the diver would be unable to breathe
because there would be nowhere for the exhaled gas to
go into, nor the inhaled gas to come from (analogous
to trying to breathe in and out of a soda bottle).
Thus, there must be some sort of collapsible bag
attached to the breathing loop that inflates when a
diver exhales, and deflates when a diver inhales. This
bag is referred to as, appropriately enough, a
counterlung. |
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If a diver was to continue breathing in
and out from this breathing loop, the carbon dioxide
(CO2) exhaled by the diver would soon build up to
dangerous levels. Therefore, the breathing loop must
also include a CO2 absorbent canister containing some
sort of chemical (e.g., HP Sodasorb, Sofnolime®, or
lithium hydroxide) that absorbs CO2, removing it from
the breathing gas.
Of course, the CO2 absorbent
canister alone will not permit the diver to continue
breathing from the rebreather indefinitely; the oxygen
in the breathing loop will eventually be consumed by
diver via metabolism. Therefore, the rebreather must
have some means to allow oxygen to be injected into
the breathing loop in order to continue sustaining the
diver. Furthermore, to prevent the diver from simply
inhaling the same gas that was just exhaled, the
rebreather must be designed to ensure that gas
continues to circulate in one direction around the
breathing loop. This is usually accomplished with an
upstream check-valve, and a downstream check-valve,
located on either side of the mouthpiece; these allow
inhaled gas to come from only one direction in the
breathing loop, and allow exhaled gas to go only in
the opposite direction. Another feature common to most
rebreather designs is some sort of shut-off valve in
the mouthpiece which can be shut if the mouthpiece is
removed underwater, to prevent water from flooding the
breathing loop. |
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Semi-closed
rebreathers are a form of mixed-gas rebreather, in
that they incorporate gas mixtures other than pure
oxygen. There are two fundamentally different
categories of semi-closed rebreathers: active-addition,
and passive-addition. By far, the most common are the
active-addition systems. The supply gas is usually
injected into the breathing loop at a constant-mass
rate. In other words, regardless of the depth, a
constant number of molecules of gas are injected into
the loop in a given period of time. |
The rate of
injection in such systems must be adjusted according
to the fraction of oxygen in the supply gas, such that
the rate of oxygen addition to the breathing loop
meets or exceeds the rate at which the diver consumes
oxygen in the breathing loop. |
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What
are the advantages of rebreathers?
Rebreathers provide two
fundamental advantages over open-circuit scuba systems:
More efficient use of gas and near-silent
operation.
- Gas Efficiency:
Perhaps the most significant advantage that
rebreathers offer is greatly increased gas efficiency.
Under normal circumstances, a diver only uses a small
fraction of the oxygen of each inhaled breath; most of
the oxygen leaves the lungs unused when the diver
exhales. When using open-circuit scuba, the oxygen and
other gases in the exhaled gas are wasted in the form
of bubbles. As the depth of the dive increases, this
inefficiency of open-circuit systems is compounded:
because of the increased pressure at greater depths,
more gas molecules are lost with each exhaled breath.
A rebreather, on the other hand, retains most or all
of the exhaled breath, processes it, and returns it to
the diver. The deeper the dive, the more advantageous
(from a gas efficiency perspective) rebreathers
become.
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 10 meters underwater, 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 rebreather, the diver could theoretically
stay underwater for two days -- regardless of the
depth!
- Silence:
With each exhaled breath, a diver using conventional
scuba gear releases a
large burst of noisy bubbles. The effect of this on
the behavior of marine-life varies, but in most cases,
fishes behave warily and are reluctant to allow a
diver to approach closely. Semi-closed rebreathers
reduce the volume of exhaled bubbles. With rebreathers,
divers are able to approach marine life much more
closely while disturbing behavioral patterns much less
severely. This is especially important for
photographic activities.
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What are the disadvantages of
rebreathers?
All kinds of rebreathers have certain specific
complexities which introduce forms of risk not
experienced by scuba divers. The fundamental
difference between open-circuit scuba and rebreather
systems is that on scuba, if a diver can breathe and
is not outside well-established depth limits, the
breathing gas is going to be life-sustaining (assuming
the cylinder was filled properly). If there is a
problem with an open-circuit system, the problem is
usually very self-evident to the diver, so the diver
at least is aware of the problem and can takes steps
toward a solution.
With rebreathers, however, the breathing gas may be
dynamic, and thus the oxygen concentration may drift
out of life-sustaining range within the course of a
single dive. The oxygen concentration in the breathing
loop depends on diver workload. Under certain
circumstances, especially during high exertion
and/or
during an ascent, the oxygen concentration in a semi-closed
rebreather could drop to dangerously low levels.
These problems can be largely avoided if the gas
supply rate of rebreathers is adjusted carefully and
the breathing loop is flushed with fresh gas prior to
an ascent. Unfortunately, symptoms associated with
hypoxia and oxygen toxicity cannot be regarded as
reliable precursors to black-out. Therefore it is
ultimately up to the diver to take steps to ensure a
continuous life-sustaining gas mixture in the
breathing loop at all times. This level of discipline
requires a great deal of discipline and training. Thus
rebreather divers must have a higher dedication to
equipment maintenance and operation than is generally
required for open-circuit divers. Furthermore,
rebreathers are generally more complex devices than
open-circuit scuba gear, which also accounts for why
they require more training time. |
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| Courses |
| Dolphin
Rebreather Try-out |
1
Day |
US$
14 |
| Discover
Dolphin Rebreather |
1
Day |
US$
140 |
| Dolphin
Rebreather Diver Course |
4
Days |
US$
560 |
| Dolphin
Rebreather Instructor Course |
4
Days |
US$
520 |
| All
Courses include: Books, T-shirt, Registration
fee, Certification, Tank fills. |
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| Specials |
| Dolphin Rebreather
Liveaboard Voyage |
2
Days/
4 Nights |
US$
325 |
| Dolphin Rebreather
Liveaboard Voyage |
4
Days/
4 Nights |
US$
550 |
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The
Dräger Dolphin is a semi-closed
rebreather.
Your
exhaled breath is returned through
the system, purified, enriched with
a nitrox mixture and returned to you
pleasantly warm and moist. The
Dolphin uses a soda lime cartrige to
cleanse your exhaled breath of
carbon-dioxide, which then flows
into an inhalation bag to be
enriched with Nitrox.
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Since your
exhaled breath is not lost to the
great blue depths, your gas
consumption drops by up to 90%,
allowing you to dive all weekend on
just one 4 or 5 liter cylinder!
Not only is the Dräger Dolphin
quieter, but through the use of
Nitrox, it will allow you to stay
down longer. An average diver
usually consumes the contents of an
12 l. tank in a 20 m. dive for 55
minutes. Advanced divers who have
training in Nitrox gasses can expand
that 55 minute limit through the use
of Nitrox, but they are still
limited by the 12 l. tank unless
they use a very heavy pair of tanks.
With the Dolphin Rebreather, you
break those boundaries. You get the
benefits of Nitrox - extended bottom
times with the time to use it and a
quiet, peaceful dive (without a
hundred pounds of tanks strapped to
your back!)
The Dolphin is a semiclosed
rebreather for use with nitrox. It
uses a constant mass flow dosing
system which includes an additional
demand bypass. Depending on which
nitrox mixture you are using there
are different orifices to set the
rate of flow. The loop has a total
volume of approximately 10.5 litres
and depending on your individual
lung volume and the adjustment of
the pressure relief/dump valve the
volume can be varied by 4.5 to 7.1
litres. The scrubber holds
approx.
2.25 kg of absorbent. It comes with
either a 4 or 5 litre 200 bar steel
cylinder and a separate 2 litre, 200 bar
open circuit bailout cylinder. It is
520 mm long, 370 mm wide and 2 5m m
deep. On land it weighs
approximately 17
kg and in water the weight is about
1 kg negative. |
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