I
have logged almost ever dive possible or could imagine
(cold water, shipwrecks, caves, coral). I have logged
many dives below recreational depth limits and dives
with different mixes of gas other then air. However,
when I started diving with a rebreather, I quickly realized
that I was hopping into the water with the beginners.People
would ask me to do this dive or that dive with them
but I knew that shallow beach diving should be my only
dives for a while.
Many of the skills and procedures we have become accustomed
to while using open circuit do not apply while using
a rebreather. Simple sensations such as hearing the
air move through the hoses to help me control my breathing
are absent while using a rebreather. There are no sounds
to listen to. The trick of fine-tuning buoyancy by inhaling
or exhaling on open circuit does not work with rebreathers.
With
a rebreather, inhaling and exhaling simply transfers
the gas from one compartment (the lungs) to a another
(the counter-lungs or breathing-bag). There is no change
in volume therefore there is no change in buoyancy.
I spent many hours bouncing off the sandy bottom before
I got accustomed to this difference.
Anybody
starting to dive with a rebreather, no matter whether
you have logged 3000 dives or 300, make sure that you
take it slow and gradually build up your abilities.
There have been to many accidents with rebreathers that
could have been avoided if the diver would have taken
more time to gather experience with the unit first.
You will learn that when diving with a rebreather you
have to become part of the unit itself. You must monitor
your body for different signs and symptoms more frequently
than on open circuit as your body becomes part of the
gas-loop. One area that is common to both systems is
a clear brain. It is one of the best pieces of equipment
you need to take with you, always. Remember, take the
attitude that you are starting over. Begin SLOWLY.
Rebreathers
have been around a lot longer then people would believe.
First attempts date as far back as 1680 by a gentleman
named Giovanni Borelli. In current use there are three
general types of rebreathers (the simple oxygen rebreather,
the semi-closed circuit rebreather and the fully-closed
circuit electronically controlled rebreather). Lets
look at the similarities and differences between the
three general systems:
ALL
REBREATHERS- Using a rebreather, the gas we breathe
is actually recycled from our last exhalation. Since
the diver is inhaling and exhaling into, and from, a
closed loop, most of the gas volume is retained instead
of lost. The divers exhalation gas, which has high levels
of carbon dioxide, is "scrubbed" out as it
passes through a chemical medium. At the same time,
oxygen (from some source) is replenished through an
oxygen injection system. The mechanism of this oxygen
injection system actually helps define the specific
type of rebreather.
OXYGEN REBREATHERS- Henry Fleuss developed the
first oxygen rebreather in 1878. Presently, oxygen rebreathers
are mostly used by the military, as they are limited
to very shallow depths. Oxygen is provided to the loop
through a constant bleed or by a mechanical regulator
that senses a volume drop in the system (due to the
oxygen metabolized by the body). For constant flow-rate
units, the set flow is determined by anticipated metabolic
consumption needs. Work and effort increase oxygen demands
therefore flow rates would need to be higher. Generally
speaking, flow rates are set anywhere between 0.5 l/min
(on a low workload dive) to 3.0 l/min.
SEMI
CLOSED REBREATHERS (SCR) - The semi-closed circuit
rebreather operates under a similar principal to that
of the oxygen rebreather with a constant flow. The difference
is that instead of utilizing pure oxygen as the supply
source, a pre-mixed nitrox is used. Since the oxygen
percentage is diluted, a higher flow rate must be maintained
to ensure enough oxygen is always present. There are
four basic components of an SCR.
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1.
The counter lung: Often called the ‘breathing bag’,
it is this that the diver inhales and exhales into via
the mouthpiece and hoses. The counter lung is mostly empty
after the diver inhales and mostly full after the diver
exhales, yet the amount of gas contained within the loop
remains fairly stable (the divers lungs are part of the
loop). SCR's have an over-pressure valve attached to the
counter lung which bleeds off excess gas being added by
the constant flow valve.
2. The scrubber canister: Here is where the CO2 absorption
medium is contained. It too, must be part of the closed-circuit
loop but the exhaled gas mostly passes through this medium
and space to remove CO2 from the loop. From the canister,
the gas passes into the counter lung.
3. Supply gas cylinder(s): The oxygen supply to revitalize
the loop gas with oxygen is stored here. While some SCR's
utilize two cylinders (one with pure O2 and the other
with air), most utilize a single cylinder with a nitrox
mixture instead. The actual fO2 that the diver chooses
will be determined by what depth they plan on diving to.
Even though it is guaranteed that the loop fO2 will not
be exactly the fO2 in the supply cylinder, one must assume
that it could be. Deeper depths require a lower oxygen
percentage and a correspondingly higher constant flow
rate.
4. Mass flow control orifice or valve: It is through this
mechanism that a constant gas flow from the supply cylinder
is regulated. It must provide a uniform flow rate at all
times to ensure appropriate levels of oxygen are available
in the loop. The actual flow rate is used in a mathematical
calculation to determine equivalent air depths.
CLOSED
CIRCUIT, ELECTRONICALY CONTROLLED SYSTEMS (CCR)- Most
of the major components in this system are shared with
the SCR with a couple of changes and additions. There
is still a counter lung (many CCR's utilize two), there
is still a CO2 canister and there are still supply cylinders
(note: at least two). The primary differences are that
instead of a constant bleed of gas through a mass flow
orifice, pure Oxygen is added (in small amounts) only
as you metabolize oxygen from the loop. To achieve this,
an oxygen sensor (or sensors) constantly monitor the PO2
in the loop. With this information, the controller will
activate a solenoid to manage an injection and maintain
the desired level. Since pure oxygen is supplied, a second
supply cylinder is required to provide a diluent. For
sport diving purposes, the diluent cylinder would contain
air. The fully closed, electronically controlled rebreather
will give the longest available underwater times because
they are the most efficient at utilizing supply gases
and they provide the lowest exposure to inert gases (i.e.
nitrogen).
DISADVANTAGES
of using open circuit.
Open Circuit
Poor Gas efficiency - when the diver exhales, all gas
is released into the water. Your body only utilizes approx
4-6% of the total volume lost. The deeper you go the greater
the volume loss with each exhalation.
Poor Weight to Duration Ratio -A large waste in volume
requires that more gas be carried. This means more and
larger tanks must be strapped to the diver causing more
physical exercise.
Limited Mix Optimization- Since the fO2 is fixed; it can
only be optimized at a single depth. This usually means
only at the deepest depth planned. While traveling ‘to
or from’ or, in circumstances where you elect to
stay shallower than planned, you are stuck with the mix
that is in your cylinder.
Maximum Noise & Environmental Disruption: Noise from
the release of bubbles as you exhale and from high-pressure
gas moving through the hoses scare away marine life. Some
environmental destruction also occurs in caves and shipwrecks
as this bubble volume collects on the roof literally exposing
marine life to air instead of water.
Dehydration- Gas from compressed cylinders is dried as
much as possible before filling. This means that the diver
is always breathing a gas that is super-dry. Physiologically,
this is a negative.
Heat Loss- Not only is the supply gas at equilibrium with
the ambient temperature of the water, it becomes super-cooled
as it passes through the first stage regulator due to
gas physics. The cooled gas robs the diver of heat energy
and it is physiologically undesirable.
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BENEFITS OF A REBREATHER
Increased Gas efficiency- Since gas used is re-circulated,
a vast improvement in efficiency is realized. Additionally
this efficiency is totally depth independent. CCR's have
the best gas efficiency.
Good Weight to Duration Ratio - Since the gas we use with
a rebreather is reused, the volume we are required to
carry becomes reduced. This translates into smaller, fewer
and much lighter weight tanks for support. CCR's have
the best ratio (i.e. a 3-liter cylinder, filled with Oxygen
can sustain the average diver-workload for 400+ minutes).
Little Noise- No more gas whistling through regulators.
No more (or very few) bubbles to annoy the fish. Simply
put, it is now quiet.
Little Heat Loss- The gas that the diver is breathing
is actually warmer than the ambient surroundings due to
the CO2 removal reaction. Heat is created in this process.
Reduced Dehydration- The same chemical reaction that makes
the gas warmer also adds moisture. This coupled with the
fact that the normally moist exhalation of a human being
is also trapped within the loop. Together, this provides
a comfortably moist inhalation gas.
What Are The Training Options & Where Can you Get
it?
American Nitrox Divers International (ANDI) has a full
line of rebreather courses. These courses can be available
to you at NEPTUNE DIVE CENTER in Pattaya.
LEVEL 1 -Introduction to Closed Circuit Rebreathers-
This is for people who want to try rebreather diving.
It gives them a chance to use a unit with the help of
a qualified instructor. You only need to be Open water
certified to get a chance to do a shallow dive with a
rebreather unit. This is a very safe course and it is
done in a controlled environment. This can be accomplished
in one short day. It is a good way to see how a rebreather
works. If you do like what you have done, you can progress
to the next level.
Level 2- Semi Closed rebreather - The length of
this course is approx. 4-days. There is one day of theory,
one day in a swimming pool and two days on the boat. This
course gives most people plenty of enough time to get
comfortable with the SCR unit. You must remember, the
buoyancy characteristics are not the same as with open
circuit. You will be shown how to assemble and disassemble
your unit to develop confidence to achieve it on your
own. The two days on the boat will provide no less than
150 minutes of bottom time, again to provide ample experience.
Don't miss out on the opportunity to dive silent and more
comfortable. The prerequisites for this course are; you
must have level-II enriched air training (ANDI, CSU certification
or equivalent) and a minimum of 50 logged open water dives.
Level 2 - Closed Circuit Rebreather - This course
is to the level 2, SCR program except that you will be
using a fully closed circuit unit. Minimum open water
time is increased to 180 minutes. The prerequisites are
the same as in level 2, SCR.
Level 3 - Technical Rebreather Diver - For the
more experienced, certified rebreather divers who would
like to do dives beyond the recreational limits. During
this course we will take you to a maximum depth of 50
metres. We will be doing deco no deeper then 9 metres
with a maximum deco of 30-minutes. During this course,
there are four to five dives with a minimum bottom time
of 180 minutes and a minimum run time of 210 minutes.
Plenty of time will be devoted to practicing decompression
procedures. This course is obviously more extensive. The
prerequisites for this are; the student needs to be TSD
qualified and also have a CCR certification with a minimum
of 100-open water dives.
Level 5- CCR/ERE- This is the highest level you
can reach. The maximum depth for this course is 100 metres
which should satisfy all needs. The minimum time using
the unit during the course is 240 minutes bottom time
with a total run time of 300 minutes. To be able to dive
these deeper depths you have to definitely be familiar
with your equipment and your specific unit. You will have
the confidence and ability to plan and execute these dives
when you finish this course. The prerequisites are, you
must be ERD & TRD certified and have a minimum of
200 open water dives.
Rebreathers are the way of the future. They have made
quite a leap in technology in the last few years. At Neptune
Dive Center and International Technical Diving Services
we have experienced instructors and our rebreathers are
in excellent shape. Just add YOU and Water.
Contact info. Bruce Konefe
Mobile # 012992968 Home 038 411292
Personnel email andibk@loxinfo.co.th
Neptune Dive Center www.neptunedive.com
post@neptunedive.com 038 361786
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