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Diving
Scuba Diving
 The
self-contained underwater breathing apparatus or scuba diving
system, as we know it today, is the result of technological
developments and innovations that began almost 300 years ago.
Scuba diving is the most extensively used
system for breathing underwater by recreational divers throughout
the world, and in various forms is also widely used to perform
underwater work for military, scientific and commercial purposes.
Advantages and Disadvantages
Scuba
diving
has many advantages over free diving, mixed gas, helmeted,
saturation, and other forms of "technical" diving. Scuba
divers have great freedom of movement under water because
they swim with fins and without heavy equipment; The gear
is relatively inexpensive, simple to operate and maintain,
and requires a small support crew, or none at all.
Despite
all of these apparent advantages, recreational scuba
also has its drawbacks. These include no direct link between
the diver and the surface; no method of communicating with
the diver or monitoring his activities; limited dive time
(since the diver must carry all of his air in a tank); and
limited depth (since decompression diving is normally avoided
due to the limited quantity of air in the tanks).
Essential Equipment
 In
addition to a mask and fins, basic recreational scuba
equipment consists of a cylinder of compressed air attached
to a two-stage "demand regulator." The regulator lowers the
air pressure in "steps" from the cylinder and dispenses it
to the diver as needed.
Cylinders
for scuba diving are made of steel or aluminum alloy,
and are designed to operate safely at pressures ranging from
2,250 to 3,500 psi (pounds per square inch). As a means of
comparison, air pressure at sea level is only about 15 psi.
One of the most commonly used types of diving cylinders is
made of aluminum alloy, and has a capacity (the quantity of
gas that can be compressed into the cylinder) of 80 cubic
ft. The amount of time that it takes a diver to use up all
of the air in the dive cylinder is dependent on several factors,
including, the diver's breathing rate, and the depth to which
the diver descends (the deeper the dive, the greater the amount
of air used). All cylinders used by scuba divers should be
inspected internally at least once a year for damage and corrosion.
 The
primary function of the "demand regulator" attached to the
diving cylinder is to reduce the high-pressure gas supplied
by the scuba cylinder to the ambient pressure surrounding
the diver at depth. If the diver were to breath compressed
air directly from the cylinder, it could easily rupture his
lungs. The reduction of air pressure from the diving cylinder
to the diver is accomplished in two steps. The first stage
of the regulator, which attaches to the cylinder valve, reduces
the high pressure in the cylinder to an intermediate pressure
approximately 140 psi over ambient pressure. This intermediate
pressure fills a low-pressure hose that connects the first
stage of the regulator to the second stage. The second stage,
contained in the diver's mouthpiece, reduces the intermediate
pressure to the ambient pressure. The regulator is known as
a "demand regulator" because it only supplies air when the
diver "demands" it; that is, gas flows through the regulator
only when the diver inhales.
Critical for Safety
 Other
devices, while not directly involved in the breathing circuit
of the recreational scuba diver, are nonetheless
critical for safety. These include a Pressure
Gauge, depth gauge, and dive timer. These
instruments inform the diver about the amount of air left
in the cylinder, their depth in the water, and how much time
has been spent underwater. A diver who exceeds the prescribed
depth or time spent underwater, may become susceptible to
nitrogen narcosis, and/or decompression sickness, which can
be fatal.
Two
additional items generally considered essential for the scuba
diver are a "BC" or "BCD" (buoyancy compensator device)
and a dive knife. Almost all BCs are worn like a vest, and
include a band for mounting the air cylinder. The BC contains
an air bladder that the diver inflates or deflates to maintain
control over buoyancy, thereby avoiding uncontrolled ascents
and descents.
In
addition to the BC, few divers enter the water without a dive
knife. Most divers carry them in the event that they become
entangled in a line, net, or some other gear, and need to
cut themselves free. A dive knife can also be used as a signaling
device by banging it against a dive cylinder.
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