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Breathing underwater
Water
normally contains dissolved oxygen from which fish and other aquatic animals extract
all their required oxygen as the water flows past their gills. Humans lack gills
and do not otherwise have the capacity to breathe underwater unaided by external
devices.
Early diving experimenters quickly discovered it is not enough
to simply supply air in order to breathe comfortably underwater. As one descends,
in addition to the normal atmospheric pressure, water exerts increasing pressure
on the chest and lungs - approximately 1 bar or 14.7 psi for every 33 feet or
10 meters of depth - so the pressure of the inhaled breath must exactly counter
the surrounding or ambient pressure in order to safely and efficiently inflate
the lungs.
By always providing the breathing gas at ambient pressure,
modern demand valve regulators ensure the diver can inhale and exhale naturally
and virtually effortlessly, regardless of depth.
Typically the diver's
nose and eyes are encapsulated in a diving mask, such that the nose cannot participate
in inhalation except when wearing a full face diving mask. However, inhaling from
a regulator's mouth-piece becomes second nature very quickly.
The most
commonly used Scuba set today is the open circuit 2-stage diving regulator, coupled
to a single pressurized gas cylinder. This 2-stage arrangement differs from Emile
Gagnan's and Jacques Cousteau's original 1942 design, known as the Aqua-lung,
in which the cylinder's pressure was reduced to ambient pressure in a single stage.
The 2-stage system has significant advantages over the original single-stage design.
In the 2-stage design, the first stage regulator reduces the cylinder pressure
of about 200 bar (3000 psi) to an intermediate level of about 10 bar (145 psi).
The second stage demand valve regulator, connected via a low pressure hose to
the first stage, delivers the breathing gas at the correct ambient pressure to
the diver's mouth and lungs. The diver's exhaled gases are exhausted directly
to the environment as waste.
Less common (but becoming increasingly so)
are the closed and/or semi-closed rebreather units. Unlike the open circuit arrangements
which vent all exhaled gases to the surrounding environment, rebreathers capture
each exhaled breath and recycle it for re-use by removing the carbon dioxide buildup
and replenishing the oxygen used up by the diver. Rebreathers release few or no
gas bubbles into the water which has advantages for research, military, photography
and other applications.
On deeper or more prolonged dives, gas mixtures
other than normal atmospheric air are used, such as air with enriched oxygen content,
known as nitrox, or oxygen with helium and a reduced percentage of nitrogen, known
as trimix. In cases of technical dives multiple cylinders may be carried, each
containing a different gas mixture for a distinct phase of the dive, typically
designated as Travel, Bottom and Decompression. These different gas mixtures may
be used to extend bottom time, reduce inert gas narcotic effects and reduce decompression
times.
Injuries due to changes in water pressure
The
diver must avoid injury caused by changes in water pressure. Pressure injuries
are called barotrauma. They are caused by pressure differences between the outside
and trapped air spaces inside the diver or the diver's equipment. To avoid them,
the diver equalizes the pressure in all air spaces with the surrounding water
pressure when changing depth.
Effects of breathing high pressure
gas
Decompression sickness
The diver must avoid the formation
of gas bubbles in the body, called decompression sickness or 'the bends', by releasing
the water pressure on the body slowly at the end of the dive. This is done by
making decompression stops and ascending slowly using dive computers or decompression
tables for guidance. Decompression sickness must be treated promptly, typically
in a recompression chamber. Administering a higher concentration of oxygen to
a decompression sickness stricken diver on the surface is a good form of first
aid for decompression sickness, although fatality or permanent disability may
still occur.
Nitrogen narcosis
Nitrogen narcosis or inert
gas narcosis is a reversible alteration in consciousness producing a state similar
to alcohol intoxication in divers who breathe high pressure gas at depth. Being
"narced" can impair judgement and make diving very dangerous. It occurs
at any depth, but in most cases doesn't become noticeable until deeper depths;
typically when breathing air at around 30m/100 ft. Jacques Cousteau famously described
it as the "rapture of the deep".
Need to see underwater
Water has a higher refractive index than air. Light entering the eye from the
water behaves differently than light entering from air. This creates a distortion
that affects normal vision. Diving masks and diving helmets solve this problem
by creating an air partition between the diver's eyes and the water. The distortion
created by the water is effectively reversed as the light travels from water to
air.
Divers who require corrective lenses to see clearly outside the
water would normally require the same prescription while wearing a mask. Some
masks can be ground to the diver's prescription to avoid the need for additional
corrective lenses.
Controlling buoyancy underwater
To dive
safely, divers need to be able to control their rate of descent and ascent in
the water. Ignoring other forces such as water currents and swimming, diver's
overall buoyancy determines whether a diver ascends or descends. Equipment such
as the diving weighting systems, diving suits (Wet, Dry & Semi-dry suits are
used depending on the water temperature) and buoyancy compensators (which go by
many different names such as BC, Stability 'Stab' Jacket they are also know as
a 'Wing' when used as part of a twin-set configuration) can be used to adjust
the overall buoyancy. When divers want to remain at constant depth, they try to
achieve neutral buoyancy. This minimises gas consumption caused by swimming to
maintain depth.
The volumes and weights of the diver and all equipment
attached to the diver, contribute to the diver's overall buoyancy. Volume creates
an upward force and weight creates a downward force. If the force due to volume
is greater than the weight, the diver ascends. If the force due to volume is less
than the weight the diver descends. Diving weighting systems can be used to reduce
the diver's weight and cause an ascent in an emergency. Diving suits, mostly being
made of compressible materials, reduce in volume as the diver descends and expand
as the diver ascends creating unwanted buoyancy changes. The diver can inject
air into some diving suits to counteract this effect and squeeze. Buoyancy compensators
allow easy and fine adjustments in the diver's overall volume and therefore buoyancy.
For open circuit divers, changes in the diver's lung volume can be used to adjust
buoyancy.
Avoiding losing body heat
Water conducts heat from
the diver 25 times[1] better than air, which can lead to hypothermia. Except in
very warm water, the diver needs the thermal insulation provided by wetsuits and
drysuits. See the main article: Diving suit. In the case of a wetsuit, the suit
is designed to minimize heat loss. Wetsuits are generally made of neoprene that
has small gas cells, generally nitrogen, trapped in it during the manufacturing
process. The poor thermal conductivity of this expanded cell neoprene means that
wetsuits reduce loss of body heat by conduction to the surrounding water. The
neoprene in this case acts as an insulator.
The second way in which wetsuits
reduce heat loss is to trap a thin layer of water between the diver's skin and
the insulating suit itself. Body heat then heats the trapped water. Provided the
wetsuit is reasonably well-sealed at all openings (neck, wrists, legs), this reduces
water flow over the surface of the skin, reducing loss of body heat by convection,
and therefore keeps the diver warm (this is the principle employed in the use
of a Semi-Dry)
In the case of a dry suit, it does exactly that... keeps
a diver dry. The suit is sealed so that frigid water cannot penetrate the suit.
Drysuit undergarments are often worn under a drysuit as well, and help to keep
layers of air inside the suit for better thermal insulation.
Drysuits
fall into two main categories neoprene and membrane; both systems have their good
and bad points but generally they can be reduced to:
Membrane: high level
of diver manoeuverability due to the thinness of the material, however that also
means that heavy weight undersuit is required if diving in cooler water.
Neoprene: low level of diver manoeuverability due to the material being considerably
thicker than membrane material (even when dealing with compressed neoprene) however
the neoprene provides a higher level of insulation for the diver.
Avoiding skin cuts and grazes
Diving suits also help prevent the diver's
skin being damaged by rough or sharp underwater objects, marine animals or coral.
Diving longer and deeper safely
There are a number of techniques
to increase the diver's ability dive deeper and longer:
technical diving
- diving deeper than 130 feet and/or using mixed gases.
surface supplied
diving - use of umbilical gas supply and diving helmets.
saturation diving
- long-term use of underwater habitats under pressure and a gradual release of
pressure over several days in a decompression chamber at the end of a dive
Being mobile underwater
The diver needs to be mobile underwater.
Streamlining dive gear will reduce drag and improve mobility. Personal mobility
is enhanced by swimfins and Diver Propulsion Vehicles. Other equipment to improve
mobility includes diving bells and diving shots.
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