Scuba equipment may be open circuit, in which exhaled gas is expelled to the surroundings, or a closed or semi-closed circuit rebreather, in which the breathing gas is scrubbed to remove carbon dioxide, and the oxygen used is replenished from a supply of feed gas before being re-breathed.
Scuba diving may be done recreationally or professionally in a number
of applications, including scientific, military and public safety
roles, but most commercial diving uses surface supplied diving equipment
when this is practicable.
A scuba diver primarily moves underwater by using fins attached to the feet, but external propulsion can be provided by a diver propulsion vehicle,
or a sled pulled from the surface.
Other equipment includes a dive mask
to improve underwater vision, a protective dive suit, equipment to
control buoyancy, and equipment related to the specific circumstances
and purpose of the dive.
Scuba divers are trained in the procedures and
skills appropriate to their level of certification by instructors
affiliated to the diver certification organisations which issue these
certifications.
These include standard operating procedures for using
the equipment and dealing with the general hazards of the underwater
environment, and emergency procedures for self-help and assistance of a
similarly equipped diver experiencing problems.
A minimum level of
fitness and health is required by most training organisations, but a
higher level of fitness may be appropriate for some applications.
Original Aqualung scuba set.
1: Air Hose, 2: Mouthpiece, 3: Regulator, 4: Harness, 5: Back plate, 6: Tank
By the early twentieth century, two basic systems for scuba (self-contained underwater breathing apparatus) had emerged: open-circuit scuba where the diver's exhaled breath is vented directly into the water, and closed-circuit scuba where the carbon dioxide is removed from the diver's exhaled breath which has oxygen added and is recirculated.
Rebreathers
The closed-circuit rebreathers
were first developed for escape and rescue purposes, and were modified
for military use, due to their stealth advantages, as they produce very
few bubbles.
The first commercially successful closed-circuit scuba was
designed and built by English diving engineer, Henry Fleuss in 1878, while working for Siebe Gorman in London.[3][4]
His SCBA (self-contained breathing apparatus) consisted of a rubber mask connected to a breathing bag, with (estimated) 50-60% O2 supplied from a copper tank and CO2 scrubbed by rope yarn soaked in a solution of caustic potash; the system giving a duration of about three hours.[4][5]
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.[citation needed]
Open-circuit scuba
The first commercially successful scuba sets were the Aqualung twin hose open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau
in 1943, in which compressed air carried in back mounted cylinders is
inhaled through a demand regulator and then exhaled into the water
adjacent to the tank.[8]
The single hose two stage scuba regulators trace their origins to Australia, where Ted Eldred developed the first example of this type of regulator, known as Porpoise scuba gear.
This was developed because patents protected the Aqualung's twin hose design.[citation needed]
The single hose regulator separates the demand valve from the cylinder,
giving the diver air at the ambient pressure at the mouth, rather than
ambient pressure at the top of the cylinder.
Etymology
The term "SCUBA" (an acronym for "self-contained underwater breathing apparatus") originally referred to United States combat frogmen's oxygen rebreathers, developed during World War II by Christian J. Lambertsen for underwater warfare.[2][9][10]
"SCUBA" was originally an acronym, but is now generally used as a common noun or adjective, "scuba".[11]
It has become acceptable to refer to "scuba equipment" or "scuba apparatus"—examples of the linguistic RAS syndrome.
Divers may be employed professionally to perform tasks underwater. Some of these tasks are suitable for scuba.[1][12][13]
There are divers who work, full or part-time, in the recreational
diving community as instructors, assistant instructors, divemasters and
dive guides.
In some jurisdictions the professional nature, with
particular reference to responsibility for health and safety of the
clients, of recreational diver instruction, dive leadership for reward
and dive guiding is recognized and regulated by national legislation.[13][14]
Other specialist areas of scuba diving include military diving, with a long history of military frogmen in various roles.
They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations.
In civilian operations, many police forces operate police diving
teams to perform "search and recovery" or "search and rescue"
operations and to assist with the detection of crime which may involve
bodies of water.
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Scuba diving in the Fiji islands in the south Pacific ocean. Underwater caves, deep blue clear water, scuba divers, sea snakes, and colorful coral reefs exploding with tropical fish and other marine life.
Scuba equipment may be open circuit, in which exhaled gas is expelled to the surroundings, or a closed or semi-closed circuit rebreather, in which the breathing gas is scrubbed to remove carbon dioxide, and the oxygen used is replenished from a supply of feed gas before being re-breathed.
A scuba diver primarily moves underwater by using fins attached to the feet, but external propulsion can be provided by a diver propulsion vehicle, or a sled pulled from the surface.
1: Air Hose, 2: Mouthpiece, 3: Regulator, 4: Harness, 5: Back plate, 6: Tank
By the early twentieth century, two basic templates for scuba (self-contained underwater breathing apparatus) had emerged: open-circuit scuba where the diver's exhaust is vented directly into the water, and closed-circuit scuba where the diver's unused oxygen is filtered from the carbon dioxide and recirculated.
Rebreathers
The closed-circuit rebreathers
were first developed for military use, due to their stealth advantages.
The first commercially successful closed-circuit scuba was designed and
built by English diving engineer, Henry Fleuss in 1878, while working for Siebe Gorman in London.[3][4]
His SCBA (self-contained breathing apparatus) consisted of a rubber mask connected to a breathing bag, with (estimated) 50-60% O2 supplied from a copper tank and CO2 scrubbed by rope yarn soaked in a solution of caustic potash; the system giving a duration of about three hours.[4][5]
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.
Open-circuit scuba
The first commercially successful scuba sets were the Aqualung twin hose open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau
in 1943, in which compressed air carried in back mounted cylinders is
inhaled through a demand regulator and then exhaled into the water
adjacent to the tank.[8]
The single hose two stage scuba regulators trace their origins to Australia, where Ted Eldred developed the first example of this type of regulator, known as Porpoise scuba gear.
This was developed, because patents protected the Aqualung's twin hose
design.
The single hose regulator separates the cylinder from the demand
valve, giving the diver air at the pressure at the mouth, not at the
pressure at the top of the cylinder.
Etymology
The term "SCUBA" (an acronym for "self-contained underwater breathing apparatus") originally referred to United States combat frogmen's oxygen rebreathers, developed during World War II by Christian J. Lambertsen for underwater warfare.[2][9][10]
"SCUBA" was originally an acronym, but is now generally used as a common noun or adjective, "scuba".[11]
It has become acceptable to refer to "scuba equipment" or "scuba apparatus"—examples of the linguistic RAS syndrome.
Divers may be employed professionally
to perform tasks underwater. Some of these tasks are suitable for
scuba. Professional scuba divers are trained to manage situations in
which they may face vertigo or entanglement.
There are divers who work, full or part-time, in the recreational
diving community as instructors, assistant instructors, divemasters and
dive guides.
In some jurisdictions the professional nature, with
particular reference to responsibility for health and safety of the
clients, of recreational diver instruction, dive leadership for reward
and dive guiding is recognised and regulated by national legislation.[12][13]
Other specialist areas of scuba diving include military diving, with a long history of military frogmen in various roles. They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations.
In civilian operations, many police forces operate police diving
teams to perform "search and recovery" or "search and rescue"
operations and to assist with the detection of crime which may involve
bodies of water.
In some cases diver rescue teams may also be part of a fire department, paramedical service or lifeguard unit, and may be classed as public service diving.
The depth range applicable to scuba diving depends on the application
and training, but most recreational dives are 30 metres (100 ft) deep
or less.
Recreational dives are limited to no-stop dives or relatively
short planned decompression stops to minimise risk of decompression
sickness.
Recreational scuba is generally limited to depths of no more
than 42 metres (140 ft) (by US-based agencies such as PADI and NAUI) or
50 metres (160 ft) (by UK-based agencies such as BSAC and SAA).
Professional diving also limits the allowed planned decompression
depending on the code of practice, operational directives, or statutory
restrictions.
Depth limits depend on the jurisdiction, and maximum
depths allowed range from 30 metres (100 ft) to more than 50 metres
(160 ft), depending on the breathing gas used and the availability of a
decompression chamber nearby or on site.
Technical diving may involve exploring the logistical and
physiological limits, and in these cases a higher risk is accepted by
the diver, and considerable effort may be made to reduce this risk by
careful planning and contingency procedures.
Depths are limited by
physiological and logistical aspects - the amount of gas that can be
carried, decompression requirements, gas toxicity at high pressure and
work of breathing constraints.
Recreational diver putting on his scuba set before diving.
The defining equipment used by a scuba diver is the eponymous scuba,
the self-contained underwater breathing apparatus which allows the
diver to breathe while diving, and is transported by the diver.
As one descends, in addition to the normal atmospheric pressure, the
water exerts increasing hydrostatic pressure of approximately 1 bar
(14.7 pounds per square inch) for every 10 m (33 feet) of depth.
The
pressure of the inhaled breath must balance the surrounding or ambient
pressure to allow inflation of the lungs.
It becomes virtually
impossible to breathe air at normal atmospheric pressure through a tube
below three feet under the water.[2]
Most recreational scuba diving is done using a half mask
which covers the diver's eyes and nose, and a mouthpiece to supply the
breathing gas from the demand valve or rebreather.
Inhaling from a
regulator's mouthpiece becomes second nature very quickly.
The other common arrangement is a full face mask
which covers the eyes, nose and mouth, and often allows the diver to
breathe through the nose.
Professional scuba divers are more likely to
use full face masks.
Aqualung Legend second stage (demand valve) regulator
Gekko dive computer with attached pressure gauge and compass
Open circuit scuba has no provision for using the breathing gas more
than once for respiration.
The gas inhaled from the scuba equipment is
exhaled to the environment, or occasionally into another item of
equipment for a special purpose, usually to increase buoyancy of a
lifting device such as a buoyancy compensator, inflatable surface marker
buoy or small lifting bag.
By always providing the appropriate breathing gas at ambient pressure, demand valve regulators ensure the diver can inhale and exhale naturally and without excessive effort, regardless of depth, as and when needed.
The most commonly used scuba set uses a "single-hose" open circuit 2-stage demand regulator,
connected to a single back-mounted high-pressure gas cylinder, with the
first stage connected to the cylinder valve and the second stage at the
mouthpiece.[1]
This arrangement differs from Emile Gagnan's and Jacques Cousteau's original 1942 "twin-hose" design, known as the Aqua-lung,
in which the cylinder pressure was reduced to ambient pressure in one
or two stages which were all in the housing mounted to the cylinder
valve or manifold.
The "single-hose" system has significant advantages
over the original system for most applications.
Aqualung 1st stage regulator
Suunto submersible pressure gauge display
In the "single-hose" two-stage design, the first stage regulator
reduces the cylinder pressure of up to about 300 bar (4350 psi) to an
intermediate level of about 10 bar (145 psi) above ambient pressure.
The
second stage demand valve
regulator, supplied by a low-pressure hose from the first stage,
delivers the breathing gas at ambient pressure to the diver's mouth. The
exhaled gases are exhausted directly to the environment as waste.
The
first stage typically has at least one outlet port delivering breathing
gas at full tank pressure which is connected to the diver's submersible
pressure gauge or dive computer, to show how much breathing gas remains
in the cylinder.
Rebreather
An Inspiration electronic fully closed circuit rebreather
Less common are closed circuit (CCR) and semi-closed (SCR) rebreathers,[14]
which unlike open-circuit sets that vent off all exhaled gases, process
all or part of each exhaled breath for re-use by removing the carbon dioxide and replacing the oxygen used by the diver.
Rebreathers release little or no gas bubbles into the water, and use
much less stored gas volume, for an equivalent depth and time because
exhaled oxygen is recovered; this has advantages for research, military,[1]
photography, and other applications.
Rebreathers are more complex and
more expensive than open-circuit scuba, and special training and correct
maintenance are required for them to be safely used, due to the larger
variety of potential failure modes.[14]
In a closed-circuit rebreather the oxygen partial pressure in the
rebreather is controlled, so it can be maintained at a safe continuous
maximum, which reduces the inert gas (nitrogen and/or helium) partial
pressure in the breathing loop.
Minimising the inert gas loading of the
diver's tissues for a given dive profile reduces the decompression
obligation. This requires continuous monitoring of actual partial
pressures with time and for maximum effectiveness requires real-time
computer processing by the diver's decompression computer.
Decompression
can be much reduced compared to fixed ratio gas mixes used in other
scuba systems and, as a result, divers can stay down longer or require
less time to decompress.
A semi-closed circuit rebreather injects a
constant mass flow of a fixed breathing gas mixture into the breathing
loop, or replaces a specific percentage of the respired volume, so the
partial pressure of oxygen at any time during the dive depends on the
diver's oxygen consumption and/or breathing rate.
Planning decompression
requirements requires a more conservative approach for a SCR than for a
CCR, but decompression computers with a real time oxygen partial
pressure input can optimise decompression for these systems.
Because rebreathers produce very few bubbles, they do not disturb
marine life or make a diver’s presence known at the surface; this is
useful for underwater photography, and for covert work.
Gas mixtures
A cylinder decal to indicate that the contents are a Nitrox mixture
Nitrox cylinder marked up for use showing maximum safe operating depth (MOD)
For some diving, gas mixtures other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% trace gases) can be used,[1][2] so long as the diver is competent in their use.
The most commonly used mixture is nitrox,
also referred to as Enriched Air Nitrox (EAN), which is air with extra
oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reducing
the risk of decompression sickness
or allowing longer exposure to the same pressure for equal risk.
The
reduced nitrogen may also allow for no stops or shorter decompression
stop times or a shorter surface interval between dives.
A common
misconception is that nitrox can reduce narcosis, but research has shown that oxygen is also narcotic.[15][16]
The increased partial pressure of oxygen due to the higher oxygen
content of nitrox increases the risk of oxygen toxicity, which becomes
unacceptable below the maximum operating depth of the mixture.
To displace nitrogen without the increased oxygen concentration, other diluent gases can be used, usually helium, when the resultant three gas mixture is called trimix, and when the nitrogen is fully substituted by helium, heliox.
For dives requiring long decompression stops, divers may carry
cylinders containing different gas mixtures for the various phases of
the dive, typically designated as Travel, Bottom, and Decompression
gases.
These different gas mixtures may be used to extend bottom time,
reduce inert gas narcotic effects, and reduce decompression times.
Diver mobility
To take advantage of the freedom of movement afforded by scuba equipment, the diver needs to be mobile underwater.
Fins have a large blade area and use the more powerful leg muscles, so
are much more efficient for propulsion and maneuvering thrust than arm
and hand movements, but require skill to provide fine control.
Several
types of fin are available, some of which may be more suited for
maneuvering, alternative kick styles, speed, endurance, reduced effort
or ruggedness.
Streamlining dive gear will reduce drag and improve mobility.
Balanced trim which allows the diver to align in any desired direction
also improves streamlining by presenting the smallest section area to
the direction of movement and allows propulsion thrust to be used more
efficiently.
Occasionally a diver may be towed using a "sled", an unpowered device
towed behind a surface vessel which conserves the diver's energy and
allows more distance to be covered for a given air consumption and
bottom time.
The depth is usually controlled by the diver by using
diving planes or by tilting the whole sled. Some sleds are faired to
reduce drag on the diver.[citation needed]
When divers want to remain at constant depth, they try to achieve
neutral buoyancy. This minimizes the effort of swimming to maintain
depth and therefore reduces gas consumption.
The buoyancy force on the diver is the weight of the volume of the liquid that they and their equipment displace minus the weight of the diver and their equipment; if the result is positive,
that force is upwards.
The buoyancy of any object immersed in water is
also affected by the density of the water. The density of fresh water is
about 3% less than that of ocean water.[17]
Therefore, divers who are neutrally buoyant at one dive destination
(e.g. a fresh water lake) will predictably be positively or negatively
buoyant when using the same equipment at destinations with different
water densities (e.g. a tropical coral reef).
The removal ("ditching" or "shedding") of diver weighting systems can
be used to reduce the diver's weight and cause a buoyant ascent in an
emergency.
Diving suits made of compressible materials decrease in volume as the
diver descends, and expand again as the diver ascends, causing buoyancy
changes.
Diving in different environments also necessitates adjustments
in the amount of weight carried to achieve neutral buoyancy.
The diver
can inject air into dry suits to counteract the compression 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 average lung volume during a breathing cycle can be used to make fine adjustments of buoyancy.
It is changed by small differences in ambient pressure caused by
a change in depth, and the change has a positive feedback effect.
A
small descent will increase the pressure, which will compress the gas
filled spaces and reduce the total volume of diver and equipment. This
will further reduce the buoyancy, and unless counteracted, will result
in sinking more rapidly.
The equivalent effect applies to a small
ascent, which will trigger an increased buoyancy and will result in
accelerated ascent unless counteracted. The diver must continuously
adjust buoyancy or depth in order to remain neutral.
This is a skill
which improves with practice until it becomes second nature.
Buoyancy changes with depth variation are proportional to the
compressible part of the volume of the diver and equipment, and to the
proportional change in pressure, which is greater per unit of depth near
the surface.
Minimizing the volume of gas required in the buoyancy
compensator will minimize the buoyancy fluctuations with changes in
depth.
This can be achieved by accurate selection of ballast weight,
which should be the minimum to allow neutral buoyancy with depleted gas
supplies at the end of the dive unless there is an operational
requirement for greater negative buoyancy during the dive.
Buoyancy and trim can significantly affect drag of a diver.
The
effect of swimming with a head up angle, of about 15° as is quite common
in poorly trimmed divers, can be an increase in drag in the order of
50%.[18]
Diving masks and helmets solve this problem by providing an air space in front of the diver's eyes.[1]
The refraction error
created by the water is mostly corrected as the light travels from
water to air through a flat lens, except that objects appear
approximately 34% bigger and 25% closer
in water than they actually are.
Therefore total field-of-view is
significantly reduced and eye–hand coordination must be adjusted.
This also affects underwater photography: a camera seeing through a
flat port in its housing is affected in the same way as its user's eye
seeing through a flat mask viewport, and so its operator must focus for
the apparent distance to target, not for the real distance.
This is only
relevant for manual focusing.
Divers who need corrective lenses to see clearly outside the water
would normally need the same prescription while wearing a mask.
Generic
and custom corrective lenses are available for some two-window masks.
Custom lenses can be bonded onto masks that have a single front window
or two windows.
Cylindrically curved faceplates such as those used for firefighting full-face masks produce severely distorted views underwater.
Commando frogmen
concerned about revealing their position when light reflects from the
glass surface of their diving masks may instead use special contact lenses to see underwater.
As a diver descends, they must periodically exhale through their nose
to equalize the internal pressure of the mask with that of the
surrounding water.
Swimming goggles are not suitable for diving because
they only cover the eyes and thus do not allow for equalization.
Failure
to equalise the pressure inside the mask may lead to a form of
barotrauma known as mask squeeze.[1][19]
A wetsuit is a garment, usually made of foamed neoprene, which
provides thermal insulation, abrasion resistance and buoyancy.
The
insulation properties depend on bubbles of gas enclosed within the
material, which reduce its ability to conduct heat.
The bubbles also
give the wetsuit a low density, providing buoyancy in water.
A good close fit and few zips helps the suit to remain waterproof and
reduce flushing - the replacement of water trapped between suit and
body by cold water from the outside.
Improved seals at the neck, wrists
and ankles and baffles under the entry zip produce a suit known as a
"semi-dry".
Suits range from a thin (2 mm or less) "shortie", covering just the
torso, to a full 8 mm semi-dry, usually complemented by neoprene boots,
gloves and hood.
A dry suit provides thermal insulation to the wearer while immersed in water,[23][24][25][26]
and normally protects the whole body except the head, hands, and
sometimes the feet. In some configurations, these are also covered.
Dry
suits are usually used where the water temperature is below 15 °C
(60 °F) or for extended immersion in water above 15 °C (60 °F), where a
wet suit user would get cold, and with an integral helmet, boots, and
gloves for personal protection when diving in contaminated water.
Dry suits are designed to prevent water entering. This generally
allows better insulation making them more suitable for use in cold
water.
They can be uncomfortably hot in warm or hot air, and are
typically more expensive and more complex to don.
For divers, they add
some degree of complexity as the suit must be inflated and deflated with
changes in depth in order to avoid "squeeze" on descent or uncontrolled
rapid ascent due to over-buoyancy.
Unless the maximum depth of the water is known, and is quite shallow,
a diver must monitor the depth and duration of a dive to avoid decompression sickness.
Traditionally this was done by using a depth gauge and a diving watch, but electronic Dive computers
are now in general use, as they are programmed to do real-time
modelling of decompression requirements for the dive, and automatically
allow for surface interval.
Many can be set for the gas mixture to be
used on the dive, and some can accept changes in the gas mix during the
dive.
Most dive computers provide a fairly conservative decompression
model, and the level of conservatism may be selected by the user within
limits.
Most decompression computers can also be set for altitude
compensation to some degree.
If the dive site and dive plan require the diver to navigate, a compass may be carried, and where retracing a route is critical, as in cave or wreck penetrations, a guide line is laid from a dive reel.
In less critical conditions, many divers simply navigate by landmarks and memory, a procedure also known as pilotage or natural navigation.
A scuba diver should always be aware of the remaining breathing gas supply, and this is usually monitored by using a submersible pressure gauge on each cylinder.
Cutting tools such as knives, line cutters or shears are often
carried by divers to cut loose from entanglement in nets or lines.
A
surface marker buoy on a line held by the diver indicates the position
of the diver to the surface personnel.
This may be an inflatable marker
deployed by the diver at the end of the dive, or a sealed float, towed
for the whole dive. A surface marker also allows easy and accurate
control of ascent rate and stop depth for safer decompression.
Various surface detection aids may be carried to help surface personnel spot the diver after ascent.
Divers may carry underwater photographic or video equipment, or tools for a specific application in addition to diving equipment.
Procedures
The underwater environment is unfamiliar and hazardous, and to ensure
diver safety simple, yet necessary procedures must be followed.
A
certain minimum level of attention to detail and acceptance of
responsibility for one's own safety and survival are required.
Most of
the procedures are simple and straightforward, and become second nature
to the experienced diver, but must be learned, and take some practice to
become automatic and faultless, just like the ability to walk or talk.
Most of the safety procedures are intended to reduce the risk of
drowning, and many of the rest are to reduce the risk of barotrauma and
decompression sickness.
In some applications getting lost is a serious
hazard, and specific procedures to minimize the risk are followed.
Preparation for the dive
Before starting any dive both the diver and their buddy do equipment
checks to ensure everything is in good working order and available.
Additionally, there is Dive planning to ensure the divers do not exceed their comfort zone or skill level, or the safe capacity of their equipment.
This includes Scuba gas planning to ensure that the amount of breathing gas to be carried is sufficient to allow for any reasonably foreseeable contingencies.
Standard diving procedures
Water entry and descent procedures are carried out first to enter
the water without injury or loss of/damage to equipment. These
procedures also cover how to descend at the right place, time, and rate;
with the correct breathing gas available; and without losing contact
with the other divers in the group.
Equalization of pressure in gas spaces to avoid barotraumas. The
expansion or compression of enclosed air spaces may cause discomfort or
injury while diving. Critically, the lungs are susceptible to
over-expansion and subsequent collapse if a diver holds their breath
while ascending: during training divers are taught to never hold their
breath while diving. Ear clearing is another critical equalization procedures, usually requiring conscious intervention by the diver.
Mask and regulator clearing may be needed to ensure ability to see
and breathe in case of flooding. This can easily happen and is not
considered an emergency.
Buoyancy control and diver trim
require frequent adjustment (particularly during depth changes) to
ensure safe and convenient underwater mobility during the dive.
Buddy checks,
breathing gas monitoring, and decompression status monitoring are
carried out to ensure that the dive plan is followed and that members of
the group are safe/available to help each other in an emergency.
Ascent, decompression,
and surfacing: to ensure that dissolved gases are safely released, that
barotraumas of ascent are avoided, and that it is safe to surface.
Water exit procedures: to leave the water again without injury or loss of/damage to equipment.
Post-dive procedures
These include debriefing where appropriate, and equipment
maintenance, to ensure that the equipment is kept in good condition for
later use.
Buddy and team diving procedures are intended to ensure that a
recreational scuba diver who gets into difficulty underwater is in the
presence of a similarly equipped person who can render assistance.
Divers are trained to assist in those emergencies specified in the
training standards for their certification, and may be required to
demonstrate competence in the prescribed skills.
Solo divers take the responsibility for their own safety and
compensate for the absence of a buddy by skill, vigilance and
appropriate equipment.
Two divers giving the sign that they are "OK" on a wreck in the Dominican Republic.
Divers cannot talk underwater unless they are wearing a full-face
mask and electronic communications equipment, but they can communicate
basic and emergency information using hand signals, light signals, and
rope signals, and more complex messages can be written on waterproof
slates.
Emergency procedures
The most urgent emergencies specific to scuba diving generally
involve loss of breathing gas: Gas supply failures, situations where
breathing air is likely to run out before the diver can surface, or
inability to ascend, and uncontrolled ascents.
Controlled emergency ascents are almost always a consequence of loss
of breathing gas, while uncontrolled ascents are usually the result of a
buoyancy control failure.
The most urgent underwater emergencies usually involve a compromised
breathing gas supply. Divers are trained in procedures for donating and
receiving breathing gas from each other in an emergency, and may carry
an alternative air source if they do not choose to rely on a buddy.
Divers may be trained in procedures which have been approved by the
training agencies for recovery of an unresponsive diver to the surface,
where it might be possible to administer first aid.
Not all recreational
divers have this training as some agencies do not include it in entry
level training.
Professional divers may be required by legislation or
code of practice to have a standby diver at any diving operation, who is
both competent and available to attempt rescue of a distressed diver.
Entrapment
Two basic types of entrapment are significant hazards for scuba
divers: Inability to navigate out of an enclosed space, and physical
entrapment which prevents the diver from leaving a location.
The first
case can usually be avoided by staying out of enclosed spaces, and when
the objective of the dive includes penetration of enclosed spaces,
taking precautions such as the use of lights and guidelines.
The most
common form of physical entrapment is getting snagged on ropes, lines or
nets, and use of a cutting implement is the standard method of dealing
with the problem.
The risk of entanglement can be reduced by careful
configuration of equipment to minimize those parts which can easily be
snagged, and allow easier disentanglement.
Other forms of entrapment
such as getting wedged into tight spaces can often be avoided, but must
otherwise be dealt with as they happen.
The assistance of a buddy may be
helpful where possible.
Emergency procedures for specific scuba applications
Scuba diving in relatively hazardous environments such as caves and
wrecks, areas of strong water movement, relatively great depths, with
decompression obligations, with equipment that has more complex failure
modes, and with gases that that are not safe to breathe at all depths of
the dive require specialized safety and emergency procedures tailored
to the specific hazards.
Divers must avoid injuries caused by changes in pressure.
The weight
of the water column above the diver causes an increase in pressure in
proportion to depth, in the same way that the weight of the column of
atmospheric air above the surface causes a pressure of 101.3 kPa (14.7 pounds-force per square inch)
at sea level.
This variation of pressure with depth will cause
compressible materials and gas filled spaces to tend to change volume,
which can cause the surrounding material or tissues to be stressed, with
the risk of injury if the stress gets too high.
Pressure injuries are
called barotrauma[2]
and can be quite painful, even potentially fatal – in severe cases
causing a ruptured lung, eardrum or damage to the sinuses.
To avoid
barotrauma, the diver equalizes the pressure in all air spaces with the
surrounding water pressure when changing depth.
The middle ear and sinus
are equalized using one or more of several techniques, which is
referred to as clearing the ears.
The scuba mask (half-mask) is equalized during descent by
periodically exhaling through the nose. During ascent it will
automatically equalise by leaking excess air round the edges.
A helmet
or full face mask will automatically equalise as any pressure
differential will either vent through the exhaust valve or open the
demand valve and release air into the low-pressure space.
If a drysuit is worn, it must be equalized by inflation and deflation, much like a buoyancy compensator.
Most dry suits are fitted with an auto-dump valve, which, if set
correctly, and kept at the high point of the diver by good trim skills,
will automatically release gas as it expands and retain a virtually
constant volume during ascent.
During descent the dry suit must be
inflated manually.
Although there are many dangers involved in scuba diving, divers can
decrease the risks through proper procedures and appropriate equipment.
The requisite skills are acquired by training and education, and honed
by practice.
Open-water certification programs highlight diving
physiology, safe diving practices, and diving hazards, but do not
provide the diver with sufficient practice to become truly adept.
The prolonged exposure to breathing gases at high partial pressure
will result in increased amounts of non-metabolic gases, usually
nitrogen and/or helium, (referred to in this context as inert gases)
dissolving in the bloodstream as it passes through the alveolar
capillaries, and thence carried to the other tissues of the body, where
they will accumulate until saturated.
This saturation process has very
little immediate effect on the diver. However when the pressure is
reduced during ascent, the amount of dissolved inert gas that can be
held in stable solution in the tissues is reduced.
As a consequence of the reducing partial pressure of inert gases in
the lungs during ascent, the dissolved gas will be diffused back from
the bloodstream to the gas in the lungs and exhaled.
The reduced gas
concentration in the blood has a similar effect when it passes through
tissues carrying a higher concentration, and that gas will diffuse back
into the bloodsteam, reducing the loading of the tissues.
As long as this process is gradual, all will go well and the diver
will reduce the gas loading by diffusion and perfusion until it
eventually re-stabilises at the current saturation pressure.
The problem
arises when the pressure is reduced more quickly than the gas can be
removed by this mechanism, and the level of supersaturation rises
sufficiently to become unstable.
At this point, bubbles may form and
grow in the tissues, and may cause damage either by distending the
tissue locally, or blocking small blood vessels, shutting off blood
supply to the downstream side, and resulting in hypoxia of those
tissues.
Divers inside a recompression chamber
This effect is called decompression sickness[2]
or 'the bends', and must be avoided by reducing the pressure on the
body slowly while ascending and allowing the inert gases dissolved in
the tissues to be eliminated while still in solution.
This process is
known as "off-gassing", and is done by restricting the ascent
(decompression) rate to one where the level of supersaturation is not
sufficient for bubbles to form.
This is done by controlling the speed of
ascent and making periodic stops to allow gases to be eliminated.
The
procedure of making stops is called staged decompression, and the stops
are called decompression stops.
Decompression stops that are not computed as strictly necessary are
called safety stops, and reduce the risk of bubble formation further.
There remains a statistical possibility of
decompression bubbles forming even when the guidance from tables or
computer has been followed exactly.
Decompression sickness must be treated as soon as practicable. Definitive treatment is usually recompression in a recompression chamber
with hyperbaric oxygen treatment.
Exact details will depend on severity
and type of symptoms, response to treatment, and the dive history of
the casualty.
Administering enriched-oxygen breathing gas or pure oxygen to a decompression sickness stricken diver on the surface is a good form of first aid for decompression sickness, although death or permanent disability may still occur.[29]
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.[2]
The mechanism is similar to that of nitrous oxide, or "laughing gas,"
administered as anesthesia. Being "narced" can impair judgment and make
diving very dangerous. Narcosis starts to affect some divers at 66 feet
(20 m).
At this depth, narcosis manifests itself as a slight giddiness.
The effects increase drastically with the increase in depth. Almost all
divers are able to notice the effects by 132 feet (40 meters).
At these
depths divers may feel euphoria, anxiety, loss of coordination and lack
of concentration. At extreme depths, hallucinogenic reaction and tunnel
vision can occur. Jacques Cousteau famously described it as the "rapture of the deep".[8]
Nitrogen narcosis occurs quickly and the symptoms typically disappear
during the ascent, so that divers often fail to realize they were ever
affected.
It affects individual divers at varying depths and conditions,
and can even vary from dive to dive under identical conditions.
However, diving with trimix or heliox dramatically reduces the effects of inert gas narcosis.
Oxygen toxicity occurs when oxygen in the body exceeds a safe partial pressure (PPO2).[2]
In extreme cases it affects the central nervous system and causes a
seizure, which can result in the diver spitting out their regulator and
drowning.
While the exact limit is idiomatic, it is generally recognized
that Oxygen toxicity is preventable if one never exceeds an oxygen partial pressure of 1.4 bar.[30]
For deep dives—generally past 180 feet (55 m), divers use "hypoxic
blends" containing a lower percentage of oxygen than atmospheric air.
For more information, see oxygen toxicity.
The underwater environment presents a constant hazard of asphyxiation
due to drowning. Breathing apparatus used for diving is life-support
equipment, and failure can have fatal consequences - reliability of the
equipment and the ability of the diver to deal with a single point of
failure are essential for diver safety.
Failure of other items of diving
equipment is generally not as immediately threatening, as provided the
diver is conscious and breathing, there may be time to deal with the
situation, however an uncontrollable gain or loss of buoyancy can put
the diver at severe risk of decompression sickness, or of sinking to a
depth where nitrogen narcosis or oxygen toxicity may render the diver
incapable of managing the situation, which may lead to drowning while
breathing gas remains available.
Water conducts heat from the diver 25 times[31] better than air, which can lead to hypothermia even in mild water temperatures.[2]
Symptoms of hypothermia include impaired judgment and dexterity,[32] which can quickly become deadly in an aquatic environment. In all but the warmest waters, divers need the thermal insulation provided by wetsuits or drysuits.[1]
In the case of a wetsuit, the suit is designed to minimize heat loss.
Wetsuits are usually made of neoprene
that has small closed 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, and to a larger
extent the nitrogen gas, in this case acts as an insulator. The
effectiveness of the insulation is reduced when the suit is compressed
due to depth, as the nitrogen filled bubbles are then smaller and
conduct heat better.
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, ankles zippers and
overlaps with other suit components), this reduces flow of cold water
over the surface of the skin, and thereby reduces loss of body heat by
convection, which helps keep the diver warm (this is the principle
employed in the use of a "Semi-Dry" wetsuit)
Spring suit (short legs and sleeves) and steamer (full legs and sleeves)
In the case of a drysuit, it does exactly what the name implies:
keeps a diver dry. The suit is waterproof and sealed so that frigid
water cannot penetrate the suit.
Drysuit undergarments are usually worn
under a drysuit to keep a layer of air inside the suit for better
thermal insulation. Some divers carry an extra gas bottle dedicated to
filling the dry suit. Usually this bottle contains argon gas, because of its better insulation as compared with air.[33] Dry suits should not be inflated with gases containing helium as it is a good thermal conductor.
Drysuits fall into two main categories: neoprene and membrane; both
systems have their good and bad points but generally their thermal
properties can be reduced to:
Membrane or Shell drysuits: usually a trilaminate
construction; owing to the thinness of the material (around 1 mm), these
require an undersuit, usually of high insulation value if diving in
cooler water.
Neoprene drysuits: a similar construction to wetsuits; these
are often considerably thicker (7–8 mm) and have sufficient insulation
to allow a lighter-weight undersuit (or none at all); however on deeper
dives the neoprene can compress to as little as 2 mm thus losing a
proportion of its insulation. Compressed or crushed neoprene may also be
used (where the neoprene is pre-compressed to 2–3 mm) which avoids the
variation of insulating properties with depth. These drysuits function
more like a membrane suit.
Injuries due to contact with the solid surroundings
Diving suits also help prevent the diver's skin being damaged by rough or sharp underwater objects, marine animals, coral, or metal debris commonly found on shipwrecks.
Some physical and psychological conditions are known or suspected to
increase the risk of injury or death in the underwater environment, or
to increase the risk of a stressful incident developing into a serious
incident culminating in injury or death.
Conditions which significantly
compromise the cardiovascular system, respiratory system or central
nervous system may be considered absolute or relative contraindications
for diving, as are psychological conditions which impair judgement or
compromise the ability to deal calmly and systematically with
deteriorating conditions which a competent diver should be able to
manage.
Inadequate learning or practice of critical safety skills may result
in the inability to deal with minor incidents, which consequently may
develop into major incidents.
Overconfidence can result in diving in conditions beyond the diver's
competence, with high risk of accident due to inability to deal with
known environmental hazards.
Inadequate strength or fitness for the conditions can result in
inability to compensate for difficult conditions even though the diver
may be well versed at the required skills, and could lead to
over-exertion, overtiredness, stress injuries or exhaustion.
Peer pressure can cause a diver to dive in conditions where they may be unable to deal with reasonably predictable incidents.
Diving with an incompetent buddy can result in injury or death while attempting to deal with a problem caused by the buddy.
Overweighting can cause difficulty in neutralising and controlling
buoyancy, and this can lead to uncontrolled descent, inability to
establish neutral buoyancy, inefficient swimming, high gas consumption,
poor trim, kicking up silt, difficulty in ascent and inability to
control depth accurately for decompression.
Underweighting can cause difficulty in neutralising and controlling
buoyancy, and consequent inability to achieve neutral buoyancy,
particularly at decompression stops.
Diving under the influence of drugs or alcohol, or with a hangover
may result in inappropriate or delayed response to contingencies,
reduced ability to deal timeously with problems, leading to greater risk
of developing into an accident, increased risk of hypothermia and
increased risk of decompression sickness.[34]
Use of inappropriate equipment and/or configuration can lead to a whole range of complications, depending on the details.
Some underwater tasks may present hazards related to the activity or
the equipment used, In some cases it is the use of the equipment, in
some cases transporting the equipment during the dive, and in some cases
the additional task loading, or any combination of these that is the
hazard.
Recreational scuba diving does not have a centralized certifying or
regulatory agency, and is mostly self regulated. There are, however,
several large diving organizations that train and certify divers and
dive instructors, and many diving related sales and rental outlets
require proof of diver certification from one of these organizations
prior to selling or renting certain diving products or services.
The following organisations publish standards for competence in recreational diving skills and knowledge:
Underwater diver training is normally given by a qualified instructor
who is a member of one of many diving training agencies or is
registered with a government agency.
Basic diver training entails the learning of skills required for the
safe conduct of activities in an underwater environment, and includes
procedures and skills for the use of diving equipment, safety, emergency
self-help and rescue procedures, dive planning, and use of dive tables.
Some of the scuba skills which an entry level diver will normally learn include:
Preparing and dressing in the diving suit
Assembly and pre-dive testing of the scuba set.
Entries and exits
Breathing from the demand valve
Recovering and clearing the demand valve.
Clearing water from the mask.
Buoyancy control using weights and buoyancy compensator
Finning techniques, underwater mobility and maneuvering.
Some knowledge of physiology and the physics of diving
is considered necessary by most diver certification agencies, as the
diving environment is alien and relatively hostile to humans.
The
physics and physiology knowledge required is fairly basic, and helps the
diver to understand the effects of the diving environment so that
informed acceptance of the associated risks is possible.
The physics mostly relates to gases under pressure, buoyancy, heat
loss, and light underwater. The physiology relates the physics to the
effects on the human body, to provide a basic understanding of the
causes and risks of barotrauma, decompression sickness, gas toxicity, hypothermia, drowning and sensory variations.
More advanced training often involves first aid and rescue skills,
skills related to specialized diving equipment, and underwater work
skills.
Endurance records
The current record for the longest continuous submergence using SCUBA gear was set by Mike Stevens of Birmingham, England at the National Exhibition Centre,
Birmingham, during the annual National Boat, Caravan and Leisure Show
between February 14 and February 23, 1986.
Mike Stevens was continuously
submerged for 212.5 hours beating his own previous record of 121.5
hours. The record was ratified by the Guinness Book of Records.[35]
Stevens used a standard regulator and mask and wore only a T-shirt and
swim shorts and an 8-pound weight belt, he had no surface breaks during
the 212.5 hours.
A team of divers attended Stevens throughout the dive.
The team was led by Diving Officer Trevor Parkes. The dive raised
£10,000 for the Birmingham Children's Hospital from donations by the
public.
Source: Wikipedia.org
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