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Scuba diving
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SCUBA (Self Contained Underwater Breathing Apparatus) diving is swimming underwater, or taking part in another activity, while using a scuba set. By carrying a source of breathing gas (usually compressed air), the scuba diver is able to stay underwater longer than with the simple breath-holding techniques used in snorkeling and free-diving, and is not hindered by air lines to a remote air source. The scuba diver typically swims underwater by using fins attached to the feet.
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Encyclopedia
SCUBA (Self Contained Underwater Breathing Apparatus) diving is swimming underwater, or taking part in another activity, while using a scuba set. By carrying a source of breathing gas (usually compressed air), the scuba diver is able to stay underwater longer than with the simple breath-holding techniques used in snorkeling and free-diving, and is not hindered by air lines to a remote air source. The scuba diver typically swims underwater by using fins attached to the feet. However, some divers also move around with the assistance of a DPV (diver propulsion vehicle), commonly called a "scooter", or by using surface-tethered devices called sleds pulled by a boat.
For the history of diving, see timeline of underwater technology.
History
Scuba today typically refers to the in-line open-circuit equipment, developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed gas (usually air) is inhaled from a tank and then exhaled into the water. However, rebreathers (both semi-closed circuit and closed circuit) are also self-contained systems (as opposed to surface-supplied systems) and are therefore classified as scuba.
Etymology
The term SCUBA (an acronym for Self-Contained Underwater Breathing Apparatus) arose during World War II, and originally referred to United States combat frogmen's oxygen rebreathers, developed by Dr. Christian Lambertsen for underwater warfare..
The word SCUBA began as an acronym, but it is now usually thought of as a regular word – scuba. It has become acceptable to refer to "scuba equipment" or "scuba apparatus" – examples of the linguistic RAS syndrome.
Types of diving
Reasons for diving may include:
| Type of diving | Classification |
|---|
| aquarium maintenance in large public aquariums | commercial, scientific | | boat and ship inspection, cleaning and maintenance | commercial, naval | | cave diving | technical, recreational | | civil engineering in harbors, water supply, and drainage systems | commercial | | crude oil industry and other offshore construction and maintenance | commercial | | demolition and salvage of ship wrecks | commercial, naval | | diver training for reward | professional | | fish farm maintenance | commercial | | fishing, e.g. for abalones, crabs, lobsters, pearls, scallops, sea crayfish, sponges | commercial | | frogman, manned torpedo | military | | harbor clearance and maintenance | commercial, military | | media diving: making television programs, etc. | professional | | mine clearance and bomb disposal, disposing of unexploded ordnance
| military, naval | | pleasure, leisure, sport | recreational | | underwater photography | professional, recreational | | policing diving to investigate or arrest unauthorized divers | police, military, naval | | search and recovery diving | commercial | | search and rescue diving | police | | spear fishing | professional (occasionally), recreational | | stealthy infiltration | military | | marine biology | scientific, recreational | | underwater tourism | recreational | | underwater archaeology (shipwrecks; harbors, and buildings) | scientific, recreational | | underwater welding | commercial |
Some professional, commercial, and police diving activities are sometimes performed by volunteer divers.
Within recreational diving there are those who are considered professional divers, because they maintain a professional standard of training and skills and may need to carry professional liability insurance.
Some consider technical diving to be a subset of recreational diving, but others separate it out due to the extensively different training equipment and knowledge needed for technical dives.
Public safety diving and military diving may be classified as commercial diving because they make a living from their pursuit of diving; however, public safety divers (police or rescue) and military divers have a different mission from the typical commercial diver. Scientific diving is used by marine scientists (including diving marine biologists and underwater archaeologists), as a tool for collecting their research data.
Physiological issues
Breathing underwater
- For more information, see Diving regulator.
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 simply to 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 almost exactly counter the surrounding or ambient pressure to inflate the lungs. It generally becomes difficult to breathe through a tube past three feet under the water.
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.
Because the diver's nose and eyes are covered by a diving mask; the diver cannot breathe in through the nose, except when wearing a full face diving mask. However, inhaling from a regulator's mouthpiece becomes second nature very quickly.
Open-circuit
The most commonly used scuba set today is the "single-hose" open circuit 2-stage diving regulator, coupled to a single pressurized gas cylinder, with the first stage on the cylinder and the second stage at the mouthpiece. 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's pressure was reduced to ambient pressure in one or two or three stages which were all on the cylinder. The "single-hose" system has significant advantages over the original system.
In the "single-hose" two-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. The first stage typically has at least one outlet delivering breathing gas at unreduced tank pressure. This is connected to the diver's pressure gauge or computer, in order to show how much breathing gas remains.
Rebreather
Less common, but becoming increasingly available, are closed and semi-closed rebreathers. Open-circuit sets vent off all exhaled gases, but rebreathers reprocess each exhaled breath for re-use by removing the carbon dioxide buildup and replacing the oxygen used by the diver. Rebreathers release few or no gas bubbles into the water, and use much less oxygen per hour because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications. The first modern rebreather was the MK-19 that was developed at S-Tron by Ralph Osterhout that was the first electronic system. Rebreathers are more complex and more expensive than sport open-circuit scuba, and need special training and maintenance to be safely used.
Gas mixtures
For some diving, gas mixtures other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% other) can be used, so long as the diver is properly trained in their use. The most commonly used mixture is Enriched Air Nitrox, which is air with extra oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reducing the likelihood of decompression sickness. The reduced nitrogen may also allow for no or less decompression stop times and a shorter surface interval between dives. A common misconception is that nitrox can reduce narcosis, but research has shown that oxygen is also narcotic.
Several other common gas mixtures are in use, and all need specialized training. The increased oxygen levels in nitrox help fend off decompression sickness, however below the maximum operating depth of the mixture, the increased partial pressure of oxygen can lead to oxygen toxicity. To displace nitrogen without the increased oxygen concentration, other diluents can be used, often helium, when the resultant mixture is called trimix.
In cases of technical dives, some of the cylinders may contain different gas mixture for each 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 air pressure
- For a full list, see Diving hazards and precautions
Divers must avoid injuries caused by changes in air pressure. The weight of the water column above the diver causes an increase in air pressure in any compressible material (wetsuit, lungs, sinus) in proportion to depth, in the same way that atmospheric air causes a pressure of 101.3 kPa (14.7 pounds-force per square inch) at sea level. Pressure injuries are called barotrauma and can be quite painful, in severe cases causing a ruptured eardrum or damage to the sinuses. To avoid them, 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 mask is equalized by periodically exhaling through the nose.
If a drysuit is worn, it too must be equalized by inflation and deflation, similar to a buoyancy compensator.
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 and allowing gases trapped in the bloodstream to gradually break solution and leave the body, called "off-gassing." This is done by making safety stops or 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 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 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. 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 meters). At , Narcosis manifests itself as 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". 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
Oxygen toxicity occurs when oxygen in the body exceeds a safe "partial pressure" (PPO2). In extreme cases it affects the central nervous system and causes a seizure, which can result in the diver spitting out his regulator and drowning. Oxygen toxicity is preventable provided one never exceeds the established maximum depth of a given breathing gas. For deep dives, (generally past 180 feet / 55 meters) "hypoxic blends" containing a lower percentage of oxygen than atmospheric air are used. For more information, see Oxygen toxicity.
Refraction and underwater vision
Water has a higher refractive index than air; it's similar to that of the cornea of the eye. Light entering the cornea from water is hardly refracted at all, leaving only the eye's crystalline lens to focus light. This leads to very severe hypermetropia. People with severe myopia, therefore, can see better underwater without a mask than normal-sighted people.
Diving masks and diving helmets and fullface masks solve this problem by creating an air space in front of the diver's eyes. 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 salt water than they actually are. Therefore total field-of-view is significantly reduced and eye-hand coordination must be adjusted.
(This affects underwater photography: a camera seeing through a flat window in its casing is affected the same as its user's eye seeing through a flat mask window, and so its user must focus for the apparent distance to target, not for the real distance.)
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.
A "double-dome mask" has curved windows in an attempt to cure these faults, but this causes a refraction problem of its own.
On rare occasions, commando frogmen use special contact lenses instead, to see underwater without the large glass surface of a diving mask, which can reflect light and give away the frogman's position.
As a diver changes depth, he must periodically exhale through his nose to equalize the internal pressure of the mask with that of the surrounding water. Swimming goggles which only cover the eyes do not allow for equalization and thus are not suitable for diving.
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, the diver's overall buoyancy determines whether he 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 can be used to adjust the overall buoyancy. When divers want to remain at constant depth, they try to achieve neutral buoyancy. This minimizes gas consumption caused by swimming to maintain depth.
The downward force on the diver is the weight of the diver and his equipment minus the weight of the same volume of the liquid that he is immersed in; if the result is negative, that force is upwards. 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, shrink 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 better than air, which can lead to hypothermia even in mild water temperatures. Symptoms of hypothermia include impaired judgment and dexterity, 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.
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 drysuit, 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. 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.
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: 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: 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 their 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.
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 to dive deeper and longer:
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.
Scuba dive training and certification agencies
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 largest international certification agencies that are currently recognized by most diving outlets for diver certification include:
See also
Reference list
Further reading
- Books published by the British Sub-Aqua Club:
- The Diving Manual, BSAC, ISBN 0-9538919-2-5
- Dive Leading, BSAC, ISBN 0-9538919-4-1
- The Club 1953-2003, BSAC, ISBN 0-9538919-5-X
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External links
- — Diving Emergencies/Hyperbaric Chamber Assistance
- — a large forum and news and information site
- — Online Scuba Diving Information
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