Scuba diving

Scuba diver with air tank

Scuba diving ("scuba" originally being an acronym for Self Contained Underwater Breathing Apparatus, although now widely considered a word in its own right)^[1] is a form of underwater diving in which a diver uses a scuba set to breathe underwater for recreation, commercial or industrial reasons.^[2]

Unlike early diving, which relied exclusively on air pumped from the surface, scuba divers carry their own source of breathing gas (usually compressed air),^[3] allowing them greater freedom than with an air line. Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding techniques as used in snorkelling and free-diving.

According to the purpose of the dive, a diver usually moves underwater by swimfins attached to the feet, but external propulsion can come from an underwater vehicle, or a sled pulled from the surface.

History

Original Aqualung SCUBA set

The first commercially successful scuba sets were the Aqualung open-circuit units 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, and the descendants of these systems are still the most popular units today.

The open circuit systems were developed after Cousteau had a number of incidents of oxygen toxicity using a rebreather system, in which exhaled air is reprocessed to remove carbon dioxide. Modern versions of rebreather systems (both semi-closed circuit and closed circuit) are still available today, and form the second main type of scuba unit, most commonly used for technical diving, such as deep diving.

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.^[3][4][5]

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

Professional diver performing underwater welding

Scuba diving may be performed for a number of reasons, both personal and professional. Most people begin through recreational diving, which is performed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving.

Divers may be employed professionally to perform tasks underwater. Most of these commercial divers are employed to perform tasks related to the running of a business involving deep water, including civil engineering tasks such as in oil exploration, underwater welding or offshore construction. Commercial divers may also be employed to perform tasks specifically related to marine activities, such as naval diving, including the repair and inspection of boats and ships, salvage of wrecks or underwater fishing, like spear fishing.

Other specialist areas of 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 or lifeguard unit.

Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater filming divers, who set out to document the underwater world, or scientific diving, including marine biology and underwater archaeology.

Reasons for diving may include:

Breathing underwater

Scuba diver on reef

Water normally contains the 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.^[3] Although the feasibility of filling and artificially ventilating the lungs with a dedicated liquid (Liquid breathing) has been established for some time,^[6] the size and complexity of the equipment allows only for medical applications with current technology.^[7]

Early diving experimenters quickly discovered it is not enough simply to supply air 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 (14.7 pounds per square inch) for every 33 feet (10 m) 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.^[3]

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.^[2] 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

An Inspiration electronic fully closed circuit rebreather

Less common are closed and semi-closed rebreathers,^[8] which unlike open-circuit sets that vent off all exhaled gases, 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^[2], 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.^[8]

Because the nitrogen in the system is kept to a minimum, decompressing is much less complicated than traditional open-circuit scuba systems and, as a result, divers can stay down longer. Because rebreathers produce very few bubbles, they do not disturb marine life or make a diver’s presence known; this is useful for underwater photography, and for covert work.

Gas mixtures

Nitrox cylinder marked up for use

For some diving, gas mixtures other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% trace gases) can be used,^[2][3] 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.^[9][10]

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.

For 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.

Hazards and dangers

According to a 1970 North American study, diving was (on a man-hours based criteria) 96 times more dangerous than driving an automobile.^[11] According to a 2000 Japanese study, every hour of recreational diving is 36 to 62 times riskier than automobile driving.^[12]

Injuries due to changes in air pressure

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^[3] 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.

If properly equalized, the sinus passages can stand the increased pressure of the water with no problems. However, congestion due to cold, flu or allergies may impair the ability to equalize the pressure. This may result in permanent damage to the eardrum. Although there are many dangers involved in scuba diving, divers can decrease the dangers through proper training and education. Open-water certification programs highlight diving physiology, safe diving practices, and diving hazards.

Effects of breathing high pressure gas

Decompression sickness

The diver must avoid the formation of gas bubbles in the body, called decompression sickness^[3] or 'the bends', by releasing the water pressure on the body slowly while ascending 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.^[13]

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.^[3] 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 66 feet (20 m), 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).^[3] 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 m), divers use "hypoxic blends" containing a lower percentage of oxygen than atmospheric air. For more information, see Oxygen toxicity).

Refraction and underwater vision

A diver wearing an Ocean Reef full face mask

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.^[2] 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.

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, he must periodically exhale through his 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.^[2][14]

Controlling buoyancy underwater

Diver under the Salt Pier in Bonaire.

To dive safely, divers must control their rate of descent and ascent in the water.^[3] 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 or semi-dry suits are used depending on the water temperature) and buoyancy compensators can be used to adjust the overall buoyancy.^[2] 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 displacing; if the result is negative, 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.^[15] Therefore, divers who are neutrally buoyant at one dive destination (e.g. a fresh water lake) will predictably be positively or negatively buoyant at destinations with different water densities (e.g. a tropical coral reef).

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 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 some diving 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 lung volume can be used to adjust buoyancy.

Avoiding losing body heat

Dry suit for reducing exposure

Water conducts heat from the diver 25 times^[16] better than air, which can lead to hypothermia even in mild water temperatures.^[3] Symptoms of hypothermia include impaired judgment and dexterity^[17], 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.^[2]

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")

Spring suit and steamer

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.^[18]

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, coral, or metal debris commonly found on shipwrecks.

Diving longer and deeper safely

There are a number of techniques to increase the diver's ability to dive deeper and longer:

• Technical diving – diving deeper than 40 metres (130 ft), using mixed gases, and/or entering overhead environments (caves or wrecks)
• surface supplied diving – use of umbilical gas supply and diving helmets.^[2]
• 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.^[2]

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

Diving lessons in Monterey Bay, California

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:

• American Canadian Underwater Certifications (ACUC) (formerly Association of Canadian Underwater Councils) – originated in Canada in 1969 and expanded internationally in 1984
• British Sub Aqua Club (BSAC) – based in the United Kingdom, founded in 1953 and is the largest dive club in the world
• European Committee of Professional Diving Instructors (CEDIP) based in Europe since 1992 (see Cedip on French Wiki pages)
• Confédération Mondiale des Activités Subaquatiques (CMAS), the World Underwater Federation
• National Association of Underwater Instructors (NAUI) – based in the United States
• Professional Diving Instructors Corporation (PDIC) – based in the United States
• Professional Association of Diving Instructors (PADI) – based in the United States, largest recreational dive training and certification organization in the world
• Scottish Sub Aqua Club (SSAC or ScotSAC) the National Governing Body for the sport of diving in Scotland.
• International Training SDI, TDI & ERDi -based in the United States, TDI is the world's largest technical diving agency, SDI is the recreational division focusing on new methods and online courses, and ERDi is the public safety component.
• Scuba Schools International (SSI) – based in the United States with 35 Regional Centers and Area Offices around the globe.
• YMCA scuba – based in the U.S., part of Young Men's Christian Association (YMCA), a Christian related organization (open to all faiths, ages and genders despite the historic name); discontinued after 2008

See also

Reference list

Scuba diving, grouped

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
• Free Scuba textbook by George D. Campbell, III called Diving With Deep-Six

External links

• Divers Alert Network—Diving Emergencies/Hyperbaric Chamber Assistance
• Scuba diving travel guide from Wikitravel

Underwater diving Types: Scuba diving · Surface supplied diving · Free-diving · Snorkelling · Saturation diving Specialities: Technical diving · Deep diving · Decompression diving · Mixed gas diving · Wreck diving · Cave diving · Ice diving · Rebreather diving · Solo diving · Altitude diving Equipment: Diving suit · Scuba set · Rebreather · Dive computer · Diver propulsion vehicle · Mask · Fins · Snorkel · Buoyancy control device Disciplines: Professional diving · Police diving · Military diving · Underwater photography · Underwater videography Hazards: Decompression sickness · Nitrogen narcosis · Oxygen toxicity · Barotrauma · Drowning · Shallow water blackout · Deep water blackout · High pressure nervous syndrome · Dysbaric osteonecrosis Related: Hyperbaric medicine