Scuba diving

Scuba diving is a form of underwater diving where the diver uses a self-contained underwater breathing apparatus (scuba) which is completely independent of surface supply, to breathe underwater.^[1] Scuba divers carry their own source of breathing gas, usually compressed air,^[2] allowing them greater independence and freedom of movement than surface-supplied divers, and longer underwater endurance than breath-hold divers.^[1] Open circuit scuba systems discharge the breathing gas into the environment as it is exhaled, and consist of one or more diving cylinders containing breathing gas at high pressure which is supplied to the diver through a regulator. They may include additional cylinders for decompression gas or emergency breathing gas.^[3] Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases. The volume of gas used is reduced compared to that of open circuit; therefore, a smaller cylinder or cylinders, may be used for an equivalent dive duration. Rebreathers extend the time spent underwater compared to open circuit for the same gas consumption, they produce fewer bubbles and less noise than scuba which makes them attractive to covert military divers to avoid detection, scientific divers to avoid disturbing marine animals, and media divers to avoid bubble interference.^[1]

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. Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers.^[4]

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 mask to improve underwater vision, exposure protection, 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.

History

The history of scuba diving is closely linked with the history of scuba equipment. By the turn of the twentieth century, two basic architectures for underwater breathing apparatus had been pioneered; open-circuit surface supplied equipment where the diver's exhaled gas is vented directly into the water, and closed-circuit breathing apparatus where the diver's carbon dioxide is filtered from unused oxygen, which is then recirculated. Closed circuit equipment was more easily adapted to scuba in the absence of reliable, portable, and economical high pressure gas storage vessels. By the mid twentieth century, high pressure cylinders were available and two systems for scuba 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.

Open circuit

These were the first systems that became popular with recreational divers. They were safer than early rebreather systems, less expensive to operate, and allowed dives to greater depths.

An important step for the development of open circuit scuba technology was the invention of the demand regulator. In 1864, the French engineers Auguste Denayrouze and Benoît Rouquayrol designed and patented their "Rouquayrol-Denayrouze diving suit" after adapting a pressure regulator and developing it for underwater use. This would be the first diving suit that could supply air to the diver on demand by adjusting the flow of air from the tank to meet the diver’s breathing and pressure requirements. The system still had to use surface supply, as the cylinders of the 1860s would not have been able to withstand the necessary high pressures.

The first open-circuit scuba system was devised in 1925 by Yves Le Prieur in France. Inspired by the simple apparatus of Maurice Fernez and the freedom it allowed the diver, he conceived an idea to make it free of the tube to the surface pump by using Michelin cylinders as the air supply, containing three litres of air compressed to 150 kilograms per square centimetre (2,100 psi; 150 bar). The "Fernez-Le Prieur" diving apparatus was demonstrated at the swimming pool of Tourelles in Paris in 1926. The unit consisted of a cylinder of compressed air carried on the back of the diver, connected to a pressure regulator designed by Le Prieur adjusted manually by the diver, with two gauges, one for tank pressure and one for output (supply) pressure. Air was supplied continually to the mouthpiece and ejected through a short exhaust pipe fitted with a valve as in the Fernez design,^[5] however, the lack of a demand regulator and the consequent low endurance of the apparatus limited the practical use of LePrieur’s device.

Fernez had previously invented the noseclip, a mouthpiece (equipped with a one-way valve for exhalation) and diving goggles, and Yves le Prieur just joined to those three Fernez elements a hand-controlled regulator and a compressed-air cylinder. Fernez's goggles didn't allow a dive deeper than ten metres due to "mask squeeze", so, in 1933, Le Prieur replaced all the Fernez equipment (goggles, noseclip and valve) by a full face mask, directly supplied with constant flow air from the cylinder.^[5]

In 1942, during the German occupation of France, Jacques-Yves Cousteau and Émile Gagnan designed the first successful and safe open-circuit scuba, known as the Aqua-Lung. Their system combined an improved demand regulator with high-pressure air tanks. Émile Gagnan, an engineer employed by the Air Liquide company, miniaturized and adapted the regulator to use with gas generators, in response to constant fuel shortage that was a consequence of German requisitioning. Gagnan's boss, Henri Melchior, knew that his son-in-law Jacques-Yves Cousteau was looking for an automatic demand regulator to increase the useful period of the underwater breathing apparatus invented by Commander le Prieur,^[6] so he introduced Cousteau to Gagnan in December 1942. On Cousteau's initiative, the Gagnan's regulator was adapted to diving, and the new Cousteau-Gagnan patent was registered some weeks later in 1943.^[7]

Closed circuit

The alternative concept, developed in roughly the same time frame was closed-circuit scuba. The body consumes and metabolises only a small fraction of inhaled oxygen—the situation is even more wasteful of oxygen when the breathing gas is compressed as it is in ambient pressure breathing systems underwater. The rebreather recycles the exhaled breathing gas, while constantly replenishing it from the supply so that the oxygen level does not get depleted. The apparatus also has to remove the exhaled carbon dioxide, as a buildup of CO₂ levels would result in respiratory distress and hypercapnia.

The first commercially practical scuba rebreather was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London.^[8] His 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.^[8][9] This apparatus was first used under operational conditions in 1880 by the lead diver on the Severn Tunnel construction project, who was able to travel 1,000 feet (300 m) in the darkness to close several submerged sluice doors in the tunnel; this had defeated the best efforts of hard hat divers due to the danger of their air supply hoses becoming fouled on submerged debris, and the strong water currents in the workings.^[8]

Fleuss continually improved his apparatus, adding a demand regulator and tanks capable of holding greater amounts of oxygen at higher pressure. Sir Robert Davis, head of Siebe Gorman, improved the oxygen rebreather in 1910^[8][9] with his invention of the Davis Submerged Escape Apparatus, the first rebreather to be made in quantity. While intended primarily as an emergency escape apparatus for submarine crews, it was soon also used for diving, being a handy shallow water diving apparatus with a thirty-minute endurance,^[9] and as an industrial breathing set.

The rig comprised a rubber breathing/buoyancy bag containing a canister of barium hydroxide to scrub exhaled CO₂ and, in a pocket at the lower end of the bag, a steel pressure cylinder holding approximately 56 litres (2.0 cu ft) of oxygen at a pressure of 120 bars (1,700 psi) which was equipped with a control valve and connected to the breathing bag. Opening the cylinder's valve admitted oxygen to the bag at ambient pressure. The rig also included an emergency buoyancy bag on the front of to help keep the wearer afloat. The DSEA was adopted by the Royal Navy after further development by Davis in 1927.^[10]

During the 1930s and all through World War II, the British, Italians and Germans developed and extensively used oxygen rebreathers to equip the first frogmen. The British adapted the Davis Submerged Escape Apparatus and the Germans adapted the Dräger submarine escape rebreathers,^[11] for their frogmen during the war.

The Italians developed similar rebreathers for the combat swimmers of the Decima Flottiglia MAS, especially the Pirelli ARO.^[12] In the U.S. Major Christian J. Lambertsen invented an underwater free-swimming oxygen rebreather in 1939, which was accepted by the Office of Strategic Services.^[13] In 1952 he patented a modification of his apparatus, this time named SCUBA, which became the generic English word for autonomous breathing equipment for diving.^[14] After World War II, military frogmen continued to use rebreathers since they do not make bubbles which would give away the presence of the divers. The high percentage of oxygen used by these early rebreather systems limited the depth at which they could be used.

Post WWII

Air Liquide started selling the Cousteau-Gagnan regulator commercially as of 1946 under the name of scaphandre Cousteau-Gagnan or CG45 ("C" for Cousteau, "G" for Gagnan and 45 for the 1945 patent). The same year Air Liquide created a division called La Spirotechnique, to develop and sell regulators and other diving equipment. To sell his regulator in English-speaking countries Cousteau registered the Aqua-Lung trademark, which was first licensed to the U.S. Divers company (the American division of Air Liquide) and later sold with La Spirotechnique and U.S. Divers to finally become the name of the company, Aqua-Lung/La Spirotechnique, currently located in Carros, near Nice.^[15]

In 1948 the Cousteau-Gagnan patent was also licensed to Siebe Gorman of England,^[16] when Siebe Gorman was directed by Robert Henry Davis.^[17] Siebe Gorman was allowed to sell in Commonwealth countries, but had difficulty in meeting the demand and the U.S. patent prevented others from making the product. This patent was curcumvented by Ted Eldred of Melbourne, Australia, who had been developing a rebreather called the Porpoise. When a demonstration resulted in a diver passing out, he developed the single-hose open-circuit scuba system, which separates the first and second stages by a low-pressure hose, and releases exhaled gas at the second stage.^[18] Eldred sold the first Porpoise Model CA single hose scuba early in 1952.

Early scuba sets were usually provided with a plain harness of shoulder straps and waist belt. The waist belt buckles were usually quick-release, and shoulder straps sometimes had adjustable or quick release buckles. Many harnesses did not have a backplate, and the cylinders rested directly against the diver's back.

Early scuba divers dived without a buoyancy aid.^[19] In an emergency they had to jettison their weights. In the 1960s adjustable buoyancy life jackets (ABLJ) became available, which can be used to compensate for loss of buoyancy at depth due to compression of the neoprene wetsuit and as a lifejacket that will hold an unconscious diver face-upwards at the surface, and that can be quickly inflated. The first versions were inflated from a small disposable carbon dioxide cylinder, later with a small direct coupled air cylinder. A low-pressure feed from the regulator first-stage to an inflation/deflation valve unit lets the volume of the ABLJ be controlled as a buoyancy aid. In 1971 the stabilizer jacket was introduced by ScubaPro. This class of buoyancy aid is known as a buoyancy control device or buoyancy compensator.

Technical diving and the backplate and wing

Technical diving is recreational scuba diving that exceeds the generally accepted recreational limits. Technical diving may expose the diver to hazards beyond those normally associated with recreational diving, and to greater risks of serious injury or death. These risks may be reduced by appropriate skills, knowledge and experience, and by using suitable equipment and procedures. The equipment often involves breathing gases other than air or standard nitrox mixtures, multiple gas sources, and different equipment configurations.^[20] Over time, several aspects of technical diving have become more widely accepted for recreational diving.

A backplate and wing is a type of scuba harness with an attached buoyancy compensation device (BCD) which establishes neutral buoyancy underwater and positive buoyancy on the surface. Unlike most BCDs, the backplate and wing is a modular system, in that it consists of separable components. The core components of this system are the backplate, usually made from metal, which is held against the diver’s back by the harness, and to which the diver’s primary cylinder or cylinders are attached, and inflatable buoyancy bladder known as a wing, sandwiched between the backplate and the cylinder(s), used for adjusting the buoyancy of the diver when in the water. This arrangement clears the front and sides of the diver for other equipment to be attached in the region where it is easily accessible. This additional equipment is usually suspended from the harness or carried in pockets on the exposure suit.^[21][22]

Restricted caves and the sidemount configuration

Sidemount is a scuba diving equipment configuration which has scuba sets mounted alongside the diver, below the shoulders and along the hips, instead of on the back of the diver. It originated as a configuration for advanced cave diving, as it facilitates penetration of tight sections of cave, allows easy access to cylinder valves, provides easy and reliable gas redundancy, and tanks can be easily removed when necessary. These benefits for operating in confined spaces were also recognized by divers who conducted technical wreck diving penetrations.

Sidemount diving is now growing in popularity within the technical diving community for general decompression diving,^[23] and is becoming an increasingly popular specialty training for recreational diving, with several diver certification agencies offering recreational and technical level sidemount training programs.^[24][25][26]

Return of the rebreather

The ready availability of oxygen sensing cells beginning in the late 1980s led to a resurgence of interest in rebreather diving. By accurately measuring the partial pressure of oxygen, it became possible to maintain a breathable gas mixture in the loop at any depth.

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][27][28]

"SCUBA" was originally an acronym, but is now generally used as a common noun or adjective, "scuba".^[29] It has become acceptable to refer to "scuba equipment" or "scuba apparatus"—examples of the linguistic RAS syndrome.

Applications

Scuba diving may be performed for a number of reasons, both personal and professional. Recreational diving is done purely for enjoyment and has a number of technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving.^[30][31][32]

Divers may be employed professionally to perform tasks underwater. Some of these tasks are suitable for scuba.^[1][3][33]

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.^[33]

Other specialist areas of scuba diving include military diving, with a long history of military frogmen in various roles. Their roles include 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.^[33]

Lastly, there are professional divers involved with underwater environment, such as underwater photographers or underwater videographers, who document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology.^[3][33]

The choice between scuba and surface-supplied diving equipment is based on both legal and logistical constraints. Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legal constraints allow. Higher risk work, particularly in commercial diving, may be restricted to surface-supplied equipment by legislation and codes of practice.^[34][33]

Diving activities commonly associated with scuba include:

Type of diving activity	Classification
Aquarium maintenance in large public aquariums	commercial, scientific[33][34]
Boat and ship inspection, cleaning and maintenance	commercial,[33][34] naval[1]
Cave diving	technical, recreational, scientific
Diver training	professional[33]
Fish farm maintenance (aquaculture)	commercial[33][34]
Fishing, e.g. for abalones, crabs, lobsters, scallops, sea crayfish	commercial,[33] recreational
Combat diver, Stealthy infiltration	military[1]
Media diving: making television programmes, etc.	professional[33][34]
Mine clearance and bomb disposal, disposing of unexploded ordnance	military, naval[1]
Pleasure, leisure, sport	recreational
Policing/security: diving to investigate or arrest unauthorised divers	police diving, military, naval[1]
Search and recovery diving	public safety, police, scientific, commercial[33][34]
Search and rescue diving	police, naval, public service[33][34]
Spear fishing	recreational
Surveys and mapping	scientific, commercial,[33][34] recreational
Scientific diving (marine biology, oceanography, hydrology, geology, palaeontology, diving physiology and medicine)	scientific[34]
Underwater archaeology (shipwrecks, harbours, buildings, artefacts and remains)	scientific,[33][34] recreational
Underwater inspections and surveys	commercial,[33] military[1]
Underwater photography and videography	professional,[33][34] recreational
Underwater tour guiding	professional, recreational
Underwater tourism	recreational

Depth range

The depth range applicable to scuba diving depends on the application and training. The major worldwide certification agencies consider 130 feet (40 m) to be the limit for recreation diving. British and European agencies, including BSAC and SAA, recommend a maximum depth of 50 metres (160 ft)^[35] Shallower limits are recommended for divers who are youthful, inexperienced, or who have not taken training for deep dives. Technical diving extends these depth limits through changes to training, equipment, and the gas mix used. The maximum depth considered safe is controversial and varies among agencies and instructors, however, there are programs that train divers for dives to 100 metres (330 ft).

Professional diving usually 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.^[34][33]

Equipment

Breathing apparatus

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 at the surface, 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, which protect the diver's airway if the diver loses consciousness.^[33]

Open-circuit

Open circuit scuba has no provision for using the breathing gas more than once for respiration.^[1] 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. The breathing gas is generally provided from a high-pressure diving cylinder through a scuba regulator. 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.^[36]

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.^[36] The "single-hose" system has significant advantages over the original system for most applications.^[37]

In the "single-hose" two-stage design, the first stage regulator reduces the cylinder pressure of up to about 300 bars (4,400 psi) to an intermediate pressure (IP) of about 8 to 10 bars (120 to 150 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 through a non-return valve on the second stage housing. The first stage typically has at least one outlet port delivering 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.^[37]

Rebreather

Less common are closed circuit (CCR) and semi-closed (SCR) rebreathers 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.^[38]

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.^[38]

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

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.^[39][2]:304

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.^[3]

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.

Diver propulsion

Personal mobility is enhanced by fins and optionally diver propulsion vehicles. Fins have a large blade area and use the more powerful leg muscles, so are much more efficient for propulsion and manoeuvring 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.^[3]

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.^[40]

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.^[41] Some sleds are faired to reduce drag on the diver.^[42]

Buoyancy control

To dive safely, divers must control their rate of descent and ascent in the water^[2] and be able to maintain a constant depth in midwater.^[43] Ignoring other forces such as water currents and swimming, the diver's overall buoyancy determines whether they ascend or descend. Equipment such as 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.^[1] When divers want to remain at constant depth, they try to achieve neutral buoyancy. This minimises the effort of swimming to maintain depth and therefore reduces gas consumption.^[43]

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.^[44] 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).^[43]

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.^[43]

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.^[43]

Neutral buoyancy in a diver is an unstable state. 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. Fine control of buoyancy can be achieved by controlling the average lung volume in open circuit scuba, but this feature is not available to the closed circuit rebreather diver, as exhaled gas remains in the breathing loop. This is a skill which improves with practice until it becomes second nature.^[43]

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. Minimising the volume of gas required in the buoyancy compensator will minimise 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%.^[40]

Underwater vision

Water has a higher refractive index than air – 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

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.

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.^[45]

Cylindrically curved faceplates such as those used for firefighting full-face masks produce severely distorted views underwater.

As a diver descends, they must periodically exhale through their nose to equalise 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 equalisation. Failure to equalise the pressure inside the mask may lead to a form of barotrauma known as mask squeeze.^[1][3]

Masks tend to fog when warm humid exhaled air condenses on the cold inside of the faceplate. To prevent fogging many divers spit into the dry mask before use, spread the saliva around the inside of the glass and rinse it out with a little water. The saliva residue allows condensation to wet the glass and form a continuous film, rather than tiny droplets. There are several commercial products that can be used as an alternative to saliva, some of which are more effective and last longer, but there is a risk of getting the anti-fog agent in the eyes.^[46]

Dive lights

Water attenuates light by selective absorption.^[47][48] Pure water preferentially absorbs red light, and to a lesser extent, yellow and green, so the colour that is least absorbed is blue light.^[49] Dissolved materials may also selectively absorb colour in addition to the absorption by the water itself. In other words, as a diver goes deeper on a dive, more colour is absorbed by the water, and in clean water the colour becomes blue with depth. Colour vision is also affected by turbidity of the water which tends to reduce contrast. Artificial light is useful to provide light in the darkness, and to restore natural colour lost to absorption.

Environmental protection

Protection from heat loss in cold water is usually provided by wet suits or dry suits. These also provide protection from sunburn, abrasion and stings from some marine organisms. Where thermal insulation is not important, lycra suits/diving skins may be sufficient.

Wetsuits

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.

Dry suits

A dry suit provides thermal insulation to the wearer while immersed in water,^{[50][51][52][53]} 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.^[54]

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.^[54]

Monitoring and navigation

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 the duration of diving time that this will safely support, taking into account the time required to surface safely and an allowance for foreseeable contingencies. This is usually monitored by using a submersible pressure gauge on each cylinder.

Safety equipment

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.

Accessories

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 minimise the risk are followed.

Preparation for the dive

The purpose of dive planning is to ensure that divers do not exceed their comfort zone or skill level, or the safe capacity of their equipment, and includes scuba gas planning to ensure that the amount of breathing gas to be carried is sufficient to allow for any reasonably foreseeable contingencies. Before starting a dive both the diver and their buddy do equipment checks to ensure everything is in good working order and available. Recreational divers are responsible for planning their own dives, unless in training, when the instructor is responsible.^[55][56][57] Divemasters may provide useful information and suggestions to assist the divers, but are generally not responsible for the details unless specifically employed to do so. In professional diving teams all team members are usually expected to contribute to planning and to check the equipment they will use, but the overall responsibility for the safety of the team lies with the supervisor.^{[58][34][33][59]}

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.
• Equalisation 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 equalisation procedure, 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 and 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, loss of, or damage to equipment.

Use of repetitive dive tables

Many repetitive dive tables, such as those based on the Bühlmann decompression algorithm and similar, use a Letter Group designation to model the amount of residual nitrogen in the diver's body after a dive. For example, in NAUI dive tables the letters range from A to L, where the letter A represents a small amount of nitrogen and the amount of nitrogen increases as the letters progress to L. At the end of a dive, a Letter Group read from the tables designates the net amount of excess nitrogen absorbed during the dive. As a diver spends time on the surface between dives, excess nitrogen is eliminated (known as "off-gassing"), and the residual nitrogen is assigned to a lower Letter Group.

At the start of a subsequent dive to a given depth, the current Letter Group determines a time penalty representing the residual nitrogen in the diver's body. This residual nitrogen time is subtracted from the time limit indicated by the tables for a dive that carried no penalty. This results in a shorter time limit for the repetitive dive. At the end of the repetitive dive, the residual nitrogen time is added to the actual dive time to determine the total equivalent nitrogen time, which is used to determine the new end-of-dive Letter Group.^[60]

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, team or solo diving

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 understands and can render assistance. Divers are trained to assist in those emergencies specified in the training standards for their certification, and are required to demonstrate competence in a set of prescribed buddy assist skills. The fundamentals of buddy/team safety are centred on diver communication, redundancy of gear and breathing gas by sharing with the buddy, and the added situational perspective of another diver.^[61]

Solo divers take responsibility for their own safety and compensate for the absence of a buddy with skill, vigilance and appropriate equipment. Like buddy or team divers, properly equipped solo divers rely on the redundancy of critical articles of dive gear which may include at least two independent supplies of breathing gas and ensuring that there is always enough available to safely terminate the dive if any one supply fails. The difference between the two practices is that this redundancy is carried and managed by the solo diver instead of a buddy. Agencies that certify for solo diving require candidates to have a high level of dive experience - usually about 100 dives or more.^[62][63]

Since the inception of scuba, there has been ongoing debate regarding the wisdom of solo diving with strong opinions on both sides of the issue. This debate is complicated by the fact that the line which separates a solo diver from a buddy/team diver is not always clear.^[64] For example, should a scuba instructor (who supports the buddy system) be considered a solo diver if their students do not have the knowledge or experience to assist the instructor through an unforeseen scuba emergency? Should the buddy of an underwater photographer consider themselves as effectively diving alone since their buddy (the photographer) is giving most or all of their attention to the subject of the photograph? This debate has motivated some prominent scuba agencies such as Global Underwater Explorers (GUE) to stress that its members only dive in teams and "remain aware of team member location and safety at all time."^[65] Other agencies such as Scuba Diving International (SDI) and Professional Association of Diving Instructors (PADI) have taken the position that divers might find themselves alone (by choice or by accident) and have created certification courses such as the "SDI Solo Diver Course" and the "PADI Self-Reliant Diver Course" in order to train divers to handle such possibilities.^[66][67]

Underwater communication

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.

Emergency 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.

Emergency air sharing

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.

Rescue of an unresponsive diver

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.^[68] 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 minimise 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.^[21]

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 are not safe to breathe at all depths of the dive require specialised safety and emergency procedures tailored to the specific hazards.

Hazards

Divers face specific physical and health risks when they go underwater with scuba equipment.

The presence of a combination of several hazards simultaneously is common in diving, and the effect is generally increased risk to the diver, particularly where the occurrence of an incident due to one hazard triggers other hazards with a resulting cascade of incidents. Many diving fatalities are the result of a cascade of incidents overwhelming the diver, who should be able to manage any single reasonably foreseeable incident.^[69]

Changes in pressure

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 equalises the pressure in all air spaces with the surrounding water pressure when changing depth. The middle ear and sinus are equalised using one or more of several techniques, which is referred to as clearing the ears.

The scuba mask (half-mask) is equalised 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 equalised 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 programmes highlight diving physiology, safe diving practices, and diving hazards, but do not provide the diver with sufficient practice to become truly adept.

Effects of breathing high-pressure gas

Decompression sickness

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. This effect is described by Henry's Law.^[70]

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.^[70] As long as this process is gradual, the tissue gas loading in the diver will reduce 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.^[70]

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 or grow. This is done by controlling the speed of ascent and making periodic stops to allow gases to be eliminated by respiration. 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. Dive computers or decompression tables are used to determine a relatively safe ascent profile, but are not completely reliable. There remains a statistical possibility of decompression bubbles forming even when the guidance from tables or computer has been followed exactly.^[70]

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 the definitive form of first aid for decompression sickness, although death or permanent disability may still occur.^[71]

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 containing nitrogen at depth.^[2] The mechanism is similar to that of nitrous oxide, or "laughing gas," administered as anaesthesia. Being "narced" can impair judgement and make diving very dangerous. Narcosis starts to affect some divers at about 66 feet (20 m) on air. At this depth, narcosis often manifests itself as a slight giddiness. The effects increase with an increase in depth. Almost all divers will notice the effects by 132 feet (40 m). At this depth divers may feel euphoria, anxiety, loss of coordination and/or lack of concentration. At extreme depths, a hallucinogenic reaction, tunnel vision or unconsciousness can occur. Jacques Cousteau famously described it as the "rapture of the deep".^[6] Nitrogen narcosis occurs quickly and the symptoms typically disappear equally quickly during the ascent, so that divers often fail to realise they were ever affected. It affects individual divers at varying depths and conditions, and can even vary from dive to dive under identical conditions. Diving with trimix or heliox reduces the effects, which are proportional to the partial pressure of nitrogen in the breathing gas.

Oxygen toxicity

Oxygen toxicity occurs when the tissues are exposed to an excessive combination of partial pressure (PPO₂) and duration.^[2] In acute 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 not reliably predictable, it is generally recognised that central nervous system oxygen toxicity is preventable if one does not exceed an oxygen partial pressure of 1.4 bar.^[72] For deep dives—generally past 180 feet (55 m), divers use "hypoxic blends" containing a lower percentage of oxygen than atmospheric air. A less immediately threatening form known as pulmonary oxygen toxicity occurs after exposures to lower oxygen partial pressures for much longer periods than generally encountered in scuba diving.

Failure of diving equipment

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.^[73]

The diving environment

Loss of body heat

Water conducts heat from the diver 25 times^[74] better than air, which can lead to hypothermia even in mild water temperatures.^[2] Symptoms of hypothermia include impaired judgment and dexterity,^[75] 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 minimise heat loss. Wetsuits are usually made of foamed neoprene that has small closed bubbles, generally containing 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, function 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 the compressed gas conducts heat better. The second way in which wetsuits can reduce heat loss is to trap the water which leaks into the suit. Body heat then heats the trapped water, and provided the suit is reasonably well-sealed at all openings (neck, wrists, ankles, zippers and overlaps with other suit components), this water remains inside the suit and is not replaced by more cold water, which would also take up body heat, and this helps reduce the rate of heat loss. This principle is applied in the "Semi-Dry" wetsuit.

A dry suit functions by keeping the diver dry. The suit is waterproof and sealed so that water cannot penetrate the suit. Special purpose undergarments are usually worn under a dry suit to keep a layer of air between the diver and the suit for thermal insulation. Some divers carry an extra gas bottle dedicated to filling the dry suit, which may contain argon gas, because it is a better insulator than air.^[76] Dry suits should not be inflated with gases containing helium as it is a good thermal conductor.

Drysuits fall into two main categories:

• Membrane or Shell dry suits are usually a trilaminate or coated textile construction. The material is thin and not a very good insulator, so the insulation is provided by the air trapped in the undersuit.^[54]
• Neoprene drysuits have a similar construction to wetsuits; these are often considerably thicker (7–8 mm) and have sufficient inherent 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, losing some 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.^[54]

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.

Dangerous marine animals

Some marine animals can be hazardous to divers. In most cases this is a defensive reaction to contact with, or molestation by the diver.

• Sharp hard coral skeleton edges can lacerate or abrade exposed skin, and contaminate the wound with coral tissue and pathogenic microorganisms.
• Stinging hydroids can cause skin rash, local swelling and inflammation by contact with bare skin.
• Stinging jellyfish can cause skin rash, local swelling and inflammation, sometimes extremely painful, occasionally dangerous or even fatal
• Stingrays have a sharp spine near the base of the tail which can produce a deep puncture or laceration that leaves venom in the wound as a result of a defensive reaction when disturbed or threatened.
• Some fish and invertebrates such as Lionfish, stonefish, crown of thorns starfish, and some sea urchins have spines which can produce puncture wounds with venom injection. These are often extremely painful and may be fatal in rare cases. Usually caused by impact with the stationary animal.
• The venomous Blue ringed octopus may on rare occasions bite a diver.
• Lacerations by shark teeth can involve deep wounds, loss of tissue and amputation, with major blood loss. In extreme cases death may result.This may result from attack or investigation by a shark with bites. The risk depends on location, conditions, and species. Most sharks do not have suitable teeth for predation on large animals, but may bite in defence when startled or molested.
• Crocodiles can injure by lacerations and punctures by teeth, brute force tearing of tissues. and the possibility of drowning.
• The tropical Indo-Pacific Titan triggerfish is very territorial during breeding season and will attack and bite divers.
• Very large groupers have been known to bite divers, resulting in bite wounds, bruising and crushing injuries. This has been linked to divers feeding fish.^{[77][78][79][80]}
• Electric shock is the defence mechanism of Electric rays, in some tropical to warm temperate seas.
• Venomous sea snakes are a minor hazard in some regions.^[81]

Hazards inherent in the diver

Pre-existing physiological and psychological conditions in the diver

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.^[82]

Diver behaviour and competence

Safety of underwater diving operations can be improved by reducing the frequency of human error and the consequences when it does occur.^[83] Human error can be defined as an individual's deviation from acceptable or desirable practice which culminates in undesirable or unexpected results.^[84] Human error is inevitable and everyone makes mistakes at some time. The consequences of these errors are varied and depend on many factors. Most errors are minor and do not cause significant harm, but others can have catastrophic consequences. Human error and panic are considered to be the leading causes of dive accidents and fatalities.^[83]

• 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.^[85]
• Use of inappropriate equipment and/or configuration can lead to a whole range of complications, depending on the details.

The dive task and special equipment

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.^[86]

Risk

The risks of dying during recreational, scientific or commercial diving are small, and on scuba, deaths are usually associated with poor gas management, poor buoyancy control, equipment misuse, entrapment, rough water conditions and pre-existing health problems. Some fatalities are inevitable and caused by unforeseeable situations escalating out of control, but the majority of diving fatalities can be attributed to human error on the part of the victim.^[87]

Equipment failure is rare in open circuit scuba, and while the cause of death is commonly recorded as drowning, this is mainly the consequence of an uncontrollable series of events taking place in water. Air embolism is also frequently cited as a cause of death, and it, too is the consequence of other factors leading to an uncontrolled and badly managed ascent, possibly aggravated by medical conditions. About a quarter of diving fatalities are associated with cardiac events, mostly in older divers. There is a fairly large body of data on diving fatalities, but in many cases the data is poor due to the standard of investigation and reporting. This hinders research which could improve diver safety.^[87]

According to death certificates, over 80% of the deaths were ultimately attributed to drowning, but other factors usually combined to incapacitate the diver in a sequence of events culminating in drowning, which is more a consequence of the medium in which the accidents occurred than the actual accident. Often the drowning obscures the real cause of death. Scuba divers should not drown unless there are other contributory factors as they carry a supply of breathing gas and equipment designed to provide the gas on demand. Drowning occurs as a consequence of preceding problems, such as cardiac disease, pulmonary barotrauma, unmanageable stress, unconsciousness from any cause, water aspiration, trauma, equipment difficulties, environmental hazards, inappropriate response to an emergency or failure to manage the gas supply.^[88]

Fatality rates are comparable with jogging (13 deaths per 100,000 persons per year) and are within the range where reduction is desirable by Health and Safety Executive (HSE) criteria,^[89] The most frequent root cause for diving fatalities is running out of or low on gas. Other factors cited include buoyancy control, entanglement or entrapment, rough water, equipment misuse or problems and emergency ascent. The most common injuries and causes of death were drowning or asphyxia due to inhalation of water, air embolism and cardiac events. Risk of cardiac arrest is greater for older divers, and greater for men than women, although the risks are equal by age 65.^[89]

Several plausible opinions have been put forward but have not yet been empirically validated. Suggested contributing factors included inexperience, infrequent diving, inadequate supervision, insufficient predive briefings, buddy separation and dive conditions beyond the diver's training, experience or physical capacity.^[89]

Based on actual exposure time, according to a 1970 North American study, diving was 96 times more dangerous than driving an automobile.^[90] and according to a 2000 Japanese study, 36 to 62 times riskier than driving.^[91] A difference between the risks of driving and diving is that the diver is less at risk from fellow divers than the driver is from other drivers.

Decompression sickness and arterial gas embolism in recreational diving are associated with certain demographic, environmental, and dive style factors. A statistical study published in 2005 tested potential risk factors: age, gender, body mass index, smoking, asthma, diabetes, cardiovascular disease, previous decompression illness, years since certification, dives in last year, number of diving days, number of dives in a repetitive series, last dive depth, nitrox use, and drysuit use. No significant associations with decompression sickness or arterial gas embolism were found for asthma, diabetes, cardiovascular disease, smoking, or body mass index. Increased depth, previous DCI, days diving, and being male were associated with higher risk for decompression sickness and arterial gas embolism. Nitrox and drysuit use, greater frequency of diving in the past year, increasing age, and years since certification were associated with lower risk, possibly as indicators of more extensive training and experience.^[92]

Risk management

Risk management has three major aspects: Risk assessment, emergency planning and insurance cover. The risk assessment for a dive is primarily a planning activity, and may range in formality from a part of the pre-dive buddy check for recreational divers, to a safety file with professional risk assessment and detailed emergency plans for professional diving projects. Some form of pre-dive briefing is customary with organised recreational dives, and this generally includes a recitation by the divemaster of the known and predicted hazards, the risk associated with the significant ones, and the procedures to be followed in case of the reasonably foreseeable emergencies associated with them. Insurance cover for diving accidents may not be included in standard policies. There are a few organisations which focus specifically on diver safety and insurance cover, such as the international Divers Alert Network^[93]

Scuba diver training and certification

Recreational scuba diving does not have a centralised certifying or regulatory agency, and is mostly self regulated. There are, however, several large diving organisations 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 organisations prior to selling or renting certain diving products or services.

The following organisations publish standards for competence in recreational diving skills and knowledge:

• CMAS - Confédération Mondiale des Activités Subaquatiques
• EUF - European Underwater Federation
• WRSTC - World Recreational Scuba Training Council

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 or a personal decompression computer.

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 between the water and the shore or boat.
• Breathing from the demand valve
• Recovering and clearing the demand valve.
• Clearing water from the mask, and replacing a dislodged mask.
• Buoyancy control using weights and [[Buoyancy compensator {diving}|buoyancy compensator]].
• Finning techniques, underwater mobility and maneuvering.
• Making safe and controlled descents and ascents.
• Equalisation of the ears and other air spaces.
• Assisting another diver by providing air from one's own supply, or receiving air supplied by another diver.
• How to return to the surface without injury in the event of a breathing supply interruption.
• Use of bailout systems (professional divers).
• Diving signals used to communicate underwater. Professional divers will also learn other methods of communication.
• Dive management skills such as monitoring depth and time and the breathing gas supply
• Buddy diving procedures.

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 specialised 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.^[94]

See also

References

Further reading

• Cousteau J.Y. (1953) Le Monde du Silence, translated as The Silent World, Hamish Hamilton Ltd., London; ASIN B000QRK890
• Ellerby D. (2002) The Diving Manual, British Sub-Aqua Club (BSAC); ISBN 0-9538919-2-5
• Dive Leading, BSAC; ISBN 0-9538919-4-1
• The Club 1953–2003, BSAC; ISBN 0-9538919-5-X
• Richardson D. (2008) Open Water Diver Manual, PADI; ASIN B004JZYO0E

External links

Wikimedia Commons has media related to Scuba diving.

Wikivoyage has a travel guide for Scuba diving.