Underwater diving is the practice of descending below the water's surface to conduct underwater activities. In ambient pressure diving, the diver is exposed to the pressure of the surrounding water, and uses breathing apparatus such as that used for scuba diving or surface supplied diving, or when freediving, will breath-hold for the duration of the dive. The saturation diving technique can be used to reduce the risk of decompression sickness after long duration deep dives. Atmospheric diving suits may be used to isolate the diver from the effects of high ambient pressure. Although not usually considered to be diving, the use of crewed submersibles can extend the range of possible diving depths, and remotely controlled and robotic diving machines can be used where the risks to human divers are unacceptable or the logistics are impractical.
The diving environment exposes the diver to a wide range of hazards, and though the risks are largely controlled by appropriate diving skills, training, types of equipment and breathing gases used depending on the mode, depth and purpose of diving, it remains a relatively dangerous activity. Diving activities are restricted to relatively shallow depths ranging from around 40 m (130 ft.) maximum for recreational Scuba diving to commercial saturation diving maximum around 534 metres (1,752 ft) and 610 metres (2,000 ft) wearing atmospheric diving suits. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable to the diver can vary considerably. Occasionally diving may be done in liquids other than water.
Recreational diving (also called sport diving or subaquatics) is a popular leisure activity . Technical diving is a form of recreational diving that achieves greater depths or endurance or addresses more challenging conditions than found in normal recreational diving. Professional diving (commercial diving, diving for scientific research purposes or diving for financial gain) takes a range of diving activities to the underwater work site. Public safety diving is the underwater work done by law enforcement, fire rescue, and search & rescue/recovery dive teams, and may be done by professionals or volunteers. Military diving includes combat diving, clearance diving and ship's husbandry diving. Underwater sports is a group of competitive sports using either free-diving, snorkeling or scuba technique, or a combination of these techniques. The term "deep sea diving" refers to underwater diving, usually with surface supplied equipment, and often refers specifically to the use of standard diving dress with the traditional copper helmet. "Hard hat" diving is any form of diving with a helmet, including the standard copper helmet, and other forms of free-flow and lightweight demand helmets.
- 1 Diving modes
- 2 Reasons for diving
- 3 History
- 3.1 Free-diving
- 3.2 Diving bells
- 3.3 Diving suits
- 3.4 Standard diving dress
- 3.5 Development of salvage diving operations
- 3.6 Self-contained air supply equipment
- 3.7 Scuba systems during World War II
- 3.8 Saturation diving
- 3.9 Atmospheric diving suits
- 3.10 Physiological discoveries
- 4 Dive sites
- 5 Underwater divers
- 6 Physiological aspects of diving
- 7 Risks and safety
- 8 Diving by other animals
- 9 See also
- 10 References
- 11 Further reading
- 12 External links
There are several modes of diving based on the diving equipment used.
The ability to dive and swim underwater while holding one's breath can be a useful emergency skill, is an important part of water sport and Navy safety training, and is also considered to be an enjoyable leisure activity. Underwater diving without breathing apparatus can be loosely categorized as underwater swimming, snorkeling and free-diving. These categories overlap considerably. Several competitive underwater sports are practiced without breathing apparatus.
Free-diving does not involve the use of external breathing devices, but relies on a diver's ability to hold his or her breath until resurfacing. It includes a range of activities from simple breath-hold diving to competitive apnea dives. Fins and a diving mask are often used in free diving to improve vision and provide more efficient propulsion. The use of a short breathing tube known as a snorkel allows the diver to breathe with the face remaining immersed, while at the surface. This may also be used when no diving is intended, and the snorkeler remains at the surface.
Scuba diving is diving with a self-contained underwater breathing apparatus, which is completely independent of surface supply, and provides the diver with the advantages of mobility and horizontal range far beyond what is possible when supplied from the surface by the umbilical hoses of surface-supplied diving equipment (SSDE). Scuba divers engaged on armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers. There are two main classes of scuba distinguished by how the breathing gas is used:
- 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 diving regulator. They may include additional cylinders for decompression gas or emergency breathing gas.
- Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases. This reduces the volume of gas used compared with open circuit, so that a smaller cylinder, or cylinders, may be used for the equivalent dive duration. They provide the capacity to spend far more time underwater compared to open circuit for the same gas consumption. Rebreathers also produce far less bubble volume and less noise than scuba. This makes them attractive to covert military divers, who wish to avoid detection, scientific divers who do not want to disturb the marine animals, and media divers who do not want their work obscured by bubbles.
Surface supplied diving
An alternative to self-contained breathing systems is to supply breathing gases from the surface through a hose. When this is combined with a communications cable, a pneumofathometer line and a safety line, with options of a hot water hose for heating, a video cable and a gas reclaim line this is called the diver's umbilical. More basic equipment may only use an air hose, and is called an airline or hookah system.
- Saturation diving lets professional divers live and work under pressure for days or weeks at a time. After working in the water, the divers rest and live in a dry pressurized underwater habitat on the bottom or a saturation life support system of pressure chambers on the deck of a diving support vessel, oil platform or other floating platform at a similar pressure to the work depth. They are transferred between the surface accommodation and the underwater workplace in a pressurised closed diving bell, also known as a personnel transfer capsule. Decompression at the end of the dive may take many days, but since it is done only once for a long period of exposure, rather than after each of many shorter exposures, the overall risk of decompression injury to the diver and the total time spent decompressing are reduced. This type of diving allows greater economy of work and enhanced safety.
- Surface oriented, or bounce diving, is how commercial divers refer to diving operations where the diver starts and finishes the diving operation at atmospheric pressure. The diver may be deployed directly, often from a diving support vessel or indirectly via a diving bell. Surface-supplied divers almost always wear diving helmets or full face diving masks. The bottom gas can be air, nitrox, heliox or trimix, the decompression gases may be similar, or may include pure oxygen. Decompression procedures include in-water decompression or surface decompression in a deck chamber.
- A wet bell with a gas filled dome provides more comfort and control than a stage and allows for longer time in water. Wet bells are used for air and mixed gas, and divers can decompress on oxygen at 12 m. Small closed bell systems have been designed that can be easily mobilized, and include a two-man bell, a handling frame and a chamber for decompression after transfer under pressure (TUP). Divers can breathe air or mixed gas at the bottom but are usually recovered with the chamber filled with air. They decompress on oxygen supplied through built in breathing systems (BIBS) towards the end of the decompression. Small bell systems support bounce diving down to 120 m and for bottom times up to 2 hours.
- A relatively portable surface gas supply system using high pressure gas cylinders for both primary and reserve gas, but using the full diver's umbilical system with pneumofathometer and voice communications is known in the industry as Scuba replacement.
- Another alternative to scuba diving, called, "airline" or "hookah" diving, has the diver supplied via an air supply hose from a cylinder or compressor at the surface. Breathing gas is supplied through a mouth-held demand valve or light full-face mask. It is used for light work such as hull cleaning and archaeological surveys, for shellfish harvesting, and as "Snuba", which is a shallow water activity typically used by tourists and those who are not scuba-certified.
- Surface-supplied diving#Compressor diving is a rudimentary method of surface-supplied diving used in some tropical regions such as the Philippines and the Caribbean. The divers swim with a half mask and fins and are supplied with air from an industrial low-pressure air compressor on the boat through plastic tubes. There is no reduction valve; the diver holds the hose end in his mouth with no demand valve or mouthpiece and allows excess air to spill out between the lips.
Atmospheric pressure diving
Submersibles and 'hard' atmospheric diving suits enable diving to be carried out in a dry environment at normal atmospheric pressure. An atmospheric diving suit (ADS) is a small one-person articulated submersible of anthropomorphic form which resembles a suit of armour, with elaborate joints to allow articulation while maintaining an internal pressure of one atmosphere. An ADS can be used for deeper dives of up to about 2,300 feet (700 m) for many hours, and eliminates the majority of physiological dangers associated with deep diving; the occupant need not decompress, there is no need for special gas mixtures, and there is no danger of nitrogen narcosis, at the expense of higher cost, complex logistics and loss of dexterity.
Robotic autonomous underwater vehicles and remotely operated underwater vehicles also carry out some functions of divers. They can be deployed at greater depths and in more dangerous environments. An autonomous underwater vehicle (AUV) is a robot which travels underwater without requiring input from an operator. AUVs constitute part of a larger group of undersea systems known as unmanned underwater vehicles, a classification that includes non-autonomous remotely operated underwater vehicles (ROVs) – controlled and powered from the surface by an operator/pilot via an umbilical or using remote control. In military applications AUVs are more often referred to simply as unmanned undersea vehicles (UUVs).
Reasons for diving
Diving may be done for a number of reasons, both personal and professional. Recreational diving is 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.
Underwater sport is also done for enjoyment and includes specific sports such as aquathlon (i.e. underwater wrestling), finswimming, free-diving, spearfishing, sport diving, underwater football, underwater hockey, underwater ice hockey, underwater orienteering, underwater photography, underwater rugby, underwater target shooting and underwater videography. Many of these underwater sports can be enjoyed simply for exercise and the associated health benefits, for true recreation, or competition at varying levels.
There are various aspects of professional diving that range anywhere from part-time work to lifelong careers. Professionals in the recreational dive industry include instructor trainers, dive instructors, assistant instructors, divemasters, dive guides, and scuba technicians. Commercial diving is industry related and includes civil engineering tasks such as in oil exploration, offshore construction dam maintenance and harbour works. 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 aquaculture.
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, 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 safety diving. Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater film makers, who set out to document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology. In North Carolina wreck divers regularly visit the WWII shipwrecks to dive with sand tiger sharks that make the wrecks their home.
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 commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice.
|Diving activities||Classification||Diving modes used|
|aquarium maintenance in large public aquariums||commercial, scientific||Scuba, SSDE|
|boat and ship inspection, cleaning and maintenance||commercial, naval||SSDE, occasionally scuba|
|cave diving||technical, recreational, scientific||Scuba, occasionally SSDE|
|civil engineering in harbours, water supply, and drainage systems||commercial||Almost exclusively SSDE|
|offshore construction and maintenance in the crude oil and other industry ||commercial||SSDE, ROV, occasionally atmospheric suit|
|demolition and salvage of ship wrecks||commercial, naval||SSDE, sometimes scuba, atmospheric suit or ROV|
|professional diver training||professional||SSDE or scuba as appropriate|
|recreational diver training||professional, recreational||Scuba, breathhold|
|fish farm and other aquaculture maintenance||commercial||Scuba, SSDE|
|fishing/gathering, e.g. for abalones, crabs, lobsters, pearls, scallops, sea crayfish, sponges||commercial, recreational||Scuba, SSDE, breathhold|
|frogman, manned torpedo||military||Scuba|
|harbour clearance and maintenance||commercial, military||Almost exclusively SSDE|
|media diving: making television programs, underwater videography, underwater photography etc.||professional, recreational||Scuba, ROV, occasionally SSDE|
|mine clearance and bomb disposal, disposing of unexploded ordnance||military, naval||Scuba, ROV, occasionally SSDE|
|pleasure, leisure, sport||recreational||Scuba, breathhold, occasionally SSDE (Snuba)|
|policing/security: diving to investigate or arrest unauthorized divers||police diving, military, naval||Scuba|
|search and recovery diving||commercial, public safety, police diving, military||Scuba, SSDE, ROV|
|search and rescue diving||police, naval, public service||Scuba, occasionally SSDE|
|spear fishing||recreational(sometimes competitive), occasionally professional,||Breathhold|
|surveys and mapping||scientific, recreational||Scuba, SSDE|
|scientific diving (marine biology, oceanography, hydrology, geology, palaeontology, diving physiology and medicine||scientific ||Scuba, occasionally SSDE, atmospheric suit, ROV, AUV|
|underwater archaeology (shipwrecks; harbors, and buildings)||scientific, recreational||Scuba, SSDE, ROV, occasionally atmospheric suit|
|underwater inspections and surveys||commercial, military||SSDE, sometimes scuba,ROV|
|underwater mineral extraction (gold, diamonds, oil)||commercial||SSDE, including airline (Hookah) and saturation, ROV|
|competitive underwater sport, including underwater hockey, aquathlon, scuba orienteering, underwater rugby and others||recreational||Snorkel, breathhold and scuba|
|underwater tour guiding||professional, recreational||Scuba|
|underwater tourism||recreational||Scuba, occasionally Snuba|
|underwater welding||commercial||Almost exclusively SSDE|
Underwater diving was practised in ancient cultures to reclaim sunken valuables, and to help aid military campaigns. In ancient times free diving without the aid of mechanical devices was the only possibility, with the exception of the occasional use of reeds and possibly leather breathing bladders.
Underwater diving for commercial, rather than recreational purposes may have begun in Ancient Greece, since both Plato and Homer mention the sponge as being used for bathing. The island of Kalymnos was a main centre of diving for sponges. By using weights (skandalopetra) of as much as 15 kilograms (33 lb) to speed the descent, breath-holding divers would descend to depths up to 30 metres (98 ft) for as much as five minutes to collect sponges. Sponges weren't the only valuable harvest to be found on the sea floor; the harvesting of red coral was also quite popular. A variety of valuable shells or fish could be harvested in this way creating a demand for divers to harvest the treasures of the sea, which could also include the sunken riches of other seafarers.
The Mediterranean had large amounts of sea trade. As a result, there were many shipwrecks, so divers were often hired to salvage whatever they could from the seabed. Divers would swim down to the wreck and choose the pieces to salvage.
Divers were also used in warfare. Defenses against sea vessels were often created, such as underwater barricades aimed at sinking enemy ships. As the barricades were hidden under the water, divers were often used to scout out the sea bed when ships were approaching an enemy harbor. Once these barricades were found it was divers who were used to disassemble them, if possible. During the Peloponnesian War, divers were used to get past enemy blockades to relay messages as well as supplies to allies or troops that were cut off by the blockade. On top of all that these ancient frogmen were used as saboteurs, drilling holes in enemy hulls, cutting ships rigging and mooring. Free-diving was the primary source of income for many Gulf nationals such as Qataris, Emiratis, and Bahrainis and Kuwaitis. As a result, Qatari, Emirati and Bahraini heritage promoters have popularized recreational and serious events associated with freediving, underwater equipment and related activities such as snorkeling.
Diving bells were developed in the 16th and 17th century as the first significant mechanical aid to underwater diving. They were rigid chambers lowered into the water and ballasted so as to remain upright in the water and to be negatively buoyant so that it sinks even when full of air.
Bells were often used for salvage work. In 1658, Albrecht von Treileben was contracted by King Gustavus Adolphus of Sweden to salvage the warship Vasa, which sank in Stockholm harbor on its maiden voyage in 1628. Between 1663 and 1665 von Treileben's divers were successful in raising most of the cannon, working from a diving bell. In 1687, Sir William Phipps used an inverted container to recover £200,000-worth of treasure from a Spanish ship sunk off the coast of San Domingo.
In 1691, Dr. Edmond Halley completed plans for a greatly improved diving bell, capable of remaining submerged for extended periods of time, and fitted with a window for the purpose of undersea exploration. The atmosphere was replenished by way of weighted barrels of air sent down from the surface. In a demonstration, Halley and five companions dived to 60 feet (18 m) in the River Thames, and remained there for over an hour and a half. Improvements made to it over time, extended his underwater exposure time to over 4 hours.
In 1775, Charles Spalding, an Edinburgh confectioner, improved on Dr. Halley's design by adding a system of balance-weights to ease the raising and lowering of the bell, along with a series of ropes for signaling to the surface crew.
In 1689, Denis Papin had suggested that the pressure and fresh air inside a diving bell could be maintained by a force pump or bellows. His idea was implemented exactly 100 years later by the engineer John Smeaton who built the first workable diving air pump in 1789.
The next advance in diving technology, was the first diving dress designs in the early 18th century. Two English inventors developed the first pressure-proof diving suits in the 1710s. John Lethbridge built a completely enclosed suit to aid in salvage work. It consisted of a pressure-proof air-filled barrel with a glass viewing hole and two watertight enclosed sleeves.
After testing this machine in his garden pond (specially built for the purpose) Lethbridge dived on a number of wrecks: four English men-of-war, one East Indiaman, two Spanish galleons and a number of galleys. He became very wealthy as a result of his salvages. One of his better-known recoveries was on the Dutch Slot ter Hooge, which had sunk off Madeira with over three tons of silver on board.
At the same time, Andrew Becker created a leather-covered diving suit with a windowed helmet. The suit used a system of tubes for inhaling and exhaling, and Becker demonstrated his suit in the River Thames, London, during which he remained submerged for an hour. These suits were of limited use as there was still no practical system for replenishing the air supply during the dive.
Standard diving dress
The first successful diving helmets were produced by the brothers Charles and John Deane in the 1820s. Inspired by a fire accident he witnessed in a stable in England, he designed and patented a "Smoke Helmet" to be used by firemen in smoke-filled areas in 1823. The apparatus comprised a copper helmet with an attached flexible collar and garment. A long leather hose attached to the rear of the helmet was to be used to supply air - the original concept being that it would be pumped using a double bellows. A short pipe allowed excess air to escape. The garment was constructed from leather or airtight cloth, secured by straps.
The brothers had insufficient funds to build the equipment themselves, so they sold the patent to their employer, Edward Barnard. It was not until 1827 that the first smoke helmets were built, by German-born British engineer Augustus Siebe. In 1828 they decided to find another application for their device and converted it into a diving helmet. They marketed the helmet with a loosely attached "diving suit" so that a diver could perform salvage work but only in a full vertical position, otherwise water entered the suit.
In 1829 the Deane brothers sailed from Whitstable for trials of their new underwater apparatus, establishing the diving industry in the town. In 1834 Charles used his diving helmet and suit in a successful attempt on the wreck of Royal George at Spithead, during which he recovered 28 of the ship's cannon. In 1836, John Deane recovered from the Mary Rose shipwreck timbers, guns, longbows, and other items.
By 1836 the Deane brothers had produced the world's first diving manual, Method of Using Deane's Patent Diving Apparatus which explained in detail the workings of the apparatus and pump, plus safety precautions.
In the 1830s the Deane brothers asked Augustus Siebe to apply his skills to improve their underwater helmet design. Expanding on improvements already made by another engineer, George Edwards, Siebe produced his own design; a helmet fitted to a full length watertight canvas diving suit.
Siebe introduced various modifications on his diving dress design to accommodate the requirements of the salvage team on the wreck of the HMS Royal George, including making the bonnet of the helmet detachable from the corselet. his improved design gave rise to the typical standard diving dress which revolutionised underwater civil engineering, underwater salvage, commercial diving and naval diving.
Development of salvage diving operations
Royal George, a 100-gun first-rate ship of the line of the Royal Navy, sank undergoing routine maintenance work in 1782. Charles Spalding used a diving bell to recover six iron 12-pounder guns and nine brass 12-pounders in the same year. In 1839 Major-General Charles Pasley, at the time a Colonel of the Royal Engineers, commenced operations. Pasley had previously destroyed some old wrecks in the Thames.His operation set many diving milestones, including the first recorded use of the buddy system in diving, when he ordered that his divers operate in pairs. The Deane brothers were commissioned to perform salvage work on the wreck. Using their new air-pumped diving helmets, they managed to recover about two dozen cannons.
Following on from this success, Colonel of the Royal Engineers Charles Pasley commenced large-scale salvage operations in 1839. His plan was to break up the wreck of Royal George with gunpowder charges and then salvage as much as possible using divers.
Pasley's diving salvage operation set many diving milestones, including the first recorded use of the buddy system in diving, when he gave instructions to his divers to operate in pairs. In addition, the first emergency swimming ascent was made by a diver after his air line became tangled and he had to cut it free. A less fortunate milestone was the first medical account of a diver squeeze suffered by a Private Williams: the early diving helmets used had no non-return valves; this meant that if a hose became severed, the high-pressure air around the diver's head rapidly evacuated the helmet causing tremendous negative pressure that caused extreme and sometimes life-threatening effects. At the British Association for the Advancement of Science meeting in 1842, Sir John Richardson described the diving apparatus and treatment of diver Roderick Cameron following an injury that occurred on 14 October 1841 during the salvage operations.
Pasley recovered 12 more guns in 1839, 11 more in 1840, and 6 in 1841. In 1842 he recovered only one iron 12-pounder because he ordered the divers to concentrate on removing the hull timbers rather than search for guns. Other items recovered, in 1840, included the surgeon's brass instruments, silk garments of satin weave 'of which the silk was perfect', and pieces of leather; but no woollen clothing. By 1843 the whole of the keel and the bottom timbers had been raised and the site was declared clear.
Self-contained air supply equipment
A drawback to the equipment pioneered by Deane and Siebe was the requirement for a constant supply of air pumped from the surface. This restricted the movements and range of the diver and was also potentially hazardous as the supply could get cut off for a number of reasons. Early attempts at creating systems that would allow divers to carry a portable oxygen source did not succeed, as the compression and storage technology was not advanced enough to allow compressed air to be stored in containers at sufficiently high pressures. By the end of the nineteenth century, two basic templates for scuba, (self-contained underwater breathing apparatus), had emerged; open-circuit scuba where the diver's exhaust is vented directly into the water, and closed-circuit scuba where the diver's unused oxygen is filtered from the carbon dioxide and recirculated.
The first important step in the development of scuba technology was the invention of the demand regulator. In 1864, the French engineers Auguste Denayrouze and Benoît Rouquayrol designed their "Rouquayrol-Denayrouze diving suit" after adapting the pressure regulator mechanism for underwater use. Their suit was the first to supply 'air-on-demand' to the user by adjusting the flow of air from the tank according to the diver's requirements. However, the system still had to use surface supply, as the cylinders of the 1860s were not 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 immediately 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 kg/cm2. 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. However, the lack of a demand regulator and the low endurance of the apparatus, limited the practical use of LePrieur's device.:1–9
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, 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. After the war, in 1946, both men founded La Spirotechnique (as a division of Air Liquide) in order to mass-produce and sell their invention, this time under a new 1945 patent, and known as CG45 ("C" for Cousteau, "G" for Gagnan and "45" for 1945). This same CG45 regulator, produced for more than ten years and commercialized in France as of 1946, was the first to actually be called the "Aqua-Lung".
The alternative concept, developed in roughly the same time frame was the closed-circuit scuba. The body consumes and metabolises a small fraction of the inhaled oxygen - the situation is even more wasteful of oxygen when the breathing gas is compressed as it is in scuba systems underwater. The rebreather therefore recycles the used oxygen, while constantly replenishing it from the supply so that the oxygen level does not get depleted. The apparatus also has to chemically remove the exhaled carbon dioxide, as a buildup of CO2 levels would result in respiratory distress and hypercapnia.
The first commercially practical closed-circuit scuba was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London. 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.
His apparatus was first used under operational conditions in 1880 by the lead diver on the Severn Tunnel construction project, who was able to travel 1000 feet 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.
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, perfected the oxygen rebreather in 1910 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, 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 of oxygen at a pressure of 120 bar. The cylinder was equipped with a control valve and was connected to the breathing bag. Opening the cylinder's valve admitted oxygen to the bag and charged it to the pressure of the surrounding water. 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.
Scuba systems during World War II
In the 1930s, Italian sport spearfishers began to use the Davis rebreather; Italian manufacturers received a license from the English patent holders to produce it. This practice soon came to the attention of the Italian Navy, which developed its frogman unit Decima Flottiglia MAS and was used effectively in World War II.
During the Second World War, captured Italian frogmen's rebreathers influenced improved designs for British rebreathers. Many British frogmen's breathing sets used aircrew breathing oxygen cylinders salvaged from shot-down German Luftwaffe aircraft. The earliest of these breathing sets may have been modified Davis Submerged Escape Apparatus; their fullface masks were the type intended for the Siebe Gorman Salvus, but in later operations different designs were used, leading to a fullface mask with one big face window, at first oval and later rectangular (mostly flat, but the sides curved back to allow better vision sideways). Early British frogman's rebreathers had rectangular counterlungs on the chest like Italian frogman's rebreathers, but later designs had a square recess in the top of the counterlung so it could extend further up toward the shoulders. In front they had a rubber collar that was clamped around the absorbent canister. Some British armed forces divers used bulky thick diving suits called Sladen suits; one version of it had a flip-up single faceplate for both eyes to let the user get binoculars to his eyes when on the surface.
The first intentional saturation dive was done on December 22, 1938, by Edgar End and Max Nohl who spent 27 hours breathing air at 101 feet (30.8 m) in the County Emergency Hospital recompression facility in Milwaukee, Wisconsin. Their decompression lasted five hours leaving Nohl with a mild case of decompression sickness that resolved with recompression.
Albert R. Behnke proposed exposing divers to raised ambient pressures long enough for the tissues to saturate with inert gases in 1942. In 1957, George F. Bond began the Genesis project at the Naval Submarine Medical Research Laboratory proving that humans could withstand prolonged exposure to different breathing gases and increased environmental pressures. Once saturation is achieved, the amount of time needed for decompression depends on the depth and gases breathed and is not affected by longer exposure.
Atmospheric diving suits
The atmospheric diving suit is a small one-man submersible of anthropomorphic form with elaborate pressure joints to allow articulation while maintaining an internal pressure of one atmosphere. Although atmospheric suits were developed during the Victorian era, none of these suits were able to overcome the basic design problem of constructing a joint which would remain flexible and watertight at depth without seizing up under pressure.
Pioneering diving engineer, Joseph Salim Peress, invented the first truly usable atmospheric diving suit, the Tritonia, in 1932 and was later involved in the construction of the famous JIM suit. Having a natural talent for engineering design, he challenged himself to construct an ADS that would keep divers dry and at atmospheric pressure, even at great depth. By 1929 he solved the weight problem by using cast magnesium instead of steel. He also managed to improve the design of the suit's joints by using a trapped cushion of oil to keep the surfaces moving smoothly. In 1930, Peress revealed the Tritonia suit. By May it had completed trials and was publicly demonstrated in a tank at Byfleet. In September Peress' assistant Jim Jarret dived in the suit to a depth of 123 m (404 ft) in Loch Ness. The suit performed perfectly, the joints proving resistant to pressure and moving freely even at depth.
The Tritonia suit was upgraded into the first JIM suit, completed in November 1971. This suit underwent trials aboard HMS Reclaim in early 1972, and in 1976, the JIM suit set a record for the longest working dive below 490 feet (150 m), lasting five hours and 59 minutes at a depth of 905 feet (276 m). The first JIM suits were constructed from cast magnesium for its high strength-to-weight ratio and weighed approximately 1,100 pounds (498.95 kg) in air including the diver.
The Exosuit is a new, state-of-the-art atmospheric diving suite that will be first used in 2014 at the Bluewater and Antikythera underwater research expeditions. The Exosuit was developed by Phil Nuytten and the first operational suit is owned and operated by J.F. White Contracting, which is making it available for research projects.
By the late 19th century, as salvage operations became deeper and longer, an unexplained malady began afflicting the divers; they would suffer breathing difficulties, dizziness, joint pain and paralysis, sometimes leading to death. The problem was already well known among workers building tunnels and bridge footings operating under pressure in caissons and was initially called 'caisson disease' but later the 'bends' because the joint pain typically caused the sufferer to stoop. Early reports of the disease had been made at the time of Pasley's salvage operation, but scientists were still ignorant of its causes.
French physiologist Paul Bert was the first to understand it as decompression sickness. His classical work, La Pression barometrique (1878), was a comprehensive investigation into the physiological effects of air-pressure, both above and below the normal. He determined that inhaling pressurized air caused the nitrogen to dissolve into the bloodstream; rapid depressurization would then release the nitrogen into its natural gaseous state, forming bubbles that could block the blood circulation and potentially cause paralysis or death. Central nervous system oxygen toxicity was also first described in this publication and is sometimes referred to as the "Paul Bert effect".
John Scott Haldane designed a decompression chamber in 1907 to help make deep-sea divers safer and he produced the first decompression tables for the Royal Navy in 1908 after extensive experiments with animals and human subjects. These tables established a method of decompression in stages - it remains the basis for decompression methods to this day. Following Haldane's recommendation, the maximum safe operating depth for divers was extended to 200 feet.:1-1
Research on decompression was continued by the US Navy. The C&R tables were published in 1915, and a large number of experimental dives done in the 1930s, which led to the 1937 tables. Surface decompression and oxygen use were also researched in the 1930s, and the US Navy 1957 tables developed to deal with problems found in the 1937 tables.
In 1965 Hugh LeMessurier and Brian Hills published their paper, A thermodynamic approach arising from a study on Torres Strait diving techniques, which suggested that decompression by conventional models results in bubble formation which is then eliminated by re-dissolving at the decompression stops which is slower than off-gassing while still in solution. This indicates the importance of minimizing bubble phase for efficient gas elimination.
M.P. Spencer showed that doppler ultrasonic methods can detect venous bubbles in asymptomatic divers. and Dr Andrew Pilmanis showed that safety stops reduced bubble formation. In 1981 D.E. Yount described the Varying Permeability Model, proposing a mechanism of bubble formation. Several other bubble models followed.
The common term for a place at which one may dive is a dive site. As a general rule, professional diving is done where the work needs to be done, and recreational diving is done where conditions are suitable. As a consequence, there are many recorded and publicized recreational dive sites which are known for their convenience, points of interest, and frequently favourable conditions. Diver training facilities for both professional and recreational divers will generally use a small range of dive sites which are familiar, convenient and where conditions are predictable and the risk is relatively low.
Recreational diver service organisations may provide websites or brochures listing the sites to which they provide access, and popular dive sites in many parts of the world have been described in magazines and books, in a widely varying range of detail and accuracy. There are also travel and other specialist websites which provide the recreational diver with facilities to describe dive sites, either as blogs or as co-operative travel guides.
Dive sites are restricted by accessibility and risk, but may include water and occasionally other liquids. Most underwater diving is done in the shallower coastal parts of the oceans, and inland bodies of fresh water, including lakes, dams, quarries, rivers, springs, flooded caves, reservoirs, tanks, swimming pools, and canals, but may also be done in large bore ducting and sewers, power station cooling systems, cargo and ballast tanks of ships, and liquid-filled industrial equipment. Diving in liquids other than water may present special problems due to density, viscosity and chemical compatibility of diving equipment, as well as possible environmental hazards to the diving team.
The depth range for human diving operations is small compared with the depth of the sea: The recreational diving depth limit for EN 14153-2 / ISO 24801-2 level 2 "Autonomous Diver" standard is 20 metres (66 ft). The recommended depth limit for more extensively trained recreational divers ranges from 30 metres (98 ft) for PADI divers, (this is the depth at which nitrogen narcosis symptoms generally begin to be noticeable in adults), 40 metres (130 ft) specified by Recreational Scuba Training Council, 50 metres (160 ft) for divers of the British Sub-Aqua Club and Sub-Aqua Association breathing air, and 60 metres (200 ft) for teams of 2 to 3 French Level 3 recreational divers, breathing air.
For technical divers, the recommended maximum depths are greater on the understanding that they will use less narcotic gas mixtures. 100 metres (330 ft) is the maximum depth authorised for divers who have completed Trimix Diver certification with IANTD or Advanced Trimix Diver certification with TDI. 332 metres (1,089 ft) is the world record depth on scuba (2014). Commercial divers using saturation techniques and heliox breathing gases routinely exceed 100 metres (330 ft), but they are also limited by physiological constraints. Comex Hydra 8 experimental dives reached a record open water depth of 534 metres (1,752 ft) in 1988. Atmospheric Diving System (ADS) suits are mainly constrained by the technology of the articulation seals, and have achieved a depth of 610 metres (2,000 ft) by a US Navy diver.
Fitness to dive
Medical fitness to dive is the medical and physical suitability of a diver to function safely in the underwater environment using underwater diving equipment and procedures. Depending on the circumstances it may be established by a signed statement by the diver that he or she does not suffer from any of the listed disqualifying conditions and is able to manage the ordinary physical requirements of diving, to a detailed medical examination by a physician registered as a medical examiner of divers following a procedural checklist, and a legal document of fitness to dive issued by the medical examiner.
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.
- Equalisation of pressure differentials between the environment and the air spaces of the ear, other air spaces inside the body, and air spaces between diving equipment and the body.
- Underwater breathing – the skill of breathing through the apparatus.
- Mask clearing – the skill of clearing water from the mask.
- Air sharing – assisting another diver by providing air from one's own supply, or receiving air supplied by another diver.
- Emergency ascents - how to return to the surface without injury in the event of a breathing supply interruption.
- Use of bailout systems (professional and technical divers)
- Buoyancy control – correct buoyancy allows the diver to move about underwater safely and comfortably.
- Diving signals – used to communicate underwater. Professional divers will also learn other methods of communication.
Some knowledge of physiology and the physics of diving is considered necessary by most diver certification agencies, as the diving environment is alien and relatively hostile to humans. The physics and physiology knowledge required is fairly basic, and helps the diver to understand the effects of the diving environment so that informed acceptance of the associated risks is possible. The physics mostly relates to gases under pressure, buoyancy, heat loss, and light underwater. The physiology relates the physics to the effects on the human body, to provide a basic understanding of the causes and risks of barotrauma, decompression sickness, gas toxicity, hypothermia, drowning and sensory variations. More advanced training often involves first aid and rescue skills, skills related to specialized diving equipment, and underwater work skills.
Physiological aspects of diving
Diving medicine is the diagnosis, treatment and prevention of conditions caused by exposing divers to the underwater environment. It includes the effects on the body of pressure on gases, the diagnosis and treatment of conditions caused by marine hazards and how fitness to dive affects a diver's safety. Hyperbaric medicine is another field associated with diving, since recompression in a hyperbaric chamber with hyperbaric oxygen therapy is the definitive treatment for two of the most significant diving-related illnesses, decompression sickness and arterial gas embolism.
Diving medicine deals with medical research on issues of diving, the prevention of diving disorders, treatment of diving accident injuries and diving fitness. The field includes the effect of breathing gases and their contaminants under high pressure on the human body and the relationship between the state of physical and psychological health of the diver and safety. In diving accidents it is common for multiple disorders to occur together and interact with each other, both causatively and as complications. Diving medicine is a branch of occupational medicine and sports medicine, and first aid and recognition of symptoms of diving disorders is an important part of diver education.
Risks and safety
Hazard and vulnerability interact with likelihood of occurrence to create risk, which can be the probability of a specific undesirable consequence of a specific hazard, or the combined probability of undesirable consequences of all the hazards of a specific activity. 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.
The assessed risk of a dive would generally be considered unacceptable if the diver is not expected to cope with any single reasonably foreseeable incident with a significant probability of occurrence during that dive. Precisely where the line is drawn depends on circumstances. Commercial diving operations are constrained by occupational health and safety legislation, but also by the physical realities of the operating environment, and expensive engineering solutions are often necessary to control risk. A formal hazard identification and risk assessment is a standard and required part of the planning for a commercial diving operation, and this is also the case for offshore diving operations. The occupation is inherently hazardous, and great effort and expense are routinely incurred to keep the risk within an acceptable range. The standard methods of reducing risk are followed where possible.
Statistics on injuries related to commercial diving are normally collected by national regulators. In the UK the Health and Safety Executive (HSE) is responsible for the overview of about 5,000 commercial divers, and in Norway the corresponding authority is the Petroleum Safety Authority Norway (PSA), which has maintained the DSYS database since 1985, gathering statistics on over 50,000 diver-hours of commercial activity per year. 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. According to a North American 1972 analysis of calendar year 1970 data, diving was, based on man hours, 96 times more dangerous than driving an automobile. According to a 2000 Japanese study, every hour of recreational diving is 36 to 62 times riskier than automobile driving.
Scuba diving fatalities have a major financial impact by way of lost income, lost business, insurance premium increases and high litigation costs. Equipment failure is rare in open circuit scuba, and while the cause of death is commonly recorded as drowning, this is usually 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, occasionally 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.
Artisanal fishermen and gatherers of marine organisms in less developed countries may expose themselves to relatively high risk using diving equipment if they do not understand the physiological hazards, particularly if they use inadequate equipment. 
Divers operate in an environment for which the human body is not well suited. They face specific physical and health risks when they go underwater or use high pressure breathing gas. The consequences of diving incidents range from merely annoying through to rapidly fatal, and the actual result often depends on the equipment, skill, response and fitness of the diver and diving team. The hazards can be listed under several categories:
- The aquatic environment
- Use of breathing equipment in an underwater environment
- Exposure to a pressurised environment and pressure changes, which includes:
- Pressure changes during descent
- Pressure changes during ascent
- Breathing gases at high ambient pressure
- The specific diving environment
- Pre-existing physiological and psychological conditions in the diver
- Diver behaviour and competence
- Failure of diving equipment other than breathing apparatus
- Hazards of the dive task and special equipment
- Hazards related to access to and egress from the water.
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. The assessed risk of a dive would generally be considered unacceptable if the diver is not expected to cope with any single reasonably foreseeable incident with a significant probability of occurrence during that dive. Precisely where the line is drawn depends on circumstances. Commercial diving operations tend to be less tolerant of risk than recreational, particularly technical divers, who are less constrained by occupational health and safety legislation.
Inshore and inland commercial and military diving is directly regulated by legislation in many countries. Responsibility of the employer, client and diving personnel is specified in these cases, while offshore commercial diving may take place in international waters, and is often done following the guidelines of a voluntary membership organisation such as the International Marine Contractors Association (IMCA), which publishes codes of accepted best practice which their member organisations are expected to follow.
Scientific diving may be classed as commercial diving for regulatory purposes, but in some cases, such as in the US it is specifically exempted from the federal regulations governing commercial diving as there is a non-governmental body, the American Academy of Underwater Sciences, which regulates the field.
Recreational diver training, and dive leading are industry regulated in some of those countries, and only directly regulated by government in a subset of them. In the UK, HSE legislation specifically includes recreational diver training and dive leading for reward, while the US and South Africa are examples where industry regulation is accepted, though non-specific HSE legislation would still apply.
Diving by other animals
Humans are not the only air-breathing creatures to dive. Marine mammals such as seals, dolphins and whales, dive to feed and catch prey under the sea as do penguins and many seabirds, as well as various reptiles: turtles, saltwater crocodiles, seasnakes and marine iguanas. Many mammals, birds and reptiles also dive in freshwater rivers and lakes.
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