Scuba diving
Scuba diving is an underwater diving mode where divers use breathing equipment completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The word scuba is an acronym for "Self-Contained Underwater Breathing Apparatus" and was coined by Christian J. Lambertsen in a patent submitted in 1952. Scuba divers carry their source of breathing gas, affording them greater independence and movement than surface-supplied divers, and more time underwater than freedivers. Although compressed air is commonly used, other gas blends are also employed.
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 at ambient pressure through a diving regulator. They may include additional cylinders for range extension, decompression gas or emergency breathing gas. 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, making longer dives feasible. Rebreathers extend the time spent underwater compared to open-circuit for the same metabolic gas consumption. They produce fewer bubbles and less noise than open-circuit scuba, which makes them attractive to covert military divers to avoid detection, scientific divers to avoid disturbing marine animals, and media diver to avoid bubble interference.
Scuba diving may be done recreationally or professionally in several applications, including scientific, military and public safety roles, but most commercial diving uses surface-supplied diving equipment for breathing gas security when this is practicable. Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers.
A scuba diver primarily moves underwater using fins worn on the feet, but external propulsion can be provided by a diver propulsion vehicle, or a sled towed from the surface. Other equipment needed for scuba diving includes a mask to improve underwater vision, exposure protection by means of a diving suit, ballast weights to overcome excess buoyancy, equipment to control buoyancy, and equipment related to the specific circumstances and purpose of the dive, which may include a snorkel when swimming on the surface, a cutting tool to manage entanglement, lights, a dive computer to monitor decompression status, and signalling devices. Scuba divers are trained in the procedures and skills appropriate to their level of certification by diving instructors affiliated to the diver certification organizations 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
Origins
Although underwater diving in the mode of freediving has been practised for thousands of years, the history of scuba diving, closely linked with the history of scuba equipment, began in the 19th century. By the early twentieth century, two basic architectures for underwater breathing apparatus had been developed – open-circuit surface-supplied equipment, where the diver's exhaled gas is vented directly into the water; and closed-circuit scuba, where the carbon dioxide is absorbed from exhaled gas, which is then recirculated with oxygen added as necessary to maintain the volume. By the mid-twentieth century, high pressure gas cylinders were available and another system for scuba had emerged: open-circuit demand scuba, where the diver is supplied with gas on demand, and their exhaled breath is vented directly into the water. Oxygen rebreathers are severely depth-limited due to oxygen toxicity risk, which increases with depth, and the available systems for mixed gas rebreathers were fairly bulky, designed for use with diving helmets and often relied on a constant flow gas supply for driving recirculation.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. His self-contained breathing apparatus consisted of a rubber mask connected to a breathing bag, with an estimated 50–60% oxygen supplied from a copper tank and carbon dioxide scrubbed by passing it through a bundle of rope yarn soaked in a solution of caustic potash, the system giving a dive duration of up to about three hours. This apparatus had no way of measuring the gas composition during use. 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, for their frogmen during the war. 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. 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, and later for the activity using the equipment. 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 due to the risk of convulsions caused by acute oxygen toxicity.
Although a working demand regulator system had been invented in 1864 by Auguste Denayrouze and Benoît Rouquayrol, the first open-circuit scuba system developed in 1925 by Yves Le Prieur in France was a manually adjusted free-flow system with a low endurance, which limited its practical usefulness. 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, and was patented in 1945. To sell his regulator in English-speaking countries Cousteau registered the Aqua-Lung trademark, which was first licensed to the U.S. Divers company, and in 1948 to Siebe Gorman of England. 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. The patent was circumvented by Ted Eldred of Melbourne, Australia, who developed the single-hose open-circuit scuba system in 1952, which separates the first stage and demand valve of the pressure regulator by a low-pressure hose, puts the demand valve at the diver's mouth, and releases exhaled gas through the demand valve casing.
Early equipment
Early scuba sets were usually provided with a plain harness of shoulder straps and a waist belt, and early scuba divers dived without a buoyancy aid. In an emergency they had to jettison their weights. In the 1960s adjustable buoyancy life jackets 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, and eventually with a low-pressure feed from the regulator first-stage to an inflation/deflation valve unit with an oral inflation valve for backup. The inflation system and a dump valve 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.Configuration development
A backplate and wing is an alternative configuration of a scuba harness with a buoyancy compensation bladder mounted behind the diver, sandwiched between the backplate and the cylinder or cylinders. This arrangement became popular with cave divers making long or deep dives, who needed to carry several extra cylinders, as it clears the front and sides of the diver for other equipment to be attached in the region where it is easily accessible. Sidemount is a scuba diving equipment configuration which has basic scuba sets, each comprising a single cylinder with a dedicated regulator and pressure gauge, mounted alongside the diver, clipped to the harness 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 caves since sets can be easily removed and remounted when necessary. The configuration allows easy access to cylinder valves and provides easy and reliable gas redundancy. These benefits for operating in confined spaces were also recognized by divers who made wreck diving penetrations. Sidemount diving has grown in popularity within the technical diving community for general decompression diving, and has become a popular specialty for recreational diving.Alternative breathing gases and technical diving
In the 1950s the United States Navy documented enriched oxygen gas procedures for military use of what is today called nitrox, and in 1970, Morgan Wells of NOAA began instituting diving procedures for oxygen-enriched air. In 1979 NOAA published procedures for the scientific use of nitrox in the NOAA Diving Manual. In 1985 IAND began teaching nitrox use for recreational diving. This was considered dangerous by some, and met with heavy skepticism by the diving community. Nevertheless, in 1992 NAUI became the first existing major recreational diver training agency to sanction nitrox, and eventually, in 1996, the Professional Association of Diving Instructors announced full educational support for nitrox. The use of a single nitrox mixture has become part of recreational diving, and multiple gas mixtures are common in technical diving to reduce overall decompression time.Technical diving is recreational scuba diving that exceeds the generally accepted recreational limits and 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 concept and term are both relatively recent advents, although divers had already been engaging in what is now commonly referred to as technical diving for decades. One reasonably widely held definition is that any dive in which at some point of the planned profile it is not physically possible or physiologically acceptable to make a direct and uninterrupted vertical ascent to surface air is a technical dive. The equipment often involves breathing gases other than air or standard nitrox mixtures, multiple gas sources, and different equipment configurations. Over time, some equipment and techniques developed for technical diving have become more widely accepted for recreational diving.
In 1924 the US Navy started to investigate the possibility of using helium in breathing gases, and after animal experiments, human subjects breathing heliox 20/80 % were successfully decompressed from deep dives, In 1963 saturation dives using trimix were made during Project Genesis, and in 1979 a research team at the Duke University Medical Center Hyperbaric Laboratory started work which identified the use of trimix to prevent the symptoms of high-pressure nervous syndrome. Cave divers started using trimix to allow deeper dives and it was used extensively in the 1987 Wakulla Springs Project and spread to the north-east American wreck diving community and then worldwide.
The challenges of deeper dives and longer penetrations and the large amounts of breathing gas necessary for these dive profiles and 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 and accurately monitor a breathable gas mixture in the loop at any depth. In the mid-1990s semi-closed circuit rebreathers became available for the recreational scuba market, followed by closed circuit rebreathers around the turn of the millennium. Rebreathers are currently manufactured for the military, technical and recreational scuba markets, but remain less popular, less reliable, more complex to operate, and more expensive than open-circuit equipment.