Scuba diving - AbsoluteAstronomy.com

Scuba diving is a form of underwater diving

Underwater diving

Underwater diving is the practice of going underwater, either with breathing apparatus or by breath-holding .Recreational diving is a popular activity...

in which a diver uses a scuba set

Scuba set

A scuba set is an independent breathing set that provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving. It is much used for sport diving and some sorts of work diving....

to breathe underwater

Underwater

Underwater is a term describing the realm below the surface of water where the water exists in a natural feature such as an ocean, sea, lake, pond, or river. Three quarters of the planet Earth is covered by water...

.

Unlike early diving, which relied either on breath-hold or on air pumped from the surface, scuba divers carry their own source of breathing gas

Breathing gas

Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...

(usually compressed air

Compressed air

Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....

), allowing them greater freedom of movement than with an air line

Air line

An air line is a tube that carries a compressed air supply, e.g. to inflate tyres or power compressed-air tools. Air line is most commonly used for suppling compressed air to air tools in workshops and in air brake systems on larger vehicles...

. Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding

Apnea

Apnea, apnoea, or apnœa is a term for suspension of external breathing. During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged...

techniques as used in snorkelling and free-diving

Free-diving

Freediving is any of various aquatic activities that share the practice of breath-hold underwater diving. Examples include breathhold spear fishing, freedive photography, apnea competitions and, to a degree, snorkeling...

.
Depending on the purpose of the dive, a diver usually moves underwater by swimfin

Swimfin

Swimfins, swim fins, fins or flippers are worn on the foot or leg and made from finlike rubber or plastic, to aid movement through the water in water sports activities such as swimming, bodyboarding, bodysurfing, kneeboarding, riverboarding, and various types of underwater diving.Scuba divers use...

s attached to the feet, but external propulsion can come from an underwater vehicle

Diver Propulsion Vehicle

A diver propulsion vehicle is an item of diving equipment used by scuba and rebreather divers to increase range underwater...

, or a sled pulled from the surface.

History

The first commercially successful scuba set

Scuba set

s were the Aqualung

Aqua-lung

Aqua-Lung was the original name of the first open-circuit free-swimming underwater breathing set in reaching worldwide popularity and commercial success...

open-circuit units developed by Emile Gagnan

Emile Gagnan

Émile Gagnan was a French engineer and co-inventor of the diving regulator used for the first Scuba equipment in 1943...

and Jacques-Yves Cousteau

Jacques-Yves Cousteau

Jacques-Yves Cousteau was a French naval officer, explorer, ecologist, filmmaker, innovator, scientist, photographer, author and researcher who studied the sea and all forms of life in water...

, in which compressed gas (usually air) is inhaled from a tank and then exhaled into the water adjacent to the tank. However, the scuba regulators of today trace their origins to Australia, where Ted Eldred developed the first mouth piece regulator, known as the Porpoise. This regulator was developed because patents protected the Aqualung's double hose design. It separated the cylinder from the demand valve giving the diver air at the same pressure surrounding his mouth, not surrounding the tank.

The open circuit systems were developed after Cousteau had a number of incidents of oxygen toxicity

Oxygen toxicity

Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at elevated partial pressures. It is also known as oxygen toxicity syndrome, oxygen intoxication, and oxygen poisoning...

using a rebreather

Rebreather

A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...

system, in which exhaled air is reprocessed to remove carbon dioxide. Modern versions of rebreather systems (both semi-closed circuit and closed circuit) are still available today, and form the second main type of scuba unit, most commonly used for technical diving, such as deep diving

Deep diving

The meaning of the term deep diving is a form of technical diving. It is defined by the level of the diver's diver training, diving equipment, breathing gas, and surface support:...

Etymology

The term "SCUBA

Scuba set

" (an acronym for self-contained underwater breathing apparatus) arose during World War II, and originally referred to United States combat frogmen's oxygen rebreather

Rebreather

s, developed by Dr. Christian Lambertsen

Christian J. Lambertsen

Christian James Lambertsen was an American environmental medicine and diving medicine specialist who was principally responsible for developing the United States Navy frogmen's rebreathers in the early 1940s for underwater warfare...

for underwater warfare.

The word "SCUBA" began as an acronym, but it is now usually thought of as a regular word—"scuba". It has become acceptable to refer to "scuba equipment" or "scuba apparatus"—examples of the linguistic RAS syndrome

RAS syndrome

RAS syndrome , also known as PNS syndrome or RAP phrases , refers to the use of one or more of the words that make up an acronym or initialism in conjunction with the abbreviated form, thus in effect repeating one or more words...

Types of diving

Scuba diving may be performed for a number of reasons, both personal and professional. Most people begin through recreational diving

Recreational diving

Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment. In some diving circles, the term "recreational diving" is used in contradistinction to "technical diving", a more demanding aspect of the sport which requires greater levels...

, which is performed purely for enjoyment and has a number of distinct technical disciplines

Technical diving

Technical diving is a form of scuba diving that exceeds the scope of recreational diving...

to increase interest underwater, such as cave diving

Cave diving

Cave diving is a type of technical diving in which specialized equipment is used to enable the exploration of caves which are at least partially filled with water. In the United Kingdom it is an extension of the more common sport of caving, and in the United States an extension of the more common...

, wreck diving

Wreck diving

Wreck diving is a type of recreational diving where shipwrecks are explored. Although most wreck dive sites are at shipwrecks, there is an increasing trend to scuttle retired ships to create artificial reef sites...

, ice diving

Ice diving

Ice diving is a type of penetration diving where the dive takes place under ice. Because diving under ice places the diver in an overhead environment typically with only a single entry/exit point, it is considered an advanced type of diving requiring special training...

and deep diving

Deep diving

The meaning of the term deep diving is a form of technical diving. It is defined by the level of the diver's diver training, diving equipment, breathing gas, and surface support:...

.

Divers may be employed professionally to perform tasks underwater. Most of these commercial divers are employed to perform tasks related to the running of a business involving deep water, including civil engineering

Civil engineering

Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including works like roads, bridges, canals, dams, and buildings...

tasks such as in oil exploration

Oil exploration

Hydrocarbon exploration is the search by petroleum geologists and geophysicists for hydrocarbon deposits beneath the Earth's surface, such as oil and natural gas...

, underwater welding or offshore construction

Offshore construction

Offshore construction is the installation of structures and facilities in a marine environment, usually for the production and transmission of electricity, oil, gas and other resources....

. 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

Marine salvage

Marine salvage is the process of rescuing a ship, its cargo, or other property from peril. Salvage encompasses rescue towing, refloating a sunken or grounded vessel, or patching or repairing a ship...

of wrecks or underwater fishing, like spear fishing.

Other specialist areas of diving include military diving, with a long history of military frogmen in various roles. They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal

Bomb disposal

Bomb disposal is the process by which hazardous explosive devices are rendered safe. Bomb disposal is an all encompassing term to describe the separate, but interrelated functions in the following fields:*Military:...

or engineering operations. In civilian operations, many police forces operate police diving

Police diving

Police diving is a branch of professional diving carried out by police services. Police divers are usually sworn police officers, and may either be employed full-time as divers or as general water police officers, or be volunteers who usually serve in other units but are called in if their 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

Diver rescue

thumb|right|Beaching a casualty while providing artificial respirationDiver rescue, following an accident, is the process of avoiding or limiting further exposure to diving hazards and bringing a SCUBA diver to safety...

teams may also be part of a fire department

Fire department

A fire department or fire brigade is a public or private organization that provides fire protection for a certain jurisdiction, which typically is a municipality, county, or fire protection district...

, paramedical service or lifeguard

Lifeguard

A lifeguard supervises the safety and rescue of swimmers, surfers, and other water sports participants such as in a swimming pool, water park, or beach. Lifeguards are strong swimmers and trained in first aid, certified in water rescue using a variety of aids and equipment depending on...

unit, and may be classed as public service diving.

Lastly, there are professional divers involved with the water itself, such as underwater photography

Underwater photography

Underwater photography is the process of taking photographs while under water. It is usually done while scuba diving, but can be done while snorkeling or swimming.-Overview:...

or underwater filming divers, who set out to document the underwater world, or scientific diving, including marine biology

Marine biology

Marine biology is the scientific study of organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather...

, geology

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

, hydrology

Hydrology

Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...

, oceanography

Oceanography

Oceanography , also called oceanology or marine science, is the branch of Earth science that studies the ocean...

and underwater archaeology

Underwater archaeology

Underwater archaeology is archaeology practised underwater. As with all other branches of archaeology it evolved from its roots in pre-history and in the classical era to include sites from the historical and industrial eras...

.

Reasons for diving may include:

Type of diving	Classification
Aquarium Aquarium An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants... maintenance in large public aquarium Public aquarium A public aquarium is the aquatic counterpart of a zoo, housing living aquatic species for viewing. Most public aquariums feature tanks larger than those kept by home aquarists, as well as smaller tanks. Since the first public aquariums were built in the mid-19th century, they have become popular... s	Commercial Commercial Diving Professional diving is a type of diving where the divers are paid for their work. There are several branches of professional diving, the most well known of which is probably commercial diving... , scientific
Boat and ship inspection, cleaning and maintenance	Commercial, naval
Cave diving Cave diving Cave diving is a type of technical diving in which specialized equipment is used to enable the exploration of caves which are at least partially filled with water. In the United Kingdom it is an extension of the more common sport of caving, and in the United States an extension of the more common...	Technical Technical diving Technical diving is a form of scuba diving that exceeds the scope of recreational diving... , recreational Recreational diving Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment. In some diving circles, the term "recreational diving" is used in contradistinction to "technical diving", a more demanding aspect of the sport which requires greater levels... , scientific
Civil engineering in harbors, water supply, and drainage systems	Commercial
Crude oil industry and other offshore construction Offshore construction Offshore construction is the installation of structures and facilities in a marine environment, usually for the production and transmission of electricity, oil, gas and other resources.... and maintenance	Commercial
Demolition and salvage Marine salvage Marine salvage is the process of rescuing a ship, its cargo, or other property from peril. Salvage encompasses rescue towing, refloating a sunken or grounded vessel, or patching or repairing a ship... of ship wrecks	Commercial, naval
Diver training Diver training Diver training is the process of developing skills and building experience in the use of diving equipment and techniques so that the diver is able to dive safely and have fun.... for reward	Professional
Fish farm maintenance	Commercial
Fishing, e.g. for abalone Abalone Abalone , from aulón, are small to very large-sized edible sea snails, marine gastropod molluscs in the family Haliotidae and the genus Haliotis... s, crab Crab True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax... s, lobster Lobster Clawed lobsters comprise a family of large marine crustaceans. Highly prized as seafood, lobsters are economically important, and are often one of the most profitable commodities in coastal areas they populate.Though several groups of crustaceans are known as lobsters, the clawed lobsters are most... s, pearl Pearl A pearl is a hard object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other... s, scallop Scallop A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source... s, sea crayfish, sponges	Commercial
Frogman Frogman A frogman is someone who is trained to scuba diving or swim underwater in a military capacity which can include combat. Such personnel are also known by the more formal names of combat diver or combatant diver or combat swimmer.... , manned torpedo	Military
Harbor clearance and maintenance	Commercial, military
Media diving: making television programs, etc.	Professional
Mine Naval mine A naval mine is a self-contained explosive device placed in water to destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an enemy vessel... clearance and bomb disposal Bomb disposal Bomb disposal is the process by which hazardous explosive devices are rendered safe. Bomb disposal is an all encompassing term to describe the separate, but interrelated functions in the following fields:*Military:... , disposing of unexploded ordnance Unexploded ordnance Unexploded ordnance are explosive weapons that did not explode when they were employed and still pose a risk of detonation, potentially many decades after they were used or discarded.While "UXO" is widely and informally used, munitions and explosives of...	Military, naval
Pleasure, leisure, sport	Recreational
Policing: diving to investigate or arrest unauthorized divers	Police diving Police diving Police diving is a branch of professional diving carried out by police services. Police divers are usually sworn police officers, and may either be employed full-time as divers or as general water police officers, or be volunteers who usually serve in other units but are called in if their diving... , military, naval
Search and recovery diving	Commercial, Public service, Police diving Police diving Police diving is a branch of professional diving carried out by police services. Police divers are usually sworn police officers, and may either be employed full-time as divers or as general water police officers, or be volunteers who usually serve in other units but are called in if their diving...
Search and rescue Search and rescue Search and rescue is the search for and provision of aid to people who are in distress or imminent danger.The general field of search and rescue includes many specialty sub-fields, mostly based upon terrain considerations... diving	Police, naval, Public service
Spear fishing	Professional (occasionally), recreational
Stealthy infiltration Infiltration tactics In warfare, infiltration tactics involve small, lightly equipped infantry forces attacking enemy rear areas while bypassing enemy front line strongpoints and isolating them for attack by follow-up troops with heavier weapons.-Development during World War I:...	Military
Surveys and mapping	Scientific, recreational
Marine biology Marine biology Marine biology is the scientific study of organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather...	Scientific, recreational
Underwater archaeology Underwater archaeology Underwater archaeology is archaeology practised underwater. As with all other branches of archaeology it evolved from its roots in pre-history and in the classical era to include sites from the historical and industrial eras... (shipwreck Shipwreck A shipwreck is what remains of a ship that has wrecked, either sunk or beached. Whatever the cause, a sunken ship or a wrecked ship is a physical example of the event: this explains why the two concepts are often overlapping in English.... s; harbors, and buildings)	Scientific, recreational
Underwater inspections and surveys	Commercial, military
Underwater photography Underwater photography Underwater photography is the process of taking photographs while under water. It is usually done while scuba diving, but can be done while snorkeling or swimming.-Overview:...	Professional, recreational
Underwater tour guiding	Professional, recreational
Underwater tourism	Recreational
Underwater welding	Commercial

Breathing underwater

Water normally contains the dissolved oxygen from which fish and other aquatic animals extract all their required oxygen as the water flows past their gills. Humans lack gills and do not otherwise have the capacity to breathe underwater

Underwater

unaided by external devices. Although the feasibility of filling and artificially ventilating the lungs with a dedicated liquid (liquid breathing

Liquid breathing

Liquid breathing is a form of respiration in which a normally air-breathing organism breathes an oxygen-rich liquid , rather than breathing air....

) has been established for some time, the size and complexity of the equipment allows only for medical applications with current technology.

Early diving experimenters quickly discovered it is not enough simply to supply air to breathe comfortably underwater. As one descends, in addition to the normal atmospheric pressure, water exerts increasing pressure on the chest and lungs—approximately 1 bar

Bar (unit)

The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

(14.7 pounds per square inch) for every 33 feet (10 m) of depth—so the pressure of the inhaled breath must almost exactly counter the surrounding or ambient pressure to inflate the lungs. It generally becomes difficult to breathe through a tube past three feet under the water.

By always providing the breathing gas

Breathing gas

Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...

at ambient pressure, modern demand valve regulators

Diving regulator

A diving regulator is a pressure regulator used in scuba or surface supplied diving equipment that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases...

ensure the diver can inhale and exhale naturally and virtually effortlessly, regardless of depth.

Because the diver's nose and eyes are covered by a diving mask

Diving mask

A diving mask is an item of diving equipment that allows scuba divers, free-divers, and snorkelers to see clearly underwater. When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable...

; the diver cannot breathe in through the nose, except when wearing a full face diving mask

Full face diving mask

A full-face diving mask is a type of diving mask that seals the whole of the diver's face from the water and contains a mouthpiece or demand valve that provides the diver with breathing gas...

. However, inhaling from a regulator's mouthpiece

Mouthpiece (scuba)

In breathing sets, a mouthpiece is a part that the user grips in his mouth, to make a watertight seal between the breathing set and his mouth. It is composed of a short flattened-oval tube that goes in between the lips, with on its free end a flange that fits between the lips and the tooth and gums...

becomes second nature very quickly.

Open-circuit regulator

The most commonly used scuba set

Scuba set

today is the "single-hose" open circuit 2-stage diving regulator

Diving regulator

, coupled to a single pressurized gas cylinder, with the first stage on the cylinder and the second stage at the mouthpiece

Mouthpiece (scuba)

. This arrangement differs from Emile Gagnan

Emile Gagnan

Émile Gagnan was a French engineer and co-inventor of the diving regulator used for the first Scuba equipment in 1943...

's and Jacques Cousteau's original 1942 "twin-hose" design, known as the Aqua-lung

Aqua-lung

Aqua-Lung was the original name of the first open-circuit free-swimming underwater breathing set in reaching worldwide popularity and commercial success...

, in which the cylinder's pressure was reduced to ambient pressure in one or two or three stages which were all on the cylinder. The "single-hose" system has significant advantages over the original system.
In the "single-hose" two-stage design, the first stage regulator reduces the cylinder pressure of about 200 bar (3000 psi) to an intermediate level of about 10 bar (145 psi) above ambient pressure. The second stage demand valve regulator, connected via a low pressure hose to the first stage, delivers the breathing gas at the correct ambient pressure to the diver's mouth and lungs. The diver's exhaled gases are exhausted directly to the environment as waste. The first stage typically has at least one outlet delivering breathing gas at unreduced tank pressure. This is connected to the diver's pressure gauge or computer, in order to show how much breathing gas remains.

Rebreather

Less common are closed (CCR) and semi-closed (SCR) rebreathers, which unlike open-circuit sets that vent off all exhaled gases, reprocess each exhaled breath for re-use by removing the carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

buildup and replacing the oxygen used by the diver.

Rebreathers release few or no gas bubbles into the water, and use much less oxygen per hour because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications. The first modern rebreather was the MK-19 that was developed at S-Tron by Ralph Osterhout

Ralph Osterhout

Ralph Osterhout is an inventor, designer and entrepreneur. During his 40+ year career he has developed a range of products spanning toys, consumer electronics, dive equipment, furniture to devices for the Department of Defense.-Early life:...

that was the first electronic system. Rebreathers are more complex and more expensive than sport open-circuit scuba, and need special training and maintenance to be safely used due to the larger variety of potential failure modes.

In a closed-circuit rebreather the oxygen partial pressure in the rebreather is controlled, so it can be increased to 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. A semi-closed circuit rebreather injects a constant flow of a fixed nitrox mixture in the breathing loop, so the partial pressure of oxygen at any time during the dive depends on the diver's oxygen consumption. Planning decompression requirements requires a more conservative approach for a SCR than for a CCR.

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

Underwater photography

Underwater photography is the process of taking photographs while under water. It is usually done while scuba diving, but can be done while snorkeling or swimming.-Overview:...

, and for covert work

Frogman

A frogman is someone who is trained to scuba diving or swim underwater in a military capacity which can include combat. Such personnel are also known by the more formal names of combat diver or combatant diver or combat swimmer....

Gas mixtures

For some diving, gas mixtures other than normal atmospheric air (21% oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, 78% nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, 1% trace gases) can be used, so long as the diver is properly trained 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 likelihood of decompression sickness

Decompression sickness

Decompression sickness describes a condition arising from dissolved gases coming out of solution into bubbles inside the body on depressurization...

or allowing longer exposure to the same depth for equal risk. The reduced nitrogen may also allow for no stops or shorter decompression stop times and a shorter surface interval between dives. A common misconception is that nitrox can reduce narcosis

Nitrogen narcosis

Narcosis while diving , is a reversible alteration in consciousness that occurs while scuba diving at depth. The Greek word ναρκωσις is derived from narke, "temporary decline or loss of senses and movement, numbness", a term used by Homer and Hippocrates...

, but research has shown that oxygen is also narcotic.

Several other common gas mixtures are in use, and all need specialized training for safe use. The increased oxygen levels in nitrox help reduce the risk of decompression sickness; however, below the maximum operating depth

Maximum operating depth

In technical diving and nitrox diving, the maximum operating depth of a breathing gas is the depth at which the partial pressure of oxygen of the gas mix exceeds a safe limit...

of the mixture, the increased partial pressure of oxygen can lead to an unacceptable risk of oxygen toxicity

Oxygen toxicity

. To displace nitrogen without the increased oxygen concentration, other diluents can be used, usually helium

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

, when the resultant three gas mixture is called trimix, and when the nitrogen is fully substituted by helium, heliox

Heliox

Heliox is a breathing gas composed of a mixture of helium and oxygen .Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of...

.

For technical dives

Technical diving

Technical diving is a form of scuba diving that exceeds the scope of recreational diving...

, some of the cylinders may contain 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

Decompression (diving)

Decompression in the context of diving derives from the reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the...

times.

Refraction and underwater vision

Water has a higher refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

than air; it's similar to that of the cornea

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

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

Myopia

Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

, therefore, can see better underwater without a mask than normal-sighted people.

Diving mask

Diving mask

s and diving helmet

Diving helmet

Diving helmets are worn mainly by professional divers engaged in surface supplied diving, though many models can be adapted for use with scuba equipment....

s and fullface masks solve this problem by creating an air space in front of the diver's eyes. The refraction error created by the water is mostly corrected as the light travels from water to air through a flat lens, except that objects appear approximately 34% bigger and 25% closer

Underwater vision

Light rays bend when they travel from one medium to another; the amount of bending is determined by the refractive indices of the two media. If one medium has a particular curved shape, it functions as a lens. The cornea, humours, and crystalline lens of the eye together form a lens that focuses...

in salt water than they actually are. Therefore total field-of-view is significantly reduced and eye–hand coordination must be adjusted.

(This affects underwater photography: a camera seeing through a flat window in its casing is affected the same as its user's eye seeing through a flat mask window, and so its user must focus for the apparent distance to target, not for the real distance.)

Divers who need corrective lenses to see clearly outside the water would normally need the same prescription while wearing a mask. Generic and custom corrective lenses are available for some two-window masks. Custom lenses can be bonded onto masks that have a single front window.

A "double-dome mask" has curved windows in an attempt to cure these faults, but this causes a refraction problem of its own.

Commando frogmen concerned about revealing their position when light reflects from the glass surface of their diving masks may instead use special contact lens

Contact lens

A contact lens, or simply contact, is a lens placed on the eye. They are considered medical devices and can be worn to correct vision, for cosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people use contact lenses worldwide, including 28 to 38 million in the United...

es to see underwater.

As a diver descends, he must periodically exhale through his nose to equalize the internal pressure of the mask with that of the surrounding water. Swimming goggles are not suitable for diving because they only cover the eyes and thus do not allow for equalization. Failure to equalise the pressure inside the mask may lead to a form of barotrauma known as mask squeeze.

Light underwater

Water preferentially absorbs red light, and to a lesser extent, yellow and green light, so the color that is least absorbed by water is blue light.

Table of Light Absorption in pure water
Color	Average wavelength	Approximate depth of total absorption
Ultraviolet	300 nm	25 m
Violet	400 nm	100 m
Blue	475 nm	275 m
Green	525 nm	110 m
Yellow	575 nm	50 m
Orange	600 nm	20 m
Red	685 nm	5 m
Infra-red	800 nm	3 m

Controlling buoyancy underwater

To dive safely, divers must control their rate of descent and ascent in the water. Ignoring other forces such as water currents and swimming, the diver's overall buoyancy

Buoyancy

In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

determines whether he ascends or descends. Equipment such as diving weighting system

Diving weighting system

Divers wear weighting systems, weight belts or weights, generally made of lead, to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders...

s, diving suit

Diving suit

A diving suit is a garment or device designed to protect a diver from the underwater environment. A diving suit typically also incorporates an air-supply .-History:...

s (wet

Wetsuit

A wetsuit is a garment, usually made of foamed neoprene, which is worn by surfers, divers, windsurfers, canoeists, and others engaged in water sports, providing thermal insulation, abrasion resistance and buoyancy. The insulation properties depend on bubbles of gas enclosed within the material,...

, dry

Dry suit

A dry suit or drysuit provides thermal insulation or passive thermal protection to the wearer while immersed in water, and is worn by divers, boaters, water sports enthusiasts, and others who work or play in or near cold water. A dry suit normally protects the whole body except the head, hands, and...

or semi-dry suits are used depending on the water temperature) and buoyancy compensators can be used to adjust the overall buoyancy. When divers want to remain at constant depth, they try to achieve neutral buoyancy. This minimizes gas consumption caused by swimming to maintain depth.

The downward force on the diver is the weight of the diver and his equipment minus the weight of the volume of the liquid that he and his equipment are displacing

Displacement (fluid)

In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced .An object that sinks...

; if the result is negative, that force is upwards. The buoyancy of any object immersed in water is also affected by the density of the water. The density of fresh water is about 3% less than that of ocean water. 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

Coral reef

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

).

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.

Diving suits made of compressible materials, decrease in volume as the diver descends, and expand again as the diver ascends, creating buoyancy changes. Diving in different environments also necessitates adjustments in the amount of weight carried to achieve neutral buoyancy. The diver can inject air into dry suits to counteract the compression effect and squeeze

Barotrauma

Barotrauma is physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding fluid...

. Buoyancy compensators allow easy and fine adjustments in the diver's overall volume and therefore buoyancy. For open circuit

Scuba set

divers, changes in the diver's lung volume can be used to make fine adjustments of buoyancy.

Neutral buoyancy in a diver is a metastable state. It is changed by small differences in ambient pressure caused by a chandge 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. This is a skill which improves with practice.

Being mobile underwater

The diver needs to be mobile underwater. Streamlining dive gear will reduce drag and improve mobility. Personal mobility is enhanced by swimfin

Swimfin

s and Diver Propulsion Vehicle

Diver Propulsion Vehicle

A diver propulsion vehicle is an item of diving equipment used by scuba and rebreather divers to increase range underwater...

s. Other equipment to improve mobility includes diving bell

Diving bell

A diving bell is a rigid chamber used to transport divers to depth in the ocean. The most common types are the wet bell and the closed bell....

s and diving shot

Diving shot

thumb|right|Diagram of a diving shot upcurrent of a dive siteA diving shot is an item of diving equipment consisting of a weight, a line and a buoy. The weight is dropped on the dive site. The line connects the weight and the buoy and is used by divers to move between the surface and the dive site...

Underwater communication

A diver cannot talk underwater unless he is wearing a full-face mask, but divers can communicate, using hand signals. It is said that if a diver "signals to his buddy regularly during a dive, this will ensure that they remain in close contact and that they can easily notify each other in case problems occur."; a diver should repeat any sign that is not clear to his buddy, and acknowledge every signal that he or she makes with an "OK" signal to show that he understands.

Table of Hand Signals

No.	Signal	Meaning	Comment
1.	Hand raised, fingers pointed up, palm to receiver.	STOP	Transmitted in the same way as a traffic police Traffic police Traffic police or Traffic cops may refer to:Police controlling traffic, see rules of the roadTraffic Cops, a BBC real-life documentaryHighway patrolRoad Policing UnitTraffic GuardSri Lankan Traffic Police*Municipal police... officer’s STOP
2.	Thumb extended downward from clenched fist.	GO DOWN or GOING DOWN
3.	Thumb extended upward from clenched fist.	GO UP or GOING UP
4.	Thumb and forefinger making a circle with three remaining fingers extended (if possible).	OK! or OK?	Divers wearing mittens may not be able to extend 3 remaining fingers distinctly.
5.	Two arms extended overhead with finger tips touching above head to make a large O shape.	OK! or OK?	A diver with only one free arm may make this signal by extending that arm overhead with finger tips touching top of head to make the O shape. Signal is for long-range use.
6.	Hand flat, fingers together, palm down, thumb sticking out, then hand rocking back and forth on axis of forearm.	SOMETHING IS WRONG	This is the opposite of OK! The signal does not indicate emergency.
7.	Hand waving over head (may also thrash hand on water).	DISTRESS	Indicates immediate aid required.
8.	Fist pounding on chest.	LOW ON AIR	Indicates signaler's air supply is reduced.
9.	Hand slashing or chopping throat.	OUT OF AIR	Indicates that the signaler cannot breathe.
10.	Clenched fist on arm extended in direction of danger.	DANGER

All signals are to be answered by the receivers repeating the signal as sent. When answering signals 7 & 9, the receiver should approach to offer aid to signaler.(Miller 6.11)

Hazards and dangers

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

Injuries due to changes in pressure

Divers must avoid injuries caused by changes in air 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

Barotrauma

Barotrauma is physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding fluid...

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

Ear clearing

Ear clearing or clearing the ears is any of various maneuvers to equalize the pressure in the middle ear with the outside pressure, by letting air enter along the Eustachian tubes, as this does not always happen automatically when the pressure in the middle ear is lower than the outside pressure...

.

The scuba mask (half-mask) is equalized 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. A more serious hazard with surface supplied helmet is a break in the supply hose at a shallower depth then the diver at the same time as a non-return valve failure at the helmet inlet. On a helmet with a neck dam, the pressure difference will cause the helmet to flood past the neck dam, but on a helmet sealed to the suit a helmet squeeze will result which can cause serious, possibly fatal injury.

If a drysuit is worn, it must be equalized 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. The buoyancy compensator will not have the automatic dumping characteristic and must be manually vented during ascent to retain correct volume for a controlled ascent rate, During descent both dry suit and buoyancy compensator 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 programs highlight diving physiology, safe diving practices, and diving hazards, but do not provide sufficient practice to become truly adept.

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

Henry's law

In physics, Henry's law is one of the gas laws formulated by William Henry in 1803. It states that:An equivalent way of stating the law is that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid...

.

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.

As long as this process is gradual, all will go well and the diver will reduce the gas loading 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.

This effect is called decompression sickness 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. This is done by controlling the speed of ascent and making periodic stops to allow gases to be eliminated. 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 computer

Dive computer

A dive computer or decompression meter is a device used by a scuba diver to measure the time and depth of a dive so that a safe ascent profile can be calculated and displayed so that the diver can avoid decompression sickness.- Purpose :...

s 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.

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

Oxygen

to a decompression sickness stricken diver on the surface is a good form of first aid

First aid

First aid is the provision of initial care for an illness or injury. It is usually performed by non-expert, but trained personnel to a sick or injured person until definitive medical treatment can be accessed. Certain self-limiting illnesses or minor injuries may not require further medical care...

for decompression sickness, although death or permanent disability may still occur.

Nitrogen narcosis

or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in divers who breathe high pressure gas at depth. The mechanism is similar to that of nitrous oxide, or "laughing gas," administered as anesthesia. Being "narced" can impair judgment and make diving very dangerous. Narcosis starts to affect some divers at 66 feet (20 m). At this depth, narcosis manifests itself as a slight giddiness. The effects increase drastically with the increase in depth. Almost all divers are able to notice the effects by 132 feet (40 meters). At these depths divers may feel euphoria, anxiety, loss of coordination and lack of concentration. At extreme depths, hallucinogenic reaction and tunnel vision can occur. Jacques Cousteau famously described it as the "rapture of the deep". Nitrogen narcosis occurs quickly and the symptoms typically disappear during the ascent, so that divers often fail to realize they were ever affected. It affects individual divers at varying depths and conditions, and can even vary from dive to dive under identical conditions. However, diving with trimix or heliox

Heliox

dramatically reduces the effects of inert gas narcosis.

Oxygen toxicity

Oxygen toxicity occurs when oxygen in the body exceeds a safe "partial pressure

Partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

" (PPO₂). In extreme cases it affects the central nervous system and causes a seizure, which can result in the diver spitting out his regulator and drowning. Oxygen toxicity is preventable provided one never exceeds the established maximum depth of a given breathing gas. For deep dives—generally past 180 feet (55 m), divers use "hypoxic blends" containing a lower percentage of oxygen than atmospheric air. For more information, see oxygen toxicity

Oxygen toxicity

Avoiding losing body heat

Water conducts heat from the diver 25 times better than air, which can lead to hypothermia

Hypothermia

Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...

even in mild water temperatures. Symptoms of hypothermia include impaired judgment and dexterity, which can quickly become deadly in an aquatic environment. In all but the warmest waters, divers need the thermal insulation

Thermal insulation

Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...

provided by wetsuit

Wetsuit

s or drysuits.

In the case of a wetsuit, the suit is designed to minimize heat loss. Wetsuits are usually made of neoprene

Neoprene

Neoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...

that has small closed gas cells, generally nitrogen, trapped in it during the manufacturing process. The poor thermal conductivity of this expanded cell neoprene means that wetsuits reduce loss of body heat by conduction to the surrounding water. The neoprene, and to a larger extent the nitrogen gas, in this case acts 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 conduct heat better.

The second way in which wetsuits reduce heat loss is to trap a thin layer of water between the diver's skin and the insulating suit itself. Body heat then heats the trapped water. Provided the wetsuit is reasonably well-sealed at all openings (neck, wrists, ankles zippers and overlaps with other suit components), this reduces flow of cold water over the surface of the skin, and thereby reduces loss of body heat by convection, which helps keep the diver warm (this is the principle employed in the use of a "Semi-Dry" wetsuit)

In the case of a drysuit, it does exactly what the name implies: keeps a diver dry. The suit is waterproof and sealed so that frigid water cannot penetrate the suit. Drysuit undergarments are usually worn under a drysuit to keep a layer of air inside the suit for better thermal insulation. Some divers carry an extra gas bottle dedicated to filling the dry suit. Usually this bottle contains argon

Argon

Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

gas, because of its better insulation as compared with air. Dry suits should not be inflated with gases containing helium as it is a good thermal conductor.

Drysuits fall into two main categories: neoprene and membrane; both systems have their good and bad points but generally their thermal properties can be reduced to:

Membrane or Shell drysuits: usually a trilaminate construction; owing to the thinness of the material (around 1 mm), these require an undersuit, usually of high insulation value if diving in cooler water.
Neoprene drysuits: a similar construction to wetsuits; these are often considerably thicker (7–8 mm) and have sufficient insulation to allow a lighter-weight undersuit (or none at all); however on deeper dives the neoprene can compress to as little as 2 mm thus losing a proportion of 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.

Avoiding skin cuts and grazes

Diving suit

A diving suit is a garment or device designed to protect a diver from the underwater environment. A diving suit typically also incorporates an air-supply .-History:...

s also help prevent the diver's skin being damaged by rough or sharp underwater objects, marine animals, coral

Coral

Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

, or metal debris commonly found on shipwrecks.

Diving longer and deeper safely

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

Technical diving
Technical diving
Technical diving is a form of scuba diving that exceeds the scope of recreational diving...

– diving deeper than 40 metres (130 ft), using mixed gases, and/or entering overhead environments (caves or wrecks)
Surface supplied diving
Surface supplied diving
Surface supplied diving refers to divers using equipment supplied with breathing gas using a diver's umbilical from the surface, either from the shore or from a diving support vessel sometimes indirectly via a diving bell...

– use of umbilical gas supply and diving helmet
Diving helmet
Diving helmets are worn mainly by professional divers engaged in surface supplied diving, though many models can be adapted for use with scuba equipment....

s.
Saturation diving
Saturation diving
Saturation diving is a diving technique that allows divers to reduce the risk of decompression sickness when they work at great depth for long periods of time....

– long-term use of underwater habitats under pressure and a gradual release of pressure over several days in a decompression chamber at the end of a dive.

Scuba diver training and certification agencies

Recreational scuba diving does not have a centralized certifying or regulatory agency, and is mostly self regulated. There are, however, several large diving organizations that train and certify divers and dive instructors, and many diving related sales and rental outlets require proof of diver certification from one of these organizations prior to selling or renting certain diving products or services.

The largest international certification agencies that are currently recognized by most diving outlets for diver certification include:

American Canadian Underwater Certifications (ACUC) (formerly Association of Canadian Underwater Councils) – originated in Canada in 1969 and expanded internationally in 1984
British Sub Aqua Club (BSAC)
British Sub Aqua Club
The British Sub-Aqua Club or BSAC has been recognised since 1954 by the Sports Council as the governing body of recreational diving in the United Kingdom....

– based in the United Kingdom, founded in 1953 and is the largest dive club in the world
European Committee of Professional Diving Instructors (CEDIP) based in Europe since 1992 (see Cedip on French Wiki pages)
Confédération Mondiale des Activités Subaquatiques (CMAS)
Confédération Mondiale des Activités Subaquatiques
The Confédération Mondiale des Activités Subaquatiques is the World Underwater Federation. CMAS is the international umbrella organisation for recreational diver training organisations represented in the CMAS Technical Committee and underwater sports governed by the CMAS Sport Committee...

, the World Underwater Federation
National Association of Underwater Instructors (NAUI)
National Association of Underwater Instructors
The National Association of Underwater Instructors is a non-profit 501 association of SCUBA instructors. It was officially CE and ISO certified in May 2007 in all three diver levels and both instructor levels.-History:...

– based in the United States
Professional Diving Instructors Corporation (PDIC)
Professional Diving Instructors Corporation
The Professional Diving Instructors Corporation is an international SCUBA training and certification agency. It has an estimated 5 million active recreational divers....

– based in the United States
Professional Association of Diving Instructors (PADI)
Professional Association of Diving Instructors
The Professional Association of Diving Instructors is the world's largest recreational diving membership and diver training organization founded in 1966 by John Cronin and Ralph Erickson...

– based in the United States, largest recreational dive training and certification organization in the world
Scottish Sub Aqua Club (SSAC or ScotSAC) the National Governing Body for the sport of diving in Scotland.
International Training SDI, TDI & ERDi - based in the United States, TDI is the world's largest technical diving agency, SDI is the recreational division focusing on new methods and online courses, and ERDi is the public safety component.
Scuba Schools International (SSI)
Scuba Schools International
Scuba Schools International or SSI is an organization that teaches the skills involved in scuba diving and supports Dive Businesses and Dive Resorts.SSI has well over 2,500 authorized dealers and 35 Regional Centers and Area Offices all over the World....

– based in the United States with 35 Regional Centers and Area Offices around the globe.
YMCA Scuba – based in the United States, provided by Young Men's Christian Association (YMCA); discontinued after 2008.

External links

Divers Alert Network—Diving Emergencies/Hyperbaric Chamber Assistance

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

History

Etymology

Types of diving

Breathing underwater

Open-circuit regulator

Rebreather

Gas mixtures

Refraction and underwater vision

Light underwater

Controlling buoyancy underwater

Being mobile underwater

Underwater communication

Table of Hand Signals

Hazards and dangers

Injuries due to changes in pressure

Decompression sickness

Nitrogen narcosis

Oxygen toxicity

Avoiding losing body heat

Avoiding skin cuts and grazes

Diving longer and deeper safely

Scuba diver training and certification agencies

See also

Further reading

External links