Scuba diving

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Scuba diving is swimming underwater while using self-contained breathing equipment. By carrying a source of compressed air with him, the scuba diver is able to stay underwater longer than he can with the simple breath-holding techniques used in Snorkeling and Free-diving, and is not hindered by air-lines to a remote air source. The scuba diver typically swims underwater using fins attached to the feet. However, some divers also move around with the assistance of a DPV (Diver Propulsion Vehicle), commonly referred to as a "scooter", or by using surface-tethered devices called sleds, which are pulled by a boat.

The term SCUBA arose during World War II and originally referred to USA combat frogmen's oxygen rebreathers, developed by Dr. Christian Lambertsen for underwater warfare. Today, SCUBA typically usually refers to the in-line open-circuit equipment, developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed gas (usually air) is inhaled from a tank and then exhaled into the water. However, rebreathers (both semi-closed circuit and closed circuit) are also self-contained systems (as opposed to surface-supplied systems) and are therefore classified as SCUBA.

Although the word 'SCUBA' is an acronym for "Self Contained Underwater Breathing Apparatus", it has also become acceptable to refer to SCUBA as 'scuba equipment' or 'scuba apparatus'.

Scuba divers exploring fish and coral.
The international diver-down flag. see International maritime signal flags.	The dive flag

[edit] Diving classification

Scuba diving is still evolving, but general classifications have grown up to describe various diving activities. These classifications include but are not limited to:

recreational diving
public safety diving
technical diving (also called Tech Divers)
military diving: this includes combat divers and armed forces work divers.
commercial diving.
Scientific diving.

Within recreational diving there are those who are considered professional divers, because they maintain a professional standard of training and skills (and must, in theory, according to the laws of the area, carry professional liability insurance).

Some consider technical diving to be a subset of recreational diving, but others separate it out due to the extensively different training equipment and knowledge needed for technical dives.

Public safety diving and military diving might likewise be classified as commercial diving because public safety divers and military divers make a living from their pursuit of diving. However, public safety divers (police or rescue) and military divers have a different mission from the typical commercial diver.

Scientific diving is used by marine scientists (including diving marine biologists and underwater archaeologists), as a tool for collecting their research data.

A scuba diver, as seen from below.

[edit] History of diving

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Early ideas of autonomous under-water systems appear in Twenty Thousand Leagues Under the Sea

Men and women have practiced breath-hold diving (free-diving) for centuries. Indirect evidence comes from ancient artifacts of undersea origin found on land (e.g. mother-of-pearl ornaments), and depictions of divers in ancient drawings. In ancient Greece, breath-hold divers are known to have hunted for sponges and engaged in military exploits. Of the latter, the story of Scyllis (sometimes spelled Scyllias; about 500 B.C.) is perhaps the most famous, as told by the 5th century B.C. Greek historian Herodotus (and quoted in numerous modern texts).

During a naval campaign the Greek Scyllis was taken aboard ship as prisoner by the Persian King Xerxes I. When Scyllis learned that Xerxes was to attack a Greek flotilla, he seized a knife and jumped overboard. The Persians could not find him in the water and presumed he had drowned. Scyllis surfaced at night and made his way among all the ships in Xerxes' fleet, cutting each ship loose from its moorings; he used a hollow reed as snorkel to remain unobserved. Then he swam nine miles (15 kilometers) to rejoin the Greeks off Cape Artemisium.

The desire to go under water has probably always existed: to hunt for food, uncover artifacts, repair ships (or sink them), and observe marine life. Until humans found a way to breathe underwater, however, each dive was necessarily short and frantic.

One of the major hurdles of diving is to stay under water for a longer period of time. Breathing through a hollow reed allows the body to be submerged, but reeds more than two feet long do not work well; difficulty inhaling against water pressure effectively limits snorkel length. Breathing from an air-filled bag brought under water was also tried, but it failed due to rebreathing of carbon dioxide and the buoyancy of the air bag.

In the 16th century people began to use diving bells supplied with air from the surface, the first effective means of staying under water for any length of time. The bell was held stationary a few feet from the surface, its bottom open to water and its top portion containing air compressed by the water pressure. A diver standing upright would have his head in the air. He could leave the bell for a minute or two to collect sponges or explore the bottom, then return for a short while until air in the bell was no longer breathable.

In 16th century England and France, full diving suits made of leather were used to depths of 60 feet. Air was pumped down from the surface with the aid of manual pumps. Soon helmets were made of metal to withstand even greater water pressure and divers went deeper. By the 1830s the surface-supplied air helmet was perfected well enough to allow extensive salvage work.

Starting in the 19th century, two main avenues of investigation - one scientific, the other technological - greatly accelerated underwater exploration. Scientific research was advanced by the work of Paul Bert and John Scott Haldane, from France and Scotland, respectively. Their studies helped explain effects of water pressure on the body, and also defined safe limits for compressed air diving. At the same time, improvements in technology - compressed air pumps, carbon dioxide scrubbers, regulators, etc., - made it possible for people to stay underwater for long periods.

[edit] Diving Issues

Divemaster ready to dive Shark and Yolanda reef at Rās Muhammad, Sharm el-Sheikh.

This section looks at some of the physiological issues posed by diving. See Diving hazards and precautions.

[edit] Breathing underwater

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

Early diving experimenters quickly discovered it is not enough to simply supply air in order 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 or 14.7 psi for every 33 feet or 10 meters of depth — so the pressure of the inhaled breath must exactly counter the surrounding or ambient pressure in order to safely and efficiently inflate the lungs.

By always providing the breathing gas at ambient pressure, modern demand valve regulators ensure the diver can inhale and exhale naturally and virtually effortlessly, regardless of depth.

As the diver's nose and eyes are inside a diving mask, the diver cannot breathe in through his nose, except when wearing a full face diving mask. However, inhaling from a regulator's mouthpiece becomes second nature very quickly.

The most commonly used scuba set today is the open circuit 2-stage diving regulator, coupled to a single pressurized gas cylinder. This 2-stage arrangement differs from Emile Gagnan's and Jacques Cousteau's original 1942 design, known as the Aqua-lung, in which the cylinder's pressure was reduced to ambient pressure in a single stage. The 2-stage system has significant advantages over the original single-stage design.

In the 2-stage design, the first stage regulator reduces the cylinder pressure of about 200 bar (3000 psi) to an intermediate level of about 10 bar (145 psi). The second stage demand valve regulator, connected via a low pressure hose to the first stage, delivers the breathing gas at the correct ambient pressure to the diver's mouth and lungs. The diver's exhaled gases are exhausted directly to the environment as waste. The first stage typically has at least one "high pressure" outlet, which delivers 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.

For more information, see diving regulator.

Less common (but becoming increasingly so) are the closed and/or semi-closed rebreathers. Open-circuit sets vent off all exhaled gases, but rebreathers reprocess each exhaled breath for re-use by removing the carbon dioxide buildup and replacing the oxygen used by the diver. Rebreathers release few or no gas bubbles into the water, and use much less oxygen per hour because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications see frogman. Modern rebreathers (rather than old-type plain oxygen rebreathers) are more complex and more expensive than sport open-circuit scuba, and need special training and maintenance to safely use. See rebreather.

For some diving, gas mixtures other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% other) can be used, so long as the diver is properly trained in their use. The most commonly used mixture is Enriched Air Nitrox, which is air with extra oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reducing the effect of decompression sickness and nitrogen narcosis.

Several other common gas mixtures are in use, and all need specialized training. Oxygen with helium and a reduced percentage of nitrogen is known as trimix, for example.

In cases of technical dives more than one cylinder may be carried, each containing a different gas mixture for a distinct phase of the dive, typically designated as Travel, Bottom, and Decompression. These different gas mixtures may be used to extend bottom time, reduce inert gas narcotic effects, and reduce decompression times.

[edit] Injuries due to changes in air pressure

Divers must avoid injuries caused by changes in air pressure. The weight of the water column above the diver causes an increase in air pressure in any compressible material (wetsuit, lungs, sinus) in proportion to depth, in the same way that atmospheric air causes a pressure of 14.7 lbs per square inch at sea level. Pressure injuries are called barotrauma and can be quite painful, in severe cases causing a ruptured eardrum or damage to the sinuses. To avoid them, the diver equalizes the pressure in all air spaces with the surrounding water pressure when changing depth. The inner ear and sinus are equalized using one of two techniques.

The first technique is known as the "Valsalva maneuver", which involves pinching the nose and gently attempting to exhale through it.

The second technique is known as the "Frenzel maneuver", which involves using your throat muscles in a swallowing motion to equalize the airspace. The Frenzel maneuver reduces the possibility of a round window rupture but is more difficult to master.

The mask is equalized by periodically exhaling through the nose. If a drysuit is worn, it too must be equalized by inflation and deflation, similar to a buoyancy compensator.

[edit] Effects of breathing high pressure gas

[edit] Decompression sickness

The diver must avoid the formation of gas bubbles in the body, called decompression sickness or 'the bends', by releasing the water pressure on the body slowly at the end of the dive and allowing gases trapped in the bloodstream to gradually break solution and leave the body, called "off-gassing." This is done by making safety stops or decompression stops and ascending slowly using dive computers or decompression tables for guidance. Decompression sickness must be treated promptly, typically in a recompression chamber. Administering enriched-oxygen breathing gas or pure oxygen to a decompression sickness stricken diver on the surface is a good form of first aid for decompression sickness, although fatality or permanent disability may still occur.

[edit] Nitrogen narcosis

Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in divers who breathe high pressure gas at depth. 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 the diver at 66 feet, or 3 atmospheres of pressure. At 66 feet, Narcosis manifests itself as slight giddiness. The effects increase drastically with the increase in depth. 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 prevents narcosis from occurring.

[edit] Oxygen toxicity

Oxygen toxicity occurs when oxygen in the body exceeds a safe "partial pressure" (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 130 feet / 39 meters) "hypoxic blends" containing a lower percentage of oxygen than atmospheric air are used. For more information, see Oxygen toxicity.

[edit] Refraction and underwater vision

A diver wearing an Ocean Reef full face mask

Water has a higher refractive index than air. Light entering the eye from the water behaves differently than light entering from air. This creates a distortion that affects normal vision by causing very severe hypermetropia. That is why people with severe myopia can see better underwater without a mask than normal-sighted people.

Diving masks and diving helmets and fullface masks solve this problem by creating an air space in front of the diver's eyes. The refraction error created by the water is mostly corrected as the light travels from water to air through a flat lens, except that objects appear approximately 34% bigger and 25% closer in salt water than they actually are. Therefore total field-of-view is significantly reduced and eye-hand coordination must be adjusted.

A HydroOptix Double-Dome mask

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.

Double-dome[1] masks restore panoramic vision, allowing objects to be seen at their true size and distance. One must be nearsighted to use a double-dome mask, however divers with normal vision wear disposable contact lenses, making themselves temporarily nearsighted.

On rare occasions, commando frogmen used special contact lenses instead, to see underwater without the large glass surface of a diving mask, which can reflect light and give away the frogman's position.

As a diver changes depth, he must periodically exhale through his nose to equalize the internal pressure of the mask with that of the surrounding water. Swimming goggles which only cover the eyes do not allow for equalization and thus are not suitable for diving.

[edit] Controlling buoyancy underwater

Diver under the Salt Pier in Bonaire.

To dive safely, divers need to be able to control their rate of descent and ascent in the water. Ignoring other forces such as water currents and swimming, the diver's overall buoyancy determines whether he ascends or descends. Equipment such as the diving weighting systems, diving suits (Wet, Dry & 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 same volume of the liquid that he is immersed in; if the result is negative, that force is upwards. Diving weighting systems can be used to reduce the diver's weight and cause an ascent in an emergency. Diving suits, mostly being made of compressible materials, shrink as the diver descends, and expand as the diver ascends, creating unwanted buoyancy changes. The diver can inject air into some diving suits to counteract this effect and squeeze. Buoyancy compensators allow easy and fine adjustments in the diver's overall volume and therefore buoyancy. For open circuit divers, changes in the diver's lung volume can be used to adjust buoyancy.

[edit] Avoiding losing body heat

Water conducts heat from the diver 25 times[2] better than air, which can lead to hypothermia 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, the diver needs the thermal insulation provided by wetsuits or drysuits. See the main articles: Diving suit, wetsuit and drysuit. In the case of a wetsuit, the suit is designed to minimize heat loss. Wetsuits are generally made of neoprene that has small gas cells, generally nitrogen, trapped in it during the manufacturing process. The poor thermal conductivity of this expanded cell neoprene means that wetsuits reduce loss of body heat by conduction to the surrounding water. The neoprene in this case acts as an insulator.

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

Spring Suit and Steamer.

In the case of a drysuit, it does exactly that: keeps a diver dry. The suit is sealed so that frigid water cannot penetrate the suit. Drysuit undergarments are often worn under a drysuit as well, and help to keep layers of air inside the suit for better thermal insulation. Some divers carry an extra gas bottle dedicated to filling the dry suit. Usually this bottle contains argon gas, because of its better insulation as compared with air.

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

Membrane: high level of diver maneuverability due to the thinness of the material, however that also means that heavy weight undersuit is required if diving in cooler water.
Neoprene: low level of diver maneuverability due to the material being considerably thicker than membrane material (even when dealing with compressed neoprene) however the neoprene provides a higher level of insulation for the diver.

[edit] Avoiding skin cuts and grazes

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

[edit] Diving longer and deeper safely

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

technical diving - diving deeper than 130 feet and/or using mixed gases.
surface supplied diving - use of umbilical gas supply and diving helmets.
saturation diving - long-term use of underwater habitats under pressure and a gradual release of pressure over several days in a decompression chamber at the end of a dive

[edit] Being mobile underwater

The diver needs to be mobile underwater. Streamlining dive gear will reduce drag and improve mobility. Personal mobility is enhanced by swimfins and Diver Propulsion Vehicles. Other equipment to improve mobility includes diving bells and diving shots.

[edit] Reasons for scuba diving

There are a variety of reasons why people go scuba diving. Below is a list why increasing numbers of people are getting engrossed in this sport:

It allows interested individuals to be close to various marine life: The large scale of underwater wildlife as well as the coral reefs leave an amusing and wonderful experience, not mentioning the thousands of fishes and marine creatures in varying kinds, colors, and shapes swimming around.
Each dive has something unusual to offer: Besides the fact that there are a lot of diving variations, it is great to know that every dive is a different underwater experience. The variety of marine life is so interesting that such an amazing adventure is sometimes unanticipated.
It is an enormous approach of discovery: It's truly a different world down there. Aside from the marine creatures which come in stunning colors and amazing features, there could still be many things to see and discover underwater.
It is very educational: This is not just going underwater and exploring the marine world. This is a sport where the diver experiences fun while learning a lot of information regarding underwater wildlife at the same time. The first part itself before going underwater is already informative; this is where the preparation and getting oneself equipped through proper training. In general, there is long list of things that should be learned when going scuba diving.
It allows people to execute definite tasks and services: People who are into professional diving are paid for a particular purpose such as underwater researches, engineering and scientific surveys and inspections, searches and recovery operations, and underwater photography and cinematography.
It is a relaxing outdoor activity: It is said that looking at an aquarium is a nice way to have some peace of mind after a tiring day of work. Scuba diving is much different and better in reality. It shows a wonderful and soothing environment.
It makes naval researches and studies truly feasible: This type of sport has an enormous part in making most marine researches and investigations. It is indeed a great help to the fields of science such as marine biology and among others.

[edit] Scuba dive training and certification agencies

See main article: List of diver training organizations

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: