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SCUBA diving is the term used to describe the use of a 'Self-Contained Underwater Breathing Apparatus' to stay underwater for periods of time greater than the average individual can breath-hold. The diver self-propels underwater using fins attached to his/her feet. Some divers also move around with the assistance of a Diver Propulsion Vehicle, commonly referred to as a "scooter", or by using surface-tethered devices called sleds, which are pulled by a boat.

Divers are not limited to the use of scuba equipment in their sojourn underwater. While the 'Aqua-Lung', developed by Emile Gagnan, with assistance from Jacques-Yves Cousteau, is an "open-circuit" unit, rebreathers (both semi-closed circuit and closed circuit) and surface-supplied systems are used depending on the needs of the diver.

Although scuba diving is still evolving, general classifications have grown up to describe the pursuits a diver might follow. These classifications include, but are not limited to: recreational diving, public safety diving, technical diving (aka Techy Divers), military diving and commercial diving. Within recreational diving there are those who are considered professional divers, because they maintain a professional standard of training and skills. Some consider Technical Diving to be a subset of recreational diving, while others separate it out due to the extensively different training equipment and knowledge required to execute technical dives. Public safety diving and military diving might likewise be classified as commercial diving because the practitioners make a living from their pursuit of diving. However, public safety divers (police or rescue) and military divers have a different mission than the typical commercial diver.

The word 'SCUBA' is an acronym for "Self-Contained Underwater Breathing Apparatus", but it is grammatically acceptable to refer to 'scuba equipment' or 'scuba apparatus' in conversation.

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.

Typically the diver's nose and eyes are encapsulated in a diving mask, such that the nose cannot participate in inhalation except when wearing a full face diving mask. However, inhaling from a regulator's mouth-piece 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 valveregulator, 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.

Less common (but becoming increasingly so) are the closed and/or semi-closed rebreather units. Unlike the open circuit arrangements which vent all exhaled gases to the surrounding environment, rebreathers capture each exhaled breath and recycle it for re-use by removing the carbon dioxide buildup and replenishing the oxygen used up by the diver. Rebreathers release few or no gas bubbles into the water which has advantages for research, military, photography and other applications.

On deeper or more prolonged dives, gas mixtures other than normal atmospheric air are used, such as air with enriched oxygen content, known as nitrox, or oxygen with helium and a reduced percentage of nitrogen, known as trimix. In cases of technical dives, multiple cylinders 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.

Injuries due to changes in water pressure

The diver must avoid injury caused by changes in water pressure. Pressure injuries are called barotrauma. They are caused by pressure differences between the outside and trapped air spaces inside the diver or the diver's equipment. To avoid them, the diver equalizes the pressure in all air spaces with the surrounding water pressure when changing depth.

Effects of Breathing High Pressure Gas

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. This is done by making 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 a higher concentration of 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.

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. Being "narced" can impair judgement and make diving very dangerous. It occurs at any depth, but in most cases doesn't become noticeable until deeper depths; typically when breathing air at around 30m/100 ft. Jacques Cousteau famously described it as the "rapture of the deep".

Need to see underwater

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. Diving masks and diving helmets solve this problem by creating an air partition between the diver's eyes and the water. The distortion created by the water is effectively reversed as the light travels from water to air.

Divers who require corrective lenses to see clearly outside the water would normally require the same prescription while wearing a mask. Some masks can be ground to the diver's prescription to avoid the need for additional corrective lenses.

Occasionally commando frogmen use 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.

Controlling buoyancy underwater

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, diver's overall buoyancy determines whether a diver 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 (which go by many different names such as BC, Stability 'Stab' Jacket they are also know as a 'Wing' when used as part of a twin-set configuration) can be used to adjust the overall buoyancy. When divers want to remain at constant depth, they try to achieve neutral buoyancy. This minimises gas consumption caused by swimming to maintain depth.

The volumes and weights of the diver and all equipment attached to the diver, contribute to the diver's overall buoyancy. Volume creates an upward force and weight creates a downward force. If the force due to volume is greater than the weight, the diver ascends. If the force due to volume is less than the weight the diver descends. 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, reduce in volume 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.

Avoiding losing body heat

Water conducts heat from the diver 25 times better than air, which can lead to hypothermia. Except in very warm water, the diver needs the thermal insulation provided by wetsuits and drysuits. 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).

In the case of a dry suit, 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.

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 manoeuverability 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 manoeuverability 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.

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

Diving longer and deeper safely

There are a number of techniques to increase the diver's ability to 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.

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.

Not only is the underwater environment hazardous but diving equipment can be dangerous when used by the untrained; there are many unexpected problems that the new diver must be taught to avoid. Also, beginners need practice and a gradual increase in experience to build their confidence in their equipment and themselves, to develop the skills needed to control the equipment and to respond safely when they encounter difficulties.

Most commercial operators and dive clubs serving divers insist that each diver is able to show them "certification", evidence of a minimum level of training, for the type of diving the diver intends to do. Reputable dive operators, dive shops and compressor operators refuse to allow uncertified people to dive, hire diving equipment or fill diving cylinders.

Sources of Diver Training

Many diver training organizations exist, throughout the world, offering diver training leading to certification: the issuing of a "C-card" or qualification card.

A good dive training organisation, such as a dive school based at a dive shop, will always offer courses to the standard of a recognised certification organisation, such as those listed below. Many dive shops in popular holiday locations offer courses that can teach you to dive in a few days, and can be combined with your vacation. Upon completing the course the student is issued a certification card.

Many diver training organizations exist:

  • Entry-level recreational SCUBA diver training organisations: Using professional instructors. Examples of this type are SSI, PADI and NAUI; Amateur instructors. An example of this type is the British Sub Aqua Club;
  • Technical recreational SCUBA diving organisations. Examples of this type are ANDI, DSAT Tec (PADI), GUE, IANTD, TDI, USC (SSI), and NAUI Tec;
  • Commercial diver training organisations. Train divers for professional diving using SCUBA, surface supplied diving and saturation diving equipment and techniques;
  • National navies and armed forces. Train divers for ship maintenance, salvage and repair, rescue, mine clearance and covert operations using SCUBA and more advanced equipment and techniques.

Location of Training Lessons

Initial training typically takes place in three environments:

  • Classroom - where material is presented and reviewed
  • Swimming pool - where skills are taught and practiced in confined water
  • Open Water - where the student demonstrates the skills he or she has learned.

Typically, early open water training takes place in a local body of water such as a lake, a flooded quarry or a sheltered and shallow part of the sea. Advanced training mostly takes place at depths and locations similar to the diver's normal diving locations.

The usual sequence for learning most diving skills is to be taught the theory in the classroom, then be shown the skill and practice in the pool using the minimum equipment, then practice again in open water under supervision in full equipment and only then use the skill on real dives.

Bodies of Water for Diving

Most bodies of water can be used as dive sites:

  • Seas and Oceans - these consist of salt water and a huge variety of flora and fauna;
  • Lakes - small lakes are often used for diver training. Large lakes have many features of seas including wrecks and a variety of marine life. Man-made lakes, such as clay pits and gravel pits, often have lower visibility.
  • Caves - these are more adventurous and dangerous than normal diving.
  • Rivers - are often shallow, murky and have strong currents.
  • Quarries - abandoned rock quarries are popular in inland areas for diver training as well as recreational diving. Rock quarries also have reasonable underwater visibility - there is often little mud or sand to create mid-water particles that cause low visibility. As they are not "wild" and usually privately owned, quarries often contain objects intentionally placed for divers to explore, such as sunken boats, automobiles, aircraft, and even structures like grain silos and gravel chutes.

Dive Site Features

Many types of underwater feature make an interesting dive site, for example:

  • Wildlife at the site. Popular examples are coral, sponges, fish, sting rays, molluscs, cetaceans, seals, sharks, and crustaceans.
  • The Topography of the site. Coral reefs, drop offs (underwater cliffs), rock reefs, gullies and caves can be spectacular. Deep dive sites mean divers must reduce the time they spend because more gas is breathed at depth and decompression sickness risks increase. Shallow regions can be investigated by snorkeling.
  • Historical or cultural items at the site. Ship wrecks and sunken aircraft, apart from their historical value, form artificial habitats for marine fauna making them attractive dive sites.
  • Underwater visbility varies widely. Poor visibility is caused by particles in the water, such as mud, sand and sewage. Dive sites that are close to sources of these particles, such as human settlements and river estuaries, are more prone to poor visibility. Currents can stir up the particles. Diving close to the sediments on the seabed can result in the particles being kicked up by the divers fins.
  • Temperature. Warm water diving is comfortable and convenient. Although cold water is uncomfortable and can cause hypothermia it can be interesting because different species of underwater life thrive in cold conditions. Cold water means divers tend to prefer Dry suits with inner thermal clothing which offer greater thermal protection although require training and experience to use properly.
  • Currents. Tidal currents can transport nutrients to underwater wildlife increasing the variety and density of that life at the site. Currents can also be dangerous to divers as they can result in the diver being swept away from his or her surface support. Tidal currents that meet solid underwater vertical surfaces can cause strong up or down currents that are dangerous because they may cause the diver to lose buoyancy control risking barotrauma.



Information taken from Wikipedia - Under GNU Free Documentation License




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