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LEARN MORE OF SCUBA DIVING
Posted: 09/08/06 Scuba diving - International
 
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Scuba diving

Scuba divers.
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Scuba divers.

Scuba diving is the term used to describe the use of a self-contained breathing set to stay underwater for periods of time greater than the average individual can breath-hold. The diver carries all equipment necessary for diving and is not reliant upon equipment elsewhere (e.g. on the surface) to supply breathing gas or other support during the dive. The diver swims underwater using fins attached to his/her feet. 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 word 'SCUBA' is an acronym for "Self Contained Underwater Breathing Apparatus", but it has become acceptable to refer to 'scuba equipment' or 'scuba apparatus'. The term SCUBA in common usage usually means open-circuit equipment in which gas (usually air) is breathed from a tank of compressed gas and then exhaled into the water, usually in the line of kit development started by Emile Gagnan and Jacques-Yves Cousteau. But rebreathers (both semi-closed circuit and closed circuit) are also self-contained systems and could be classed as SCUBA. The term SCUBA arose during WWII and originally meant USA combat frogmen's oxygen rebreathers.

Or surface-supplied systems may be used depending on the needs of the diver.

Scuba diving is still evolving, but 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 (also called Tech Divers)
    • Cave diving
    • Deep diving
    • Ice diving
    • Wreck diving
  • military diving: this includes combat divers and armed forces work divers.
  • commercial 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.

History of diving

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

Diving Issues

A scuba diver.
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A scuba diver.

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 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.

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 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[1] better than air, which can lead to hypothermia. Except in very warm water, the diver needs the thermal insulation provided by wetsuits and drysuits. See the main article: Diving suit. 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.

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.

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

 

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.

 

Scuba dive 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 international certification agencies that are currently recognized by most diving outlets for diver certification include:

 

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