Scuba Diving
Published on Apr 29, 2016
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Scuba diving is a form of underwater diving in which a diver uses a SCUBA to breathe underwater. Unlike other models of underwater diving, which rely on breath-hold or air pumped from the surface, scuba divers carry their own source of breathing gas, allowing them greater freedom of movement. Scuba equipment may be open circuit, in which oxygen is expelled into the surroundings, or a closed or semi-closed circuit rebreather, in which Breathing gas is scrubbed to remove carbon dioxide, and the oxygen used is replenished from a supply of feed gas before being re-breathed.
Photo by Derek Keats
Scuba diving may be performed for a number of reasons, both professional and personal. Recreational diving is performed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater. There are divers who work full or part-time in the recreational diving community as instructors, divemasters and dive guides. Other specialist areas include military diving. They can perform roles such as direct combat, infiltration, placing mines or engineering operations. In civilian operations many police forces operate diving teams to perform "Search and Recovery". Lastly there are professional divers involved in the water itself, such as underwater photography, oceanography, marine biology, hydrology and underwater archaeology.
The defining equipment used by a scuba diver is the eponymous scuba, the self-contained underwater breathing apparatus which allows the diver to breathe while diving, and is transported by the diver. As one descends, in addition to the normal atmospheric pressure, the water exerts increasing hydrostatic pressure of approximately 1 bar for every 10 m of depth. The pressure of the inhaled breath must balance the surrounding or ambient pressure to allow inflation of the lungs. It becomes virtually impossible to breathe air at normal atmospheric pressure through a tube below three feet under the water.Most recreational scuba diving is done using a half mask which covers the diver's eyes and nose, and a mouthpiece to supply the breathing gas from the demand valve.
Photo by star5112
Less common are closed circuit (CCR) and semi-closed (SCR) rebreathers, which unlike open-circuit sets that vent off all exhaled gases, process all or part of each exhaled breath for re-use by removing the carbon dioxide and replacing the oxygen used by the diver.Rebreathers release little or no gas bubbles into the water, and use much less stored gas volume, for an equivalent depth and time because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications. Rebreathers are more complex and more expensive than open-circuit scuba, and special training and correct maintenance are required for them to be safely used, due to the larger variety of potential failure modes
Photo by tiswango
For some diving, gas mixtures other than normal atmospheric air can be used, so long as the diver is competent in their use. The most commonly used mixture is Nitrox, also referred to as Enriched Air Nitrox (EANx), which is air with extra oxygen, often with 32% or 36% oxygen, and thus, less nitrogen, reducing the risk of decompression illness or allowing longer exposure to the same pressure for equal risk. The increased partial pressure of oxygen due to the higher content of oxygen of Nitrox increases the risk of oxygen toxicity. To displace nitrogen without the increased oxygen concentration, other inert gases may be used, usually helium, the resultant of these three gas mixtures is called Trimix, and when the nitrogen is fully substituted for helium, it is named Heliox.
Photo by Joi
To dive safely, divers must control their dare of descent and ascent in the water and be able to maintain a constant depth in midwater. The diver's overall buoyancy determines wether they descend or ascend. Equipment such as diving weighting systems, diving suits and buoyancy compensators can be used to adjust the overall buoyancy. When divers try to remain at constant depth, they try to achieve neutral buoyancy. This minimises the effort of swimming and therefore reduces gas consumption. The buoyancy force on the diver is the weight of the volume of the liquid that they and their equipment displace minus the weight of the diver and their equipment; if the result is positive, the force is upwards.
Photo by AstroSamantha
Water has a higher refractive index than air - similar to that of the cornea of the eye. Light entering the cornea from water is hardly refracted at all, leaving only the eye's crystalline lens to focus light. Diving mask and helmets solve this problem by providing 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 water than they actually are. This also affects photography.
Before starting any dive both the diver and their buddy to equipment checks to ensure everything is in good working order and available. Additionally, there is a Dive planning to ensure the diver's do not exceed their skill or comfort zone, or the safe capacity of their equipment. This includes Scuba gas planning to ensure that the amount of breathing gas to be carried is sufficient to allow for any reasonably foreseeable contingencies.
Photo by Ocean Networks Canada
Water conducts heat 25 times better than air, which can lead to hypothermia even in mild water, which can quickly become deadly in an aquatic environment. In all but the warmest waters, divers need thermal protection provided by wetsuits or dry suits. In the case of a wetsuit, the suit is designed to minimise heat loss. Wetsuits are normal led made of neoprene that has small closed gas cells, trapped in during manufacture. In the case of a sry suit, it does exactly what the name implies: it keeps a diver dry. The suit has a waterproof seal so that frigid water cannot penetrate the suit. Dry suit garments are usually worn under a dry suit, to provide thermal insulation.
Photo by sebastien.barre
Recreational scuba diving does not have a centralised certifying agency, and is mostly self-regulated. There are, however, several large organisations that train and certify divers and dive instructors, one of them being PADI (Professional Association of Dive Instructors). Underwater dive training is normally given by a qualified instructor who is a member of one of the many dive agencies or is registered with a government agency. Basic diver training entails the learning skills required for the safe conduct of activities in an underwater environment and includes procedures and skills for use of diving equipment, safety, emergency self-help and rescue procedures, dive planning and use of dive tables.
Photo by jeshua.nace
