About Scuba Diving
Scuba diving (“SCUBA” now widely considered a word in its own right) is a form of underwater diving in which a diver uses a scuba set to breathe underwater.
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 (usually compressed air), allowing them greater freedom of movement than with an air line. Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding techniques as used in snorkelling and free-diving.
Depending on the purpose of the dive, a diver usually moves underwater by swim-fins attached to the feet, but external propulsion can come from an underwater vehicle, or a sled pulled from the surface.
The Origins of Scuba Diving
The first commercially successful scuba sets were the Aqualung open-circuit units 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 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 using a re-breather system, in which exhaled air is reprocessed to remove carbon dioxide. Modern versions of re-breather 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.
The term “SCUBA” (an acronym for self-contained underwater breathing apparatus) arose during World War II, and originally referred to United States combat frogmen’s oxygen re-breathers, developed by Dr. Christian Lambertsen for underwater warfare.
Scuba Diving Variations
Scuba diving may be performed for a number of reasons, both personal and professional. Most people begin through recreational diving, which is performed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving.
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 tasks such as in oil exploration,underwater welding or offshore construction.
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 of wrecks or underwater fishing, like spear fishing.
There are a fair number of divers who work, full or part time, in the recreational diving community as instructors, assistant instructors, dive-masters and dive guides. There is some controversy as to whether such recreational diving leadership personnel should be termed “professionals” or not.
Several of the recreational training agencies include the word “professional” in their name, something that is ridiculed by others, who do not feel that the training programs that recreational leaders undergo and their rather short career length warrant the title.
In some jurisdictions the professional nature, with particular reference to responsibility for health and safety of the clients, of recreational diver instruction, dive leadership for reward and dive guiding is recognised by national legislation.
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 or engineering operations.
In civilian operations, many police forces operate police 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 teams may also be part of a fire department, paramedical service or lifeguard unit, and may be classed as public service diving.
Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater filming divers, who set out to document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology.
The choice between scuba and surface supplied diving equipment is based on both legal and logistical constraints. Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legal constraints allow. Higher risk work, particularly commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice.
Scuba Diving Safety
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 determines whether he ascends or descends. Equipment such as diving weighting systems, diving suits (wet, dry or semi-dry suits are used depending on the water temperature) and buoyancy compensator scan 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 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; 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).
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.
Neutral buoyancy in a diver is a metastable state. It is changed by small differences in ambient pressure caused by a change 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.