Scuba diving is the use of independent breathing equipment to stay underwater for long periods for recreational diving and professional diving. Generally the diver swims underwater, but walking and the use of diver propulsion vehicles is possible while breathing from scuba equipment. The word 'scuba' is an acronym for Self Contained Underwater Breathing Apparatus, although it is grammatically acceptable to refer to 'scuba equipment' or 'scuba apparatus' in conversation.
The two types of scuba equipment are the "open-circuit" Aqua-lung and the "closed-circuit" rebreather.
Issues that diving poses
Also see Diving disorders for a longer description of these issues followed by an extensive discussion of physical and medical risks.
Equipment to allow underwater breathing
The two most common types of equipment are:
There is no single optimal breathing gas mix for every type of dive
A wide variety of breathing gas mixes have been used for diving. Each mix must contain sufficient oxygen to sustain life and consciousness. Mixes may contain other gases such as nitrogen and helium. As the concentration of gases increases with the depth of the dive and some gases are toxic at high concentrations, the design of breathing gas mixes depends on the depth of the dive.
Need to 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 "equalises" the pressure in all air spaces with the surrounding water pressure when changing depth.
Need to avoid decompression sickness
Decompression sickness can be prevented by decompression stops and a slow ascent using dive computers or decompression tables for guidance.
Need to see underwater
Diving masks and diving helmets solve this problem. Occasionally commando frogmen use special contact lenses instead, to avoid searchlight beams reflecting off a mask window.
Controlling buoyancy underwater
This is needed to descend and ascend safely and at will. Diving weighting systems, diving suits and buoyancy compensators all contribute to the diver's buoyancy.
Avoiding losing body heat
This causes hypothermia. Water conducts heat from the diver 25 times better than air. Except in very warm water, the diver needs the thermal insulation provided by a diving suit.
Avoiding skin cuts and grazes
Diving suits also help prevent the diver's skin being damaged by rough or sharp underwater objects and marine animals.
Diving longer and deeper safely
There are a number of techniques to increase the diver's ability dive deeper and longer:
Being mobile underwater