Scuba diving is an underwater diving mode where divers use breathing equipment completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The word scuba is an acronym for "Self-Contained Underwater Breathing Apparatus" and was coined by Christian J. Lambertsen in a patent submitted in 1952. Scuba divers carry their source of breathing gas, affording them greater independence and movement than surface-supplied divers, and more time underwater than freedivers. Although compressed air is commonly used, other gas blends are also employed.
Open-circuit scuba systems discharge the breathing gas into the environment as it is exhaled and consist of one or more diving cylinders containing breathing gas at high pressure which is supplied to the diver at ambient pressure through a diving regulator. They may include additional cylinders for range extension, decompression gas or emergency breathing gas. Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases. The volume of gas used is reduced compared to that of open-circuit, making longer dives feasible. Rebreathers extend the time spent underwater compared to open-circuit for the same metabolic gas consumption. They produce fewer bubbles and less noise than open-circuit scuba, which makes them attractive to covert military divers to avoid detection, scientific divers to avoid disturbing marine animals, and media diver to avoid bubble interference.
