Scuba diving
is a mode of underwater diving whereby divers use breathing equipment
that is completely independent of a surface breathing gas supply, and
therefore has a limited but variable endurance. The name 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 own source of breathing gas, usually compressed
air, affording them greater independence and movement than
surface-supplied divers, and more time underwater than free divers.
Although the use of compressed air is common, a gas blend with a higher
oxygen content, known as enriched air or nitrox, has become popular due
to the reduced nitrogen intake during long or repetitive dives. Also,
breathing gas diluted with helium may be used to reduce the effects of
nitrogen narcosis during deeper dives.