SCUBA is an acronym for Self-Contained Underwater Breathing Apparatus. This is a simple life support system designed for underwater exploration, consisting of one or more tanks of compressed gasses which is passed through a pressure regulator for easiest breathing.
The Oxygen Tanks used for most Scuba gear are constructed of either Aluminum or Steel, usually between 10 and 12 liters is size. They are capable of holding gas pressurized to 300 bar, giving a breathing time of around one hour, less under strenuous or deep diving conditions. When full they weigh between 10kg and 12kg each. Usually the tanks are weighted so that when full, they will sink in water but when empty they will rise.
The regulator supplied reduces the high pressure of the tank (at 300 bar) to a correctly breathable pressure depending upon depth of the diver. SCUBA pressure regulators and breathing apparatus is designed to work under normal atmosphere or higher pressures. They will not function correctly under very low (0.1 atmosphere or less) pressure conditions or in a vacuum.
For most open regulators, the tanks hold compressed air, frequently with an enhanced oxygen content to extend dive time and depths allowed. Under common conditions, this allows for dive depths of up to 30 meters. For deeper dives, the inert nitrogen is partly or entirely replaced with Helium.
The Scuba tanks are 10 liters in size, weigh 10kg when filled and cost Cr300 each. A pair of tanks is 20 liters, 21 kg (including harness) and costs Cr600. The purchase of an air tank includes one regulator and mouthpiece for breathing. Refilling the tanks is usually Cr10 and takes less than an hour. Replacing or restoring the CO2 costs Cr1 and takes less than a minute.
SCUBA regulator comes in one of three types: Open, semi-closed, and full closed. Open regulators simply exhaust the outgoing breath into the surrounding water. This type of gear is the simplest to use and maintain. The dive times estimate for a tank of gas assumes an open regulator.
Semi-closed regulator, also known as a rebreather, recycles some of the exhausted air back into the rebreather. This allows for more efficient use of the air tank and dive times of up to twice as long as the open regulators. The regulator is more complex than an open regulator and needs to be adjusted for the gas mixture to correctly balance the exhaled and new gas.
Fully closed regulators cycle the exhaled air through a CO2 absorbent material. This allows the air supply to last up to 10 times as long, though the lifetime CO2 scrubbing is usually the limiting factor. As with the rebreather, the closed regulator needs careful monitoring and adjustments not to cause problems.
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