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Scuba Tanks are very important for divers and are used to store as well as transport breathing gas. These tanks provide divers with the breathing gas or air they need to be able to go down to the depths of the sea.
Scuba tanks can be used above the water as well as oxygen for patients or athletes that need it. A Scuba Tank is one of the most important pieces of equipment that one can have in diving. Without it, you will not be able to go very deep on a dive.
Divers may carry one cylinder or multiples, depending on the requirements of the dive. In parts of the world where diving takes place in warm water and in good visibility, recreational divers usually carry only one cylinder.
An example of this type is coral reef diving where it is possible to do an interesting dive without going deep or needing long decompression.
Where diving risks are higher, for example in parts of the world where the water is cold and visibility is low or when recreational divers do deeper or decompression diving, divers routinely carry more than one gas source.
An example of this type is north European diving where the temperature is often less than 60F and visibility less than 33ft and many interesting dive sites are shipwrecks in deeper water on the sea bed.
In recreational diving, each cylinder may have a different purpose. The cylinder may be used as a primary breathing source which is intended to be breathed from for most of the dive. A cylinder used as a bail out or bale out is carried purely as an independent safety reserve. A pony is a small bail out cylinder.
Diving cylinders typically have an internal volume of between 3 and 18 liters and a maximum pressure rating of about 3000 psi to 4500 psi. The internal cylinder volume is also expressed as water capacity; the volume of water which could be contained by the cylinder.
When pressurized, a cylinder carries a volume of gas greater than its water capacity because gas is compressible. 25 square feet of gas at atmospheric pressure can be compressed into a 3-liter cylinder filled to 232 bar.
Cylinders also come in smaller sizes, such as 0.2, 1.5 and 2 liters, however these are not generally used for breathing, instead being used for purposes such as Surface Marker Buoy, dry suit and buoyancy compensator inflation.
For safety, divers sometimes carry an additional redundant aqualung (a second scuba tank and scuba valve) to mitigate out-of-air emergencies should the primary breathing source fail.
For most common recreational diving (for example dives of 20 m to examine typical coral reefs) such extra equipment is usually not needed or used.