A diving cylinder, scuba tank or diving tank is used to store and transport high pressure breathing as a component of SCUBA (Self-Contained Underwater Breathing Apparatus). It provides gas to the Scuba Diver through the demand valve of a diving regulator.
Diving cylinders typically have an internal volume of between 3 and 18 litres and a maximum pressure rating of 200 bar to 300 bar, (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. 696 (3 x 232) litres (25ft³) of gas at atmospheric pressure can be compressed into a 3-litre cylinder filled to 232 bar. Cylinders also come in smaller sizes, such as 0.2, 1.5 and 2 litres, however these are not generally used for breathing, instead being used for purposes such as marker buoy, dry suit and buoyancy compensator inflation.
Valves – A K-valve, which comes on all aluminum tanks and low pressure steel tanks, do nothing other than release or stop the flow of air from the cylinder.
Din Valve – These valves are used on all high pressure tanks – 3300 PSI and above. Regulators used on HP tanks require that your regulator be converted to DIN.
If a diver wants the freedom to dive anytime, a tank is a must. You can safely use a tank that was filled a week ago. Aluminum tanks are much lighter than steel tanks, but steel tanks take more punishment, and as a rule last longer.