Your own air supply

Diving cylinders usually have an internal volume of between 3 and 18 liters and a maximum pressure of 200 bar to 300 bar. The volume of the internal cylinder is expressed as water capacity, the number of liters that can be stored in the cylinder. When a cylinder is under pressure it carries a volume of gas which is greater than the water capacity because gas is compressible. 696 litres (25ft³) of gas at atmospheric pressure can be compressed into a 3 liter cylinder filled to 232 bar.

The cylinder contains several parts:

the pressure vessel is normally made of cold-extruded aluminium or forged steel. An especially common cylinder available at tropical dive resorts is an "aluminium-80" which is an aluminium cylinder of 0.39 cubic feet rated to hold (about) 80 ft³ of 14.7 psi gas at its rated pressure of 3000 psi (in metric units, its internal capacity is approximately 11.1 litres, to be pressurized to about 207 bar). Aluminium cylinders are also used where divers carry many cylinders, such as in technical diving, because the greater buoyancy of aluminium cylinders reduces the extra buoyancy the diver would need to achieve neutral buoyancy. In cold water diving, where a diver wearing a highly buoyant thermally insulating dive suit has a large excess of buoyancy, steel cylinders are often used because they are denser than aluminium cylinders. Kevlar wrapped composite cylinders are used in fire fighting breathing apparatus and oxygen first aid equipment, but are rarely used for diving, due to their high positive buoyancy

the pillar valve is the point at which the pressure vessel connects to the diving regulator. The purpose of the pillar valve is to control gas flow to and from the pressure vessel and to form a seal with the regulator. Some countries require that the pillar valve includes a burst disk, a type of pressure 'fuse', that will fail before the pressure vessel fails in the event of over pressurization.

a rubber o-ring forms a seal between the metal of the pillar valve and the metal of the diving regulator. Fluoroelastomer (i.e. "viton") o-rings are used with cylinders storing oxygen-rich gas mixtures to reduce the risk of fire.

The Y pillar valves. Most pillar valves only have one output and one valve. A Y valve has two outputs and two valves allowing two regulators to be connected to the cylinder. If one regulator “freeflows”, which is a common failure mode, its valve can be closed and the cylinder breathed from the regulator connected to the other valve.

Reserve lever or "J-valve" (obsolete). Until the 1970s, when submersible pressure gauges on regulators came into common use, diving cylinders often used a mechanical reserve mechanism to indicate to the diver that the cylinder was nearly empty. The gas supply was automatically cut-off when the gas pressure reached the reserve pressure. To release the reserve, the diver pulled a lever and finished the dive before the reserve (typically 500 psi) was consumed. On occasion, divers would inadvertently trigger the mechanism while donning gear or performing a movement underwater and, not realizing that the reserve had already been accessed, could find themselves out of air at depth with no warning whatsoever. The J-valve got its name from being item number J in one of the first scuba equipment manufacturer catalogs. The standard non-reserve yoke valve at the time was item K, and is often still referred to as a K-valve.

_{Source: www.wikipedia.org}