Cylinders come in a variety of air capacities, depending upon their pressure rating and size. In the metric system, you express cylinder capacity in litres or kilograms of water capacity. The most common sizes are 8, 10, 12 and 15 litres. In the imperial system, you express capacity in the number of cubic feet of air you would have if you released it all at the surface. The three most common cylinder sizes are 50, 71.2 and 80 cubic feet, although other sizes are available.
The standard 12 litre/71.2- or 80-cubic-foot cylinder contains about the same air you have in a walk-in closet, compressed into a space about 600mm/two feet long and 150mm/half a foot in diameter. As this air is compressed into the cylinder, its pressure increases. The pressure in scuba cylinders may be higher than 320 bar/4500 pounds per square inch (psi), but typical pressure ratings range from about 170 to 200 bar or 2250 and 3000 psi.
Scuba cylinders are either made of aluminium or steel. Both types are subject to regulations, usually established by governments or agencies, and must pass periodic pressure tests mandated by these agencies. Both steel and aluminium are equally acceptable materials for scuba cylinders, with different advantages and disadvantages. Aluminium cylinders resist corrosion better whereas steel ones can hold a similar quantity of air in a smaller cylinder at a higher pressure.
The regulating agencies require cylinder manufacturers to stamp specific information onto the cylinder neck. These markings indicate the type of material the cylinder is made from and the maximum pressure permitted in the cylinder (working pressure). Additional makings include a serial number identifying the cylinder, dates of all pressure tests and a manufacturer of distributor symbol, although these markings may vary internationally.