Air Cylinders

Air cylinders used for scuba diving come in various sizes and materials and are typically designated by material — usually aluminium or steel. In the U.S. the size is designated by how much air they contain when expanded to 1 atmosphere, 80, 100, 120 cubic feet, etc., with the most common being the "Aluminum 80" which will give an average experienced diver from 40 to 60 minutes of dive time under common dive conditions. In Europe the size is given as their internal volume (10 liter, 12 liter, etc.).

Air cylinder pressure will vary according to the type of material used, ranging from 200 bar (2,900 psi) up to 300 bar (4,400 psi).

Aluminium cylinders are less expensive than steel and have been known to last for 20 years with standard regular maintenance. The drawback is that an aluminium cylinder is thicker and bulkier than a steel cylinder of the same capacity, which means the diver would need to carry more weight. Many steel cylinders also accept higher pressure fills, carrying more air for the same displacement of cylinder.

Compressed air diving cylinders are sometimes colloquially called "tanks", although the proper technical term for them is "cylinder".