Air cylinders come in a variety of sizes, capabilities, and uses. Typical Scuba cylinders are either aluminum or steel. Valves will be either K valve (US) or Din (European). Smaller cylinders, commonly referred to as “pony bottles” are usually used as a supplemental air supply, some with a built-in regulator – like the Spare Air. A cylinder that has been prepared for enriched air use will have a “NITROX” label on the cylinder.