232 bar: 18L(141 ft3) and 15L(117 ft3) = 8-inch (large); 12L(95 ft3) and 10L(78 ft3) = 7-inch (standard); 7L(54 ft3) and 5L(39 ft3) = 5-inch (small).
In many parts of the world, the most popular SCUBA tank is the steel "12 liter"; the US equivalent is the "HP100". However, in North America and the Caribbean you will find the most common SCUBA tank to be the aluminum "S80" because aluminum cylinders manufactured in the US are about half the price of imported steel cylinders. Although, the buoyancy characteristics of the steel HP100 is considerably better than the aluminum S80.
The maximum service pressure for SCUBA cylinders in the US is {3442 psi | 237 bar}, there are no 300 bar service pressure cylinders. Although you may see the occasional 300 bar DIN valve, high pressure steel tanks in the US have 232 bar DIN valves and most aluminum tanks will have a "K" valve that only works with DIN regulators when using a DIN-to-Yoke adapter. For more information about tanks available in North America, read our article How to Select a SCUBA Tank .
Imperial cylinders are described by their free air capacity in cubic feet at the service pressure in pounds per square inch, while metric cylinders are usually described by their water volume (WC) in liters and working pressure (WP) in bar. Conversions between cylinders intended for metric markets and their imperial equivalents are very problematic because not everyone does their math the same way using the same conversion factors. Depending on the specific marketplace (such as US or EU), even cylinders with similar water volumes can vary slightly in length and weight due different wall thicknesses plus the working pressures can be different too. Just to make matters worse, the common names used to describe the tanks can be misleading as to their free air capacity.
Below are examples of four cylinders with European Union EN standard specifications and similar cylinders with North American DOT standard specifications.
| Water Volume |
Service Pressure |
True† Capacity |
Diameter | Common Name |
|---|---|---|---|---|
| 5.0 L | 232 bar | ≈39.2 ft3 | 140 mm | Steel "5 Litre" (Faber EN/ISO) |
| 5.7 L | 3000 psi | ≈40.0 ft3 | 5.25 in | Aluminum "S40" (Metal Impact DOT) |
| 10.0 L | 232 bar | ≈78.4 ft3 | 178 mm | Steel "10 Litre" (Faber EN/ISO) |
| 11.1 L | 3000 psi | ≈77.6 ft3 | 7.25 in | Aluminum "S80" (Metal Impact DOT) |
| 10.2 L | 3442 psi | ≈80.1 ft3 | 7.25 in | Steel "HP80" (Faber DOT) |
| 12.2 L | 232 bar | ≈95.6 ft3 | 178 mm | Steel "12 Litre" (Faber EN/ISO) |
| 12.9 L | 3442 psi | ≈101.3 ft3 | 7.25 in | Steel "HP100" (Faber DOT) |
| 15.0 L | 232 bar | ≈117.6 ft3 | 203 mm | Steel "15 Litre" (Faber EN/ISO) |
| 15.0 L | 3442 psi | ≈117.8 ft3 | 8.00 in | Steel "HP117" (Faber DOT) |
Gas capacities are nominal for air and the actual free capacity depends on the settled pressure, temperature, and gas composition. DOT cylinder neck openings are 0.750-14 NPSM imperial thread form and EN cylinder neck openings are M25 x 2.0 metric thread form; the two thread forms are not compatible.