Valves provide access to the gas in the cylinder and help control the gas flow. Most often, valves are made from chrome-plated brass. When filling a cylinder to the required pressure, it can become over-pressurized. This can happen if the cylinder is left out in direct sunlight and the gas expands. The cylinder can also be over-pressurized accidentally during the filling process. Pressure-relief valves are often included in the valve design. These valves are referred to as burst disks.
Most valves today are K-valves. These valves have a simple on/off valve, allowing gas in or out, with or without a burst disk. In the past, when submersible gas pressure gauges were not available, the cylinders were fitted with a J-valve. This valve was designed to close when the pressure within the cylinder reached a pre-determined level at which time the valve would close. The OC diver would be alerted that they were low on air. By pulling the lever, the diver would open the valve again, and he or she could ascend to the surface with the last remaining air/gas in the cylinder.
The standard DIN thread size is 5/8". The new European Union regulations SS-EN 144-3 require all oxygen and nitrox cylinders with an oxygen content of more than 22% to have the new DIN thread connection size of M26 x 2. Cylinders with compressed air will have the standard DIN M25 thread.