It is important to ensure there is a reserve of breathable compressed gases in the tank. The amount to reserve depends on the risk of the diving trip. When scuba tanks need to be refilled scuba air compressors are used. The filling process must be done very carefully as filling too fast can result in heightened pressure and heating which turns to low pressure after cooling. Fill speed is usually at the rate of 1 bar per second to keep the tanks from this risk.
