Volume boosters are classified as twin-piston pressure boosters and are intended solely for compressing air. The basic variant of the booster consists merely of a double piston system and a directional control valve for continuous operation. The input pressure is automatically doubled; it is not possible to adjust the pressure to a lower value. The use of a pressure booster is particularly recommended when the pressure in a system needs to be selectively increased. A system or sections of it do not have to be supplied with excessively high pressure, as this would lead to substantially higher operating costs.