Fluid power is the use of fluids under pressure to generate, control, and transmit power. Fluid power is subdivided into hydraulics using a liquid such as mineral oil or water, and pneumatics using a gas such as air or other gases. Compressed-air and water-pressure systems were once used to transmit power from a central source to industrial users over extended geographic areas; fluid power systems today are usually within a single building or mobile machine.
A fluid power system has a pump driven by a prime mover (such as an electric motor or internal combustion engine) that converts mechanical energy into fluid energy. This fluid flow is used to actuate a device such as:
Cost: pneumatics are considerably less expensive to build and operate. For one, air is used as the compressed medium, so there is no need to provide means to drain or recover fluid. With increasing working pressures, Hydraulics require larger parts than pneumatics.
Precision: Unlike liquids, gases change volume significantly when pressurized making it difficult to achieve precision.
Safety: Compressed gases tend to expand at high velocities when decompressed, thus pneumatics are typically limited in utilities with a working pressure up to around 100 psi (7 bar).