Pneumatic devices are principally very similar to hydraulic systems. A source of pressurized air is used to power and drive linear or rotary cylinders, controlled by manual or electrically controlled solenoid valves. Since the source of pressurized air is separate from the moving actuators, these systems have lower inertial loads. However, since pneumatic devices operate at a much lower air pressure, usually up to 100-120 psi max, their power to weight ratio is much lower than hydraulic systems.