Hydraulics International, Inc, (HII) is a manufacturer of pneumatically driven, high-pressure liquid pumps, gas boosters, air pressure amplifiers, and high pressure systems.

HII air driven liquid pumps operate on the principle of differential working areas. An air piston drives a smaller diameter plunger or piston to provide a pressure ratio. The pressure ratio is the method of determining maximum outlet pressure available. Hydraulics International model numbers reflect the nominal pressure ratio of the pumps. HII liquid pumps operate at a drive air pressure of 12-150 psi (0.8-10 Bar) and will cycle automatically when compressed air is first applied to the drive.

It will start cycling at its maximum speed thus producing maximum fluid flow. At this stage, the pump is acting as a transfer pump, filling the pressure system with fluid. As resistance increases in the system (creating backpressure to the pump), the pump will decrease its reciprocating action. Backpressure in the system finally balances with the incoming air drive pressure times the ratio. At this point, the pump will stall and maintain pressure without consuming any energy. If there is further demand on the system for flow, the pump will automatically start cycling again.