Metering pumps are positive displacement machines that use a diaphragm in combination with suction and discharge check valves to pressurize and move a fluid.

Quite simply, metering pumps are either mechanically or hydraulically actuated. In a mechanical pump, the diaphragm and the drive unit are physically connected. In a hydraulically actuated pump, the diaphragm is actuated by pressurized hydraulic oil.

Both types of pumps can be further classified by their method of volume control: either lost motion or amplitude modulation. In an amplitude modulated machine, the actual stroke length of the diaphragm is adjusted by means of a variable cam and gear arrangement. Horsepower requirements will therefore increase and decrease with flow. A lost motion machine will utilize mechanical stops, springs or a hydraulic bypass without actually adjusting the travel of the cam; horsepower consumption remains constant.