The quantity and duration is often controlled by a computer.
Metering pumps are positive displacement pumps which can deliver output at high pressures. The pump is usually driven by a piston.
For sensitive liquids which require sterility and for corrosive liquids, the pump is ideal as it does not come in contact with the liquid. A diaphragm acts as the interface between the piston and the liquid.
Metering pumps can also be designed as gear pumps.