A category of pumps suitable for applications requiring metering characteristics.

Such devices have the following characteristics.

• High self-priming ability (high suction lift; excellent ability to draw liquid from below the pump)
• High discharge pressure (high upward lift; excellent ability to push liquid above the pump)
• Usually less transferred liquid than non-positive displacement pumps

The principle behind positive displacement pumps is the conversion of machine energy directly into potential energy in the transferred liquid through the use of a partition or closed chamber inside the pump chamber. This construction prevents backflow from the discharge side to the suction side even if the pump is stopped.

The volume of the sealed pump chamber in a positive displacement pump changes through reciprocating motion such as with a piston pump, resulting in the pump liquid being forced out. Other types of positive displacement pumps maintain a set volume that is moved within the chamber through rotary motion such as with a gear pump, resulting in the liquid being forced out.