In the positive displacement pump the movement of the fluid is forced by reducing the volume of the pump chamber.

The types are divided in rotary (e.g. lobe, screw, gear pump) and reciprocating (e.g. piston, diaphragm pump) positive displacement pumps.

In opposition to the centrifugal pumps the positive displacement pumps are suitable for a very high viscosity of the pumped fluid. Furthermore the reciprocating pumps can be suitable for metering and dosing applications.

Also see:
Reciprocating Pump
Centrifugal Pump

Viscosity
Curve Corrections for Different Fluids