A positive displacement flowmeter is a type of flow meter that requires fluid to mechanically displace components in the meter in order for flow measurement. Positive displacement (PD) flow meters measure the volumetric flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes (finite increments or volumes of the fluid). A basic analogy would be holding a bucket below a tap, filling it to a set level, then quickly replacing it with another bucket and timing the rate at which the buckets are filled (or the total number of buckets for the “totalized” flow). With appropriate pressure and temperature compensation, the mass flow rate can be accurately determined.
Positive-displacement meters provide high accuracy, ±0.1% of actual flow rate is claimed, and good repeatability. Their accuracy is not affected by pulsating flow and many designs do not require a power supply nor lengths of straight pipe upstream and downstream in their installation.