Positive displacement (PD) meters are the types of flowmeters which are suitable for measurement of viscous liquid flows. These are also considered ideal for applications which require use of a simple mechanical meter
system. In general, a positive displacement flowmeter consists of a chamber or cavity which restricts the flow.
A rotating or reciprocating mechanical device is located inside the chamber to generate fixed-volume discrete parcels from the flowing liquid. Via PD meter units, liquids get separated into exactly calculated increments which are then further counted by a connecting register. Since every measured increment represents a distinct volume, these types of meters are widely used for automatic batching and accounting purposes.