Positive displacement (PD) meters measure volumetric flow directly by continuously separating (isolating) a flow stream into discrete volumetric segments and counting them. Therefore, PD meters are not inference-type meters, as are turbine meters and d/p meters. PD meters are often used in refineries, chemical plants, pipeline pump stations, marine loading terminals, and marketing truck loading racks. They are commonly used in liquid service, and in the gas industry, gas PD meters are used as domestic gas meters by utility companies. This section is primarily concerned with liquid PD meters for use by the petroleum and petrochemical industries.
The three basic subassemblies in a bare PD meter are the external housing, internal measuring element, and counter drive train.