Positive Displacement (PD) Flowmeters are volumetric flow metering instruments that measure flow by passing discreet parcels of fluid in precisely known volumes. PD meters are precision instruments whose internal moving components are mass-balanced yet remain hydraulically imbalanced. The result is that the meter can measure very low flows of both liquids and gases without using external power. The PD meter derives the power necessary to work from the energy contained in the flow process.

As illustrated in the picture below each moving chamber of fluid is separated from the next chamber by a capillary seal, the integrity of which is a function of the precision to which the meter is manufactured. This high precision allows these meters to be almost universally accepted as transfer standards when properly installed and flow-calibrated. The close tolerances of the internal metering components requires the PD meter be used only with clean fluids. The metallurgy of the meter dictates the type of process for which it can be used. Generally, these meters are used in dedicated unidirectional flow applications, such as fuel oil dispensing, natural gas or water metering.

Good instrumentation practice requires a filtering mechanism and a capacitance vessel for air removal for liquids or a coalescing filter for gases be an integral part of the metering installation. Advances in technology allow the meters to be temperature-compensated and interfaced electronically with central control systems as easily as they can be the integral part of a truck-mounted delivery system. Individual manufacturers' specifications will help guide the user to select the correct meter for the application. Examples of the more common PD meters are oscillating piston, nutating disc, oval gear, roots, vane, rotor, and multi-piston.