Positive Displacement Flow Meter

A Positive Displacement Meter is a type of Flow Meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. Positive displacement (PD) flow meters make volumetric flow measurements taking 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 "totalised" flow). With appropriate pressure and temperature compensation, the mass flow rate can be accurately determined.



Positive displacement flow meters are very accurate and have high turndown. They can be used in very viscous, dirty and corrosive fluids and essentially require no straight runs of pipe for fluid flow stream conditioning. They are widely used in custody transfer of oils and liquid fluids (gasoline) and are applied on residential home natural gas and water metering. A diaphragm meter, with which most homes are equipped, is an example of a positive displacement meter. This type of meter is appealing in certain custody transfer flow applications where it is critical that the metering be functional in order for any flow to take place.



PD flow meters are relatively expensive and produce the highest differential pressure (and subsequently greatest pressure drop head loss) of all the flow meter types.



Positive displacement flow meters measure the volume or flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes. These devices consist of a chamber that obstructs the media flow and a rotating or reciprocating mechanism that allows the passage of fixed-volume amounts. The number of parcels that pass through the chamber determines the media volume. The rate of revolution or reciprocation determines the flow rate. There are two basic types of positive displacement flow meters. Sensor-only systems or transducers are switch-like devices that provide outputs for processors, controllers, or data acquisition systems. Complete sensor systems provide additional capabilities such as an integral display and/or user interface. For both types of positive displacement flow meters, performance specifications include operating pressure, temperature range, maximum material density, connection size, and percent accuracy. Suppliers indicate whether devices are designed to move fluid or gas.

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