A positive displacement meter is a type of flow meter that requires fluid to mechanically displace components in the meter in order for flow measurement.
Positive displacement flow meter technology is the only flow measurement technology that directly measures the volume of the fluid passing through the flow meter. This is achieved by repeatedly entrapping fluid in order to measure its flow.
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 analog 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. These devices consist of a chamber(s) 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 electronic 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 the minimum and maximum measurable flow rate, operating pressure, temperature range, maximum allowable material viscosity, connection size, and percent accuracy (typically as a percentage of actual reading, not full scale).
Operation principle of PD Meter Type
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