Positive displacement flow meters measure fluid, usually liquid, flow when the flow mechanically displaces components placed in the fluid.
Within positive displacement flow meters are chambers through which fluid flows. These chambers contain a variety of mechanical components that help separate liquid or gas media into fixed, metered volumes. One such component is the rotating or reciprocating mechanism, which allows free flow only when it rotates a certain way. All components are rotated and shifted, or displaced, by the flow of liquid. The rate at which the rotating mechanism turns is directly proportional to the flow rate of the fluid. These features set positive displacement flow meters apart from other flow meters.
In order to gauge flow rate, positive displacement flow meters measure displacement. To do so, they accumulate a fixed volume of fluid, and then keep track of the number of times the volume is filled. One way to think about this is with the following analogy: A woman fills a bucket of water and then quickly grabs another bucket. She fills this bucket to the same line as the first bucket. As she does so, she times the rate at which the second bucket fills. Then, she uses the information to determine flow rate. (Assume that appropriate pressure and temperature compensations are met.)
There are two main types of positive displacement flow meters, sensor-only flow meters and transducer flow meters:
Sensor-only systems: provide integrated user interface display
Transducer systems: provide electronic outputs for controllers, processors and data acquisition systems
Both types of positive displacement flow meter are highly accurate and can be used with dirty, corrosive, or very viscous fluids. They can also be used to measure the volumetric flow rate of gases. They are popular for use in applications from chemical injection, to hydraulic testing, among other precision applications.
Each type of flow meter system has important specifications that you need to note while shopping for them. These include: temperature range, connection size, minimum and maximum measurable flow rate, percent accuracy (usually shown as a percentage of actual reading, not the full scale), operating pressure, and maximum permissible material viscosity.
For the best results, it's important that you connect with an experienced flow meter manufacturer that can guide your selection process. If you're interested, such a manufacturer can likely also design you a custom flow meter system. To learn more, check out the manufacturers we have listed on this page.