Positive-Displacement Flowmeters

About using positive-displacement flowmeters

All positive-displacement meters operate by using mechanical divisions to displace discrete volumes of fluid successively. Whilst this principle of operation is common, many different mechanical arrangements exist for putting the principle into practice. All versions of positive-displacement meter are low-friction, low-maintenance and long-life devices, although they do impose a small permanent pressure loss on the flowing fluid. Low friction is especially important when measuring gas flows, and meters with special mechanical arrangements to satisfy this requirement have been developed.

The rotary piston meter is the most common type of positive-displacement meter. It uses a cylindrical piston which is displaced around a cylindrical chamber by the flowing fluid. Rotation of the piston drives an output shaft. This can either be used with a pointer-and-scale system to give a visual flow reading or be converted into an electrical output signal.

Positive-displacement flowmeters account for nearly 10% of the total number of flowmeters used in the industry. Such devices are used in large numbers for metering domestic gas and water consumption. The cheapest instruments have an accuracy of about 1.5%, but the accuracy in more expensive ones can be as good as 0.2%. These higher-quality instruments are used extensively within the oil industry, as such applications can justify the high cost of such instruments.

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