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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