Rate-of-Flow Meter

in engineering, an instrument for measuring the flow rate, that is, the volume or mass of a medium passing through the instrument per unit time. Rate-of-flow meters are used to control and monitor liquids, vapors, and gases during production, distribution, consumption, and storage. They also find application in the automatic systems for production control and for heat-and-power engineering. Rate-of-flow meters operating over an arbitrary time interval are called fluid meters; they may be used independently or as part of the measuring unit of fuel and oil pumps. Rate-of-flow meters are sometimes fitted with integrators that enable them to totalize the mass or volume of fluid.

Rate-of-flow meters with variable and constant pressure drops are used most widely. Differential manometers, in which the pressure drop in the pipe is brought about by the restrictions of diaphragms, nozzles, and Venturi tubes, are of the variable pressure drop variety. In rate-of-flow meters with a constant pressure drop, the cross-sectional area varies, but the drop in pressure before and after remains unchanged. Such meters are equipped with a submersible float or piston. In cases where it is impossible to use rate-of-flow meters, the flow velocity is measured by means of pressure gauges, propellers, and anemometers at several points along the stream. The flow velocity is then calculated at a particular cross section, and the volume flow rate is determined by multiplying the flow velocity by the cross-sectional area. This method is applicable in testing procedures where the temporary installation of a rate-of-flow meter is inadvisable. Inductive, ultrasonic, and radioactive techniques are employed in certain specialized rate-of-flow meters.