Methods of Flow Measurement

All flowmeters may be placed in one of three basic classifications: the differential pressure type, the volumetric type, and the mass rate type. Differential pressure flowmeters incorporate a suitable restriction in the flow passage. For steady-state liquid flows, the pressure drop caused by the restriction is related by a simple square law to the velocity, so tha t when either the pressure difference or the area of the restriction is fixed, the value of the other parameter will provide an indication of the volumetric flow rate. For known fluid density, the mass flow may then be computed. This type of flowmeter is not generally suited to the measurement of pulsating flows, since the square-law relationship means that , if any averaging action occurs in the pressure measurement, an erroneous signal will be obtained. Volumetric flowmeters are distinguished from the differential type in tha t they measure fluid velocity independently of pressure. Although inherently more accurate than the differential type due to the relatively small effect of viscosity, they possess the disadvantage of requiring a knowledge of fluid density in order to obtain mass flow. Turbine meters and electromagnetic flowmeters, which can be used in pulsating flows, are some specific examples of this class. The use of the hot-wire anemometer for the measurement of liquid flows is also discussed. In the case of the true mass rate flowmeter, the signal is directly proportional to the mass flow itself. Although several designs of a mass flowmeter have been conceived, only one appears to be commercially available at this t ime. Its operation depends on the measurement of the angular momentum of a rotating fluid passage.