Flow measurement is the quantification of volume of fluid movement. Flow can be measured in a number of ways. Positive-displacement flowmeters accumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow. Other flow measurement methods rely on forces produced by the flowing stream as it overcomes a known constriction, to indirectly calculate flow. Flow may be measured by measuring the velocity of fluid over a known area. The common types of flowmeters that find industrial applications can be listed as follows: (a) obstruction type (differential pressure or variable area), (b) inferential (turbine type), (c) electromagnetic, (d) positive displacement (integrating), (e) fluid dynamic (vortex shedding), (f) anemometer, (g) ultrasonic, and (h) mass flowmeter (coriolis).
Obstruction or head type flowmeters are of two types: differential pressure type and variable area type. Orifice meter, venturimeter, and Pitot tube fall under the differential pressure type, while rotameter is of the variable area type. In all the cases, an obstruction is created in the flow passage and the pressure drop across the obstruction is related with the flow rate.
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