The vane anemometer is an instrument designed to measure velocity utilizing the kinetic energy of a flow. Its main element is a vane with blades of different shape. To measure velocity in the atmosphere, the blades are made in the form of hollow hemispheres with their concavity facing away from the flow; the plane of rotation is parallel to the direction of motion. In measuring the velocity in pipes and channels, vanes with flat blades are used, arranged at an angle of attack 40-45° to the direction of flow in the plane perpendicular to it. In hydraulic laboratories microvanes with blades diameters of 6-8 mm are used for local measurements. The speed of vane rotation is proportional to the flow velocity. The characteristic is linear within the range of from a minimum defined by friction in the mechanism to a maximum. The range of mean velocity ū for application in air is ū = 2.0-50 m/s. The accuracy of velocity measurement is ± 0.5 + 0.035ū. For exact measurements to be performed, the anemometers are calibrated against a standard velocity value. The deviation of the plane rotation from a nominal value by ± 10° causes an error of not higher than 1%. The vane rotation is transmitted to a special mechanism or an electronic counter, which counts the number of revolutions per unit time, proportional to flow velocity. If the density of gas on measuring ρ1 differs from the density of gas in calibration ρ0, the actual value of velocity u1 is defined from the measured u0 (read off from the calibration curve) u1 = u0(ρ0/ρ1)0.5.
Fluid Meters. Their Theory and Application. Report of ASME, Research Committee on Fluid Meters, New York, Published by ASME, 1971.