Introduction: The term is derived from the Greek wordanemos, meaning wind. An anemometer is a device formeasuring wind speed, and is a common weather stationinstrument. The first known description of an anemometerwas given by Leon Battista Alberti around 1450. Anemometers can be divided into two classes: thosethat measure the winds speed, and those that measure thewinds pressure; but as there is a close connection between thepressure and the speed, an anemometer designed for one willgive information about both.
Fig: A hemispherical cup anemometer of the typeinvented in 1846 by John Thomas Romney Robinson
Types: Three cup anemometers1) Windmill anemometers2) Hot Wire / Film Anemometer3) Laser Doppler anemometers4) Sonic anemometers
1) Hot Wire / Film Anemometer
Operating Principle : Hot wire anemometers use a very fine wire (mm) electricallyheated up to some temperature above the ambient. Air flowing past thewire has a cooling effect on the wire. As the electrical resistance of mostmetals is dependent upon the temperature of the metal (tungsten), arelationship can be obtained between the resistance of the wire and theflow speed. Several ways of implementing this exist, and hot-wire devicescan be further classified as•CCA (Constant-Current Anemometer)•CVA (Constant-Voltage Anemometer)•CTA (Constant-Temperature Anemometer) The voltage output from these anemometers is thus the result ofsome sort of circuit within the device trying to maintain the specificvariable (current, voltage or temperature) constant.
Visualization of Hot Wire Anemometer :
2) Windmill anemometersThe other forms of mechanical velocityanemometer may be described asbelonging to the windmill type orpropeller anemometer. An aerovanecombines a propeller and a tail on thesame axis to obtain accurate and precisewind speed and direction measurementsfrom the same instrument. In cases wherethe direction of the air motion is alwaysthe same, as in the ventilating shafts ofmines and buildings for instance, windvanes, known as air meters areemployed, and give most satisfactoryresults.
3) Laser Doppler anemometers
Drawing of a laser anemometer.The laser is emitted (1) through the front lens (6) of theanemometer and is backscattered off the air molecules (7). Thebackscattered radiation (dots) re-enter the device and arereflected and directed into a detector (12).Laser Doppler anemometers use a beam of light from a laserthat is divided into two beams, with one propagated out of theanemometer. Particulates (or deliberately introduced seedmaterial) flowing along with air molecules near where thebeam exits reflect, or backscatter, the light back into adetector, where it is measured relative to the original laserbeam. When the particles are in great motion, they produce aDoppler shift for measuring wind speed in the laserlight, which is used to calculate the speed of the particles, andtherefore the air around the anemometer.
4) Sonic anemometers (3D ultrasonic anemometer)
Sonic anemometers use ultrasonic sound waves to measure wind velocity. Theymeasure wind speed based on the time of flight of sonic pulses between pairs oftransducers. Measurements from pairs of transducers can be combined to yield ameasurement of velocity in 1-, 2-, or 3-dimensional flow.The lack of moving parts makes them appropriate for long term use in exposedautomated weather stations and weather buoys where the accuracy and reliabilityof traditional cup-and-vane anemometers is adversely affected by salty air orlarge amounts of dust.The speed of sound varies with temperature, and is virtually stable with pressurechange, sonic anemometers are also used as thermometers.Two-dimensional (wind speed and wind direction) sonic anemometers are usedin applications such as weather stations, ship navigation, wind turbines, aviationand weather buoys.Three-dimensional sonic anemometers are widely used to measure gas emissionsand ecosystem fluxes using eddy covariance method when used with fast-response infrared gas analyzer or laser-based analyzer.
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