Editing Anemometer (section)

===Ultrasonic anemometers===
[[File:WindMaster.jpg|thumb|3D ultrasonic anemometer]]
Ultrasonic anemometers, first developed in the 1950s, use [[ultrasound|ultrasonic sound waves]] to measure wind velocity. They measure wind speed based on the time of flight of sonic pulses between pairs of [[transducer]]s.<ref>{{Citation | title=Sonic Anemometers (Centre for Atmospheric Science - The University of Manchester) | url=http://www.cas.manchester.ac.uk/restools/instruments/meteorology/sonic/ | access-date=29 February 2024}}</ref>

The time that a sonic pulse takes to travel from one transducer to its pair is inversely proportionate to the speed of sound in air plus the wind velocity in the same direction: <math>t=\frac{L}{(c+v)}</math> where <math>t</math> is the time of flight, <math>L</math> is the distance between transducers, <math>c</math> is the speed of sound in air and <math>v</math> is the wind velocity. In other words, the faster the wind is blowing, the faster the sound pulse travels. To correct for the [[speed of sound]] in air (which varies according to temperature, pressure and humidity) sound pulses are sent in both directions and the wind velocity is calculated using the forward and reverse times of flight: <math>v=\frac{1}{2} L(\frac{1}{t_1}-\frac{1}{t_2})</math> where <math>t_1</math> is the forward time of flight and <math>t_2</math> the reverse.

Because ultrasonic anenometers have no moving parts, they need little maintenance and can be used in harsh environments. They operate over a wide range of wind speeds. They can measure rapid changes in wind speed and direction, taking many measurements each second, and so are useful in measuring turbulent air flow patterns.

Their main disadvantage is the distortion of the air flow by the structure supporting the transducers, which requires a correction based upon wind tunnel measurements to minimize the effect. Rain drops or ice on the transducers can also cause inaccuracies.

Since the speed of sound varies with temperature, and is virtually stable with pressure change, ultrasonic anemometers are also used as [[thermometers]].

Measurements from pairs of transducers can be combined to yield a measurement of velocity in 1-, 2-, or 3-dimensional flow. Two-dimensional (wind speed and wind direction) sonic anemometers are used in applications such as [[weather station]]s, ship navigation, aviation, [[weather buoy]]s and wind turbines. Monitoring wind turbines usually requires a refresh rate of wind speed measurements of 3&nbsp;Hz,<ref>{{cite book |last=Giebhardt |first=Jochen |editor1-last=Dalsgaard Sørensen |editor1-first=John |editor2-last=N Sørensen |editor2-first=Jens |title=Wind Energy Systems: Optimising design and construction for safe and reliable operation |publisher=Elsevier |date=December 20, 2010 |pages=329–349 |chapter=Chapter 11: Wind turbine condition monitoring systems and techniques |isbn=9780857090638 }}</ref> easily achieved by sonic anemometers. Three-dimensional sonic anemometers are widely used to measure gas emissions and ecosystem fluxes using the [[eddy covariance]] method when used with fast-response [[infrared gas analyzer]]s or [[laser]]-based analyzers.

====Acoustic resonance anemometers====
[[File:Acoustic Resonance Wind Sensor.jpg|thumb|left|Acoustic resonance anemometer]]
Acoustic resonance anemometers are a more recent variant of sonic anemometer. The technology was invented by Savvas Kapartis and patented in 1999.<ref>Kapartis, Savvas (1999) "Anemometer employing standing wave normal to fluid flow and travelling wave normal to standing wave" {{US Patent|5877416}}</ref> Whereas conventional sonic anemometers rely on time of flight measurement, acoustic resonance sensors use resonating acoustic (ultrasonic) waves within a small purpose-built cavity in order to perform their measurement.

[[File:Acoustic Resonance Anemometer.jpg|thumb|Acoustic resonance principle]]
Built into the cavity is an array of ultrasonic transducers, which are used to create the separate standing-wave patterns at ultrasonic frequencies. As wind passes through the cavity, a change in the wave's property occurs (phase shift). By measuring the amount of phase shift in the received signals by each transducer, and then by mathematically processing the data, the sensor is able to provide an accurate horizontal measurement of wind speed and direction.

Because acoustic resonance technology enables measurement within a small cavity, the sensors tend to be typically smaller in size than other ultrasonic sensors. The small size of acoustic resonance anemometers makes them physically strong and easy to heat, and therefore resistant to icing. This combination of features means that they achieve high levels of data availability and are well suited to wind turbine control and to other uses that require small robust sensors such as battlefield meteorology. One issue with this sensor type is measurement accuracy when compared to a calibrated mechanical sensor. For many end uses, this weakness is compensated for by the sensor's longevity and the fact that it does not require recalibration once installed.