Editing Anemometer (section)

==Velocity anemometers==

===Cup anemometers===
[[File:Cup-Anemometer-Animation.gif|thumb|right|Cup anemometer animation]]
A simple type of anemometer was invented in 1845 by Rev. Dr. [[Thomas Romney Robinson|John Thomas Romney Robinson]] of [[Armagh Observatory]]. It consisted of four [[Sphere#Hemisphere|hemispherical]] cups on horizontal arms mounted on a vertical shaft. The air flow past the cups in any horizontal direction turned the shaft at a rate roughly proportional to the wind's speed. Therefore, counting the shaft's revolutions over a set time interval produced a value proportional to the average wind speed for a wide range of speeds. This type of instrument is also called a ''rotational'' anemometer.

====Four cup====
With a four-cup anemometer, the wind always has the hollow of one cup presented to it, and is blowing on the back of the opposing cup. Since a hollow hemisphere has a [[drag coefficient]] of .38 on the spherical side and 1.42 on the hollow side,<ref>{{Citation|url=https://archive.org/details/FluidDynamicDragHoerner1965 |title=Sighard Hoerner's Fluid Dynamic Drag|pages=3–17, Figure 32|date=1965}} (pg 60 of 455)</ref> more force is generated on the cup that presenting its hollow side to the wind. Because of this asymmetrical force, [[torque]] is generated on the anemometer's axis, causing it to spin.

Theoretically, the anemometer's speed of rotation should be proportional to the wind speed because the force produced on an object is proportional to the speed of the gas or fluid flowing past it. However, in practice, other factors influence the rotational speed, including turbulence produced by the apparatus, increasing drag in opposition to the torque produced by the cups and support arms, and friction on the mount point. When Robinson first designed his anemometer, he asserted that the cups moved one-third of the speed of the wind, unaffected by cup size or arm length. This was apparently confirmed by some early independent experiments, but it was incorrect. Instead, the ratio of the speed of the wind and that of the cups, the ''anemometer factor'', depends on the dimensions of the cups and arms, and can have a value between two and a little over three. Once the error was discovered, all previous experiments involving anemometers had to be repeated.

====Three cup====
The three-cup anemometer developed by Canadian John Patterson in 1926, and subsequent cup improvements by Brevoort & Joiner of the United States in 1935, led to a cupwheel design with a nearly linear response and an error of less than 3% up to {{convert|60|mi/h|km/h|abbr=on}}. Patterson found that each cup produced maximum torque when it was at 45° to the wind flow. The three-cup anemometer also had a more constant torque and responded more quickly to gusts than the four-cup anemometer.

====Three cup wind direction====
The three-cup anemometer was further modified by Australian Dr. Derek Weston in 1991 to also measure wind direction. He added a tag to one cup, causing the cupwheel speed to increase and decrease as the tag moved alternately with and against the wind. Wind direction is calculated from these cyclical changes in speed, while wind speed is determined from the average cupwheel speed.

Three-cup anemometers are currently the industry standard for [[wind resource assessment]] studies and practice.

===Vane anemometers===
One of the other forms of mechanical velocity anemometer is the ''vane anemometer''. It may be described as a [[windmill]] or a propeller anemometer. Unlike the Robinson anemometer, whose axis of rotation is vertical, the vane anemometer must have its axis parallel to the direction of the wind and is therefore horizontal. Furthermore, since the wind varies in direction and the axis has to follow its changes, a [[weather vane|wind vane]] or some other contrivance to fulfill the same purpose must be employed.

A ''vane anemometer'' thus combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument.<ref name= WMO>{{cite web|url=http://www.eumetcal.org/resources/ukmeteocal/rapid_cyclo/www/english/glossary/vaneanem.htm|title=Vane anemometer|author=World Meteorological Organization|author-link=World Meteorological Organization|work=Eumetcal|access-date=6 April 2014|archive-url=https://web.archive.org/web/20140408221107/http://www.eumetcal.org/resources/ukmeteocal/rapid_cyclo/www/english/glossary/vaneanem.htm|archive-date=8 April 2014|url-status=dead}}</ref> The speed of the fan is measured by a revolution counter and converted to a windspeed by an electronic chip. Hence, volumetric flow rate may be calculated if the cross-sectional area is known.

In cases where the direction of the air motion is always the same, as in ventilating shafts of mines and buildings, wind vanes known as air meters are employed, and give satisfactory results.<ref>{{Cite book|last=Various|url=https://books.google.com/books?id=iUFjDwAAQBAJ&q=In+cases+where+the+direction+of+the+air+motion+is+always+the+same%2C+as+in+ventilating+shafts+of+mines+and+buildings%2C+wind+vanes+known+as+air+meters+are+employed%2C+and+give+satisfactory+results.&pg=PP8|title=Encyclopaedia Britannica, 11th Edition, Volume 2, Part 1, Slice 1|date=2018-01-01|publisher=Prabhat Prakashan|language=en}}</ref>

<gallery caption="Vane anemometers" class="center">
File:Wind speed and direction instrument - NOAA.jpg|Vane style of anemometer
File:Prop vane anemometer.jpg|[[Helicoid]] propeller anemometer incorporating a [[wind vane]] for orientation
File:Anemometer-IMG 4734-white.jpg|Hand-held low-speed vane anemometer
File:Digital_Handheld_Anemometer.jpg|Hand-held digital anemometer or Byram anenometer.
</gallery>

===Hot-wire anemometers===
[[File:Hd sonde.jpg|thumb|left|Hot-wire sensor]]
Hot wire anemometers use a fine wire (on the order of several micrometres) electrically heated to some temperature above the ambient. Air flowing past the wire cools the wire. As the electrical resistance of most metals is dependent upon the temperature of the metal ([[tungsten]] is a popular choice for hot-wires), a relationship can be obtained between the resistance of the wire and the speed of the air.<ref>{{cite web|url=http://www.efunda.com/designstandards/sensors/hot_wires/hot_wires_intro.cfm|title=Hot-wire Anemometer explanation|publisher=eFunda|access-date=18 September 2006|archive-url=https://web.archive.org/web/20061010125307/http://www.efunda.com/DesignStandards/sensors/hot_wires/hot_wires_intro.cfm |archive-date=10 October 2006|url-status=live}}</ref> In most cases, they cannot be used to measure the direction of the airflow, unless coupled with a wind vane.

Several ways of implementing this exist, and hot-wire devices can be further classified as CCA ([[constant current]] anemometer), CVA ([[Voltage source|constant voltage]] anemometer) and CTA (constant-temperature anemometer). The voltage output from these anemometers is thus the result of some sort of circuit within the device trying to maintain the specific variable (current, voltage or temperature) constant, following [[Ohm's law]].

Additionally, PWM ([[pulse-width modulation]]) anemometers are also used, wherein the velocity is inferred by the time length of a repeating pulse of current that brings the wire up to a specified resistance and then stops until a threshold "floor" is reached, at which time the pulse is sent again.

Hot-wire anemometers, while extremely delicate, have extremely high frequency-response and fine spatial resolution compared to other measurement methods, and as such are almost universally employed for the detailed study of turbulent flows, or any flow in which rapid velocity fluctuations are of interest.

An industrial version of the fine-wire anemometer is the [[Thermal mass flow meter|thermal flow meter]], which follows the same concept, but uses two pins or strings to monitor the variation in temperature. The strings contain fine wires, but encasing the wires makes them much more durable and capable of accurately measuring air, gas, and emissions flow in pipes, ducts, and stacks. Industrial applications often contain dirt that will damage the classic hot-wire anemometer.

[[File:Laser anemometer.png|thumb|360px|Drawing of a laser anemometer. The laser light is emitted (1) through the front lens (6) of the anemometer and is backscattered off the air molecules (7). The backscattered radiation (dots) re-enters the device and is reflected and directed into a detector (12).]]

===Laser Doppler anemometers===
In [[laser Doppler velocimetry]], laser Doppler anemometers use a beam of light from a [[laser]] that is divided into two beams, with one propagated out of the anemometer. Particulates (or deliberately introduced seed material) flowing along with air molecules near where the beam exits reflect, or backscatter, the light back into a detector, where it is measured relative to the original laser beam. When the particles are in great motion, they produce a [[Doppler shift]] for measuring wind speed in the laser light, which is used to calculate the speed of the particles, and therefore the air around the anemometer.<ref>{{cite web|last=Iten|first=Paul D.|date=29 June 1976|url=http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=3966324|title=Laser Doppler Anemometer|publisher=United States Patent and Trademark Office|access-date=18 September 2006}}</ref>
[[File:Ultrasonic Windsensor.png|thumb|Fixed mounted 2D ultrasonic anemometer with 3 paths. <br />Central spike keeps birds away.]]

===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.