Editing Anemometer (section)

===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).]]