Hot-wire Anemometer is also called Thermal Anemometer. It uses a very fine wire electrically heated up to some temperature above the ambient. Thermalanemometry is the most common method used to measure instantaneous
fluid velocity. The technique depends on the convective heat loss to the surrounding fluid from an electrically heated sensing element or probe. If only the air velocity varies, then the heat loss can be interpreted as a measure of
that variable. The same principle goes for air. When air flows past the wire, it has a cooling effect on the metal. As there is a relationship between the electrical resistance and the temperature of most metals (tungsten is widely used for this reason) which can show in current and voltage, the flow speed can be deduced from them. Hot-wire devices can be further classified as CCA (constant current anemometer), CVA (constant voltage anemometer) and CTA (constanttemperature anemometer). 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.