Thermal Anemometers






Thermal anemometers use a very fine wire (on the order of several micrometers) or element heated up to some temperature above the ambient. Air flowing past over has a cooling effect. 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 flow velocity.

Several ways of implementing this exist, and hot-wire devices can be further classified as CCA (Constant-Current Anemometer), CVA (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. 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. Thermal anemometers are available with additional functions such as temperature measurement, data logging ability.