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.