The hot-wire anemometer, principally used in
gas flow measurement, consists of an electrically heated,
fine platinum wire which is immersed into the flow. As the
fluid velocity increases, the rate of heat flow from the
heated wire to the flow stream increases. Thus, a cooling
effect on the wire electrode occurs, causing its electrical
resistance to change. In a constant-current anemometer, the
fluid velocity is determined from a measurement of the
resulting change in wire resistance. In a constant-resistance
anemometer, fluid velocity is determined from the current
needed to maintain a constant wire temperature and, thus, the
resistance constant.
