From the energy balance of the wire of the anemometer, heated by the Joule effect and cooled by a fluid stream, we have:
where Cw is the heat capacity of the wire, W is the electrical power supplied to the wire and Q is the thermal power exchanged among the wire and the environment by convection, conduction and radiation.
3.4.1 Conduction to the supports
The supports have a diameter much greater than that of the wire, partly for reasons of robustness and partly to avoid heating up at the passage of electric current; for this reason, the temperature at the end of the wire is very close to the temperature of the fluid, Ta, and thus the heat transfer by conduction along the wire is appreciable (Figure 3.6). Since this effect is quite complicated, especially when there are fluctuations of speed that over time will change the temperature distribution on the wire, it is difficult to make a theoretical evaluation valid in general and it is necessary to calibrate each individual probe.