Compensation of the stream temperature

Since the output voltage of the anemometer depends on the fluid stream temperature, Ta, this must be taken into account every time it varies during the test or between calibration and measurement, for
example, an increase in ambient temperature looks like a decrease in stream speed.

There are various methods for correcting temperature:

■ the temperature is measured separately and the data are corrected referring to a family of calibration curves plotted for different temperatures (Figure 3.17) or by using the theoretical equations of heat transfer;

■ using two active sensors operating at two different temperatures and making the difference between the output signals to obtain a signal independent of the stream temperature; in particular, the compensation sensor operates at low temperature and is therefore insensitive to velocity. The following equations can be written for the power outputs of the two sensors:

W = (+1*U){T – Ta) W2 = (+bU) – Ta)

W – w2 = (+bU) – T)

■ a temperature-sensitive resistor of appropriate value and having the same temperature coefficient of the sensor is placed in the arm of the

Calibration curves of a CTA at different temperatures of the stream

bridge opposite to the sensor and mounted in the stream near the speed sensor. Its resistance varies with temperature by forcing the speed sensor to vary of the same entity.