Measurements of absolute value and direction of average velocity

1.6.2 Calibration

To carry out any measure of average speed a calibration curve is needed. For measurements in gases at speeds between 10 and 100 m/s calibration can be made with reference to the difference in pressure read in a Venturi tube (a mini-subsonic wind tunnel) where the sensor is placed. For very low speeds, the use of a laminar flow tube is recommended. For higher speeds, a cylinder powered by compressed air and equipped with a set of interchangeable nozzles is used.

Obviously the accuracy of the calibration curve increases with the number of measured points. This involves a tedious procedure to be repeated frequently, even if the same probe is used, to check that the wear of the sensor or the presence of impurities in the stream have not altered the characteristics of the probe itself. The linearizer could alleviate the problem because it allows calibration by measuring only the voltage corresponding to the maximum speed.

In the classical analog anemometer, output voltage was measured with a DC voltmeter, that is a voltmeter with a considerable inertia, that even in the presence of fluctuations due to the turbulence of the fluid current, did not follow the fluctuations but showed an average value.

In modern anemometers, the signal passes through an analog/digital converter and is given as a large number, n, of numerical data representing the succession in time of voltage fluctuations. The average voltage is thus obtained by the equation:

– У, E

n

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