On the output frequency measurement within cup anemometer calibrations

The output frequency of five commercial anemometers (Thies Clima First-Class Advanced) was studied. The output voltage of these anemometers was measured at the calibrations process at 25 kHz sampling frequency, during 25 s. The output frequency was calculated by two different procedures: by Counting Pulses (CP) and by using the Fast Fourier Transform (FFT). The errors of the anemometers’ transfer function based on both ways for calculating the output frequency are given in relation to the reference calibration results, obtained with an ISO 17025 standard calibration process. The effect of the sampling frequency and the length of the sample are analyzed. The results of using FFT are compared to the ones resulting from using CP. Counting Pulses (CP) has proven to be the most accurate way to extract the anemometer’s output frequency from data recorded.