Anemometer Knowledge
Anemometers measure the horizontal wind speed (velocity). This parameter is crucial for any wind site assessment.
Cup anemometers are the general standard type of anemometer. They are robust and resistant to turbulence and skew winds caused by masts and traverses.
Each anemometer should be individually calibrated and equipped with a certified calibration report according to international standards (e.g. MEASNET).
At least two anemometers have to be installed on a met mast. Most anemometers can be equipped with electronically regulated heating.
Studies regarding Anemometer Behaviour
In order to analyse the accuracy of anemometers and define criterias for anemometers which are suitable for wind measurement, the EU has financed three comprehensive studies in which well-known institutes like DTU (formerly Risø) and DEWI took part. The studies SITEPARIDEN, CLASSCUP and ACCUWIND indicate that the following criterias are crutial for anemometers used in wind measurement:
- Angular response: anemometer behaviour according to variable flow angle
- Turbulence behaviour: anemometer reaction on changing velocity
- Bearing friction: influence of temperature on bearing friction
IEC Classification of Anemometers
The classification describes the uncertainty of an anemometer considering general influencing parameters like turbulence, air temperature, air density and average flow inclination.
According to the IEC 61400-12-1 the Operational Standard Uncertainty can be derived from the classification and the wind speed bin. The operational standard uncertainty describes the maximum deviation of the wind speed measured by the anemometer compared with the real wind speed in m/s, whereas the classification is only a number. The operational standard uncertainty is calculated as follows:
where Uj is the wind speed bin; k the classification