Effect of Air Temperature and Barometric Pressure on Anemometer Calibration
Anemometer calibration is usually performed in a wind tunnel at constant, prevailing air temperature and atmospheric pressure. This calibration is the basis for the wind speed measurement of the mast and can therefore also be used for site assessment of potential wind farms. Since environmental parameters can affect the performance of an anemometer, it is imperative to select an anemometer suitable for a particular site. Additionally, environmental impact measurement is important for accurate site assessment and wind turbine performance control.
Anemometers are available in different accuracy classes. The latest describes the different influencing parameters that need to be investigated when classifying anemometers. Different levels correspond to different operating ranges of these influencing parameters. These include air temperature and air density.
It is assumed that changes in air temperature only affect bearing friction. Higher air temperature reduces friction and therefore increases RPM, and vice versa.
There are no further recommendations on how to assess changes in air density. Air density is a function of air temperature and air pressure. To study the effect of changes in air density, either the air temperature or the air pressure can be varied. If temperature changes are used to adjust air density during calibration, it can be difficult to distinguish the effects of changing air temperature and density. The most accurate results are obtained if both parameters, air temperature and air pressure, can be varied independently.
In standard wind tunnels, air temperature and pressure cannot usually be set precisely. Therefore, the influence of these environmental conditions cannot be reliably estimated. A wind tunnel for research in a "Göttinger" layout is designed here, in which the ambient pressure and temperature of the tunnel channels can be varied independently.
Calibration measurement methods for cup anemometers, propeller anemometers and ultrasonic anemometers. The result of the calibration is the wind speed indicated by the anemometer at different temperatures and/or different barometric pressures. Calibration results can be used to estimate the uncertainty of wind sensors at very low or high temperatures and/or high altitudes (lower temperatures and lower densities). These results are also essential for the classification of anemometers according to IEC [1].
A previous investigation of the effect of air temperature variation on cup anemometer bearing friction is presented. In Chapter 3, a technical description of a variable air volume density wind tunnel will be given, followed by an anemometer calibration procedure for variable air temperature and air pressure. Uncertainty calculations are introduced in Chapter 4. Finally, the results and final conclusions are shown for two different cup anemometers, commonly used and with frequency signal output, with similar shapes.
