9th June 2021
Calibration Notes for a Rotating Cup Anemometer
Rotating Cup Anemometers can be calibrated to the ISO/IEC 61400-12-1 “Wind turbines – Part 12-1: Power performance measurements of electricity producing wind turbines” industry standard. The 1 m2 closed jet wind tunnel coupled with an automated traversing laser doppler anemometer and multiple pitot probes provides the reference hardware for the required calibration specification. The wind tunnel provides an air velocity test profile of less than 0.1% uniformity with reference air velocity speeds measure across the test section to a 0.15% spot point measurement accuracy. Further UKAS accredited reference sensors provide traceable measurements for the air stream atmospheric pressure, air humidity, air temperature, differential pitot probe pressures and rotating cup frequency output.
Ensuring the repeatability and drift of the wind tunnel facility regular “Rotating Cup Master Meter” calibrations are run and form part of the overall measurement uncertainty contributions. All uncertainty assessments are conducted using the UKAS M3003 document and are independently assessed and verified via external audits and yearly UKAS re-assessments. Similar documents for the expression of uncertainty may also be used, such as GUM or EA-04/02:2013. The calibration process has over 30 components of uncertainty evaluated in the derivation of the overall measurement uncertainty of the calibration.
Anemometer Calibration Procedure
Before the starting the calibration procedure the rotating cup anemometer is installed on its mounting tube, aligned, and positioned as close as possible to its’ in service mounting arrangement to minimise any installation sensitivities effects. Once fixed and the instrumentation checks have been completed the wind tunnel is operated for a period of time to gain thermal stability within the wind tunnel, provide instrument warm up and stabilise any mechanical friction effects from the cup anemometer. Calibration, as per the ISO standard and MEASNET laboratory procedures is performed as a rising and falling air velocity across a speed range of 4 m/s to 16 m/s and in 1 m/s increments to help determine cup anemometer hysteresis effects. Measurement sampling is taken for 30 seconds at > 1 Hz frequency, allowing for stable measurements to be taken across all instrumentation which is then logged and processed into an accredited calibration certificate. The present wind tunnel facility for this type of anemometer rotating cup calibration covers 1 to 21 m/s with a measurement CMC of 0.23%.