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Enhanced traceability of Prandtl tubes by Laser-Doppler Anemometry

30.10.2012

In Germany accredited calibration laboratories in the wind industry predominantly use Prandtl tubes indicated as Pitot static tubes as reference standards for the calibration of far over 10.000 anemometers per year. Comparison measurements with Laser Doppler-Anemometers as transfer standards have shown that the traceability of air speed measurements given by Prandtl tubes can be realized in the order of a few per mill.

 

In the field of regenerative energy sources wind resource assessment is most important for cost-effective wind energy utilization. For this purpose calibrated anemometers traceable to the SI units with increasingly higher demands are used. Up to now these anemometers are calibrated with uncertainties of approx. 1 % in wind tunnels with nozzle sections of 1 m² in a velocity range from below 5 m/s up to over 15 m/s. In these facilities usually Prandtl tubes are used as reference standards according to the ISO 3966 standard including relevant geometries and calibration factors.  Due to a recent resolution of the WGFF (Working Group on Fluid Flow CIPM CCM) and the IEC 61400-12-1 Prandtl tubes to be used as reference standards require a previous calibration to ensure the metrological traceability to the SI units.

Measurements in wind tunnels have shown that for flow velocities above 4 m/s the measurement results based on Prandtl tubes according to the ISO 3966 are within a view per mill consistent with those obtained with Laser-Doppler-Anemometers as transfer standards. The increasing demand for minimal measurement uncertainties requires more and more the application of calibrated Laser-Doppler-Anemometers with lowest uncertainties in the range of 1 ‰ up to 2 ‰ which currently allow best possible calibrations as well as the validation of recommendations for the application of Prandtl tubes based on theoretical and experimental experience.

With improved refinement of differential pressure measurement capabilities for the application of Prandtl tubes and increased requirements on precise measurement procedures and their validation Laser-Doppler anemometers play a key role. Therefore a long-term reduction of the Laser-Doppler-Anemometer uncertainty of usually above 0,12 % will be required.

Literature:

[1] International Standard IEC 61400-12-1 Wind turbines - Part 12-1: Power performance measurements of electricity producing wind turbines, First edition 2005-1

[2] International Standard ISO 3966 Measurement of fluid flow in closed conduits - Velocity area method using Pitot static tubes, Second edition 2008-07-15

[3] Pitot, Doppler und Prandtl – Wegbereiter für heutige Windpotenzialanalysen. Proceedings der 20. GALA-Fachtagung „Lasermethoden in der Strömungsmesstechnik“, 2012

Contact person:

Harald Müller, FB 1.4, AG 1.41, E-Mail: Opens window for sending emailharald.mueller(at)ptb.de