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Improvement of the measurement uncertainty of the primary standard for LDA calibrations


Increasingly, Laser Doppler Anemometers (LDA) which are retraced by PTB are used by calibration laboratories, by test benches for air and gas flow rate, and by large test rigs for fluidic and calorimetric flow metering as a transfer standard for the measurement of flow velocity. The primary standard for the calibration of LDA systems – a precisely rotating glass disk – has now been subjected to a further uncertainty analysis.

The different sources of uncertainty contributions and both their static and their dynamic properties were re-investigated and documented in a measurement uncertainty budget [1]. The roundness of the glass disk was analysed using the coordinate measuring technique, and the movement of the rotating disk was examined with an interferometer. Similarly, the uniformity of the rotation was checked with an incremental angle encoder, and the coupling between the encoder and the disk, as well as the geometric alignment and the eccentricity of the moving disk, were inspected. As a result, the velocity on the disk of the primary standard can now be realized with an expanded uncertainty of 1·10-4. Compared to the uncertainty of the fringe distance of a typical LDA, the contribution of the primary standard is smaller by about two orders of magnitude.

Rotating disk for the calibration of Laser Doppler Anemometers (LDA)

Figure 1: Rotating disk for the calibration of Laser Doppler Anemometers (LDA)

[1] V. Strunck et al: „Messunsicherheit der Geschwindigkeitsdarstellung zur LDA-Kalibrierung mit rotierender Scheibe”, Fachtagung „Lasermethoden in der Strömungsmesstechnik”, 7.-9. 9. 2010, ISBN 978-3-9805613-6-5, Cottbus, 4.1-4.8

Contact person:

Volker Strunck, Dept. 1.4, WG 1.41, E-mail: volker.strunck@ptb.de