18.09.2013
Laser Doppler anemometers (LDAs) in backscattering arrangement are used in many measuring devices as reference standards. An alternative calibration method, which is presented here, now also allows, for the first time, LDAs in forward scattering direction to be calibrated with small uncertainties.
Laser Doppler anemometers (LDAs) in backscattering arrangement are used in many measuring devices as reference standards. An alternative calibration method, which is presented here, now also allows, for the first time, LDAs in forward scattering direction to be calibrated with small uncertainties.When calibrating an LDA measuring system, the lower limit of the measurement uncertainty is given by the uncertainty of the realization and by the determination of the velocity using a rotating disk as a primary standard. Whereas the standard calibration procedure uses the lateral surface of a glass cylinder to realize the velocity, the difference method applied here uses a glass carrier applied to the plane area with a matrix of lithographically printed scattering particles. With positional uncertainties of the scattering particles on the matrix of 25 nm at most, the relative uncertainty contribution of the smallest distance difference of 2 mm for two adjacent scatterers is smaller than 10-5 and, thus, comparable to the uncertainty of the radius of the lateral surface. Of advantage is the fixed application of the matrix to the plane surface of the glass cylinder, which does not require any correction of the eccentricity of the rotating disk. Including the uncertainty contributions to installation errors (height errors / phase angle errors) when the LDA system is aligned to the glass disk and in the alignment of the matrix, the expanded measurement uncertainty can be estimated – with correction of the eccentricity – to smaller than 0.02 % for the realization of the velocity both when the plane surface of the cylinder or the lateral surface are used, see Strunck et al. 2013. With the measurement on the plane surface, a method is, for the first time, also available for the calibration of LDAs in forward-scattering direction (e.g. reference-beam LDAs).
For the dissemination of the velocity from the rotating disk to the velocity measuring signal of the LDA, scattering particles in the shape of a Siemens star with 16 beams are used, see Figure 1.
Figure 1: Particles with a diameter of 10 µm in the shape of a Siemens star.
With eight scattering particles used (Siemens star), the expanded uncertainty of the mean value of the LDA fringe distance becomes less than 0.1% when the method applied here is used and lies, thus, below the uncertainty of the fringe distance inside the measuring volume of common LDAs (0.2 %).
References:
[1] Strunck, V., Pape, N., Eggert, M., Weimann, T., Müller, H.: „Messunsicherheit der Geschwindigkeitsdarstellung zur LDA-Kalibrierung mit normierten Streuteilchen“, 21st Technical Meeting, laser methods in fluid flow measuring techniques (GALA' 2013), 3 - 5 September 2013, UniBw Munich, ISBN 978-3-9805613-9-6, contribution 8, pp. 1 - 8
Contact person:
Volker Strunck, Dept. 1.4, WG 1.41, E-Mail: volker.strunck@ptb.de