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Measurement technique and data acquisition
Figure 4.18: Data flux between the central command unit and the data acquisition programs
ON TOP: schematic view; BOTTOM: illustrated with program interfaces
4.4.3 The level acquisition unit
The level data (voltage between 0 and 10V), recorded with an ultrasonic gauge, was directly written
into an Excel file (Version 7.0a). In order to increase the precision of the measurement,
32 data samples (sampling rate 10 Hz) were averaged at each measurement point. The voltage was
transformed into a distance to a reference level located about 50 cm below the ground of the
channel. The error of the level measurement is less than 1 mm (average ±0.5 mm). Additional
information on the used probes and on the level acquisition can be found in Appendix 2.1.1 and
2.2.2.
4.4.4 The Ultrasonic Doppler Velocity Profiler (UVP)
The velocities were measured with an Ultrasonic Doppler Velocity Profiler (Metflow SA, Model
UVP-XW, Version 1.1b, see also Appendix 2.1 and 2.2.3) allowing to obtain instantaneously a 1Dvelocity
profile over the whole channel depth (METFLOW, 2000). To measure the 3D-flow field,
three probes were mounted on a probe support plate (Fig. 4.17 and Fig. 4.21). Since the number
of measurement points was very high (about 1500 points over the whole channel), three plates
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 85