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Streaming fluid modulates the ultrasound as well in phase as in amplitude. The complex bandpass sampling separates the phase and the amplitude information. Because of unknown zero phase adjustment point of the receiving sensor, the demodulated phase signal has to be reconstructed. The reconstruction is based on the analysis of the density distribution function of the phase angle. To determine the transit time of the structures between two ultrasonic barriers two methods can be used. The maximum of the cross-correlation function corresponds to the transit time. The other way is to add the demodulated signal of both barriers. In the spectrum of the added signals after a filtering a cosine function can be determined. The frequency of this function is directly proportional to the transit time.