PROBLEM TO BE SOLVED: To eliminate a sudden failure of data due to disturbances of a flow velocity distribution of a water path or bubbles, eliminate a malfunction because of a surface wave and eliminate measurement errors due to dust and mud accumulated on a front face of a sensor. ;SOLUTION: A transmission signal and a detection signal are mixed at a mixing part 11, which is detected at a wave detection circuit 5. A deflection frequency signal which is a difference signal of the signals is taken out at a filter part 6 and spectrally transformed at an FFT(fast Fourier transform) part 7. A transformed result is processed by a moving average method at an operating part 8. A comparison for detection of a peak is started from a maximum frequency value Nmax. After a first peak Np1 is detected, the comparison is stopped when the power value mean fft(N) becomes not larger than half a power value mean fft(Np1) of the peak Np1. The frequency of the first peak Np1 is converted to an average flow velocity, thereby operating a flow rate.;COPYRIGHT: (C)1998,JPO
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