a liquid flowmeter and method for determining the velocity of flow of fluid in a conduit , including a sensor adapted for mounting in the bottom of the conduit , and including acoustic energy transmitting and receiving devices , an in - phase reflected doppler energy signal detector connected with the receiving device , an anti - alias low pass filter for producing from the in - phase reflected energy a filtered signal , a digital - to - analog converter for digitalizing the in - phase signal to produce a sampled array , an accumulator for storing a first number of mean zeroed power spectra from the sampled array , a zeroing device for zeroing a second number of the initial samples of the power spectra , thereby to produce a first modified spectra , a median filter of rank r for filtering the first modified spectra to produce a second modified spectra , a limiting device for limiting the second modified spectra to produce samples limited to the upper portion of a range of amplitudes , thereby to produce a third modified spectra , a device for determining the high - frequency edge of the third modified spectra , a first converting device operable when the third modified spectra have a wide band width for producing as a multiple of the high - frequency edge an estimate of mean fluid velocity , and a second converting device operable when the third modified spectra have a narrow band width for producing as a function of the spectral peak an estimate of mean fluid velocity .