Virtual-instrument based ultrasonic Doppler flowmeter for low velocities measurement

Conventional ultrasonic Doppler flowmeters can not easily measure low flow velocities and directions. A low speed Doppler flowmeter was developed using virtual-instrument technology to demodulate the echo signal to an intermediate frequency of about 15 kHz. A Zoom-FFT estimator is used to analyze the spectrum of the demodulated Doppler shift signal with high frequency resolution, and a weighted averaging algorithm is used to calculate the average Doppler frequency. The flow direction and average velocity can then be determined from the results. The new Doppler system was tested with transducers clamped on a stainless-steel pipe to measure the flow. The results indicate that the system can measure the flow direction and velocity at velocities as low as 0.02 m/s. Therefore, the system is more effective than a conventional ultrasonic Doppler flowmeter for measuring low flow velocities.