A method of extending the lower measured value of ultrasonic Doppler Flowmeter

In order to improve the measure performance of gas ultrasonic Dooppler Flowmeter in lower velocity, the optimized algorithm of autocorrelation filtering is applied to increase signal to noise ratio. Then by applying Fast Fourier Transform (FFT), frequency is derived and the invalidation problem of Fourier Transform when the velocity is under 0.2m/s is avoided. The result shows that, when the velocity is under 0.08m/s, this method is very efficient in rejecting interference noise and improving successful rate of identifying signals, therefore extends the lower measure limit of ultrasonic Doppler Flowmeter.