Dual Frequency Ultrasonic Flowmeter

Abstract

As to an ultrasonic flowmeter which deduces flowrate from the propagating time of ultrasonic pulse wave in the flow, it is very important to accurately detect the pulse wave reaching the ultrasonic transducer. In a conventional ultrasonic flowmeter, a constant threshold is set to detect a certain peak in the leading edge of the received pulse wave. But when the amplitude of the received pulse wave fluctuates due to a disturbance in the flow, ‘trigger-miss’ often happens as it is difficult to identify the same peak by a constant threshold.
In the present study a new method has been developed to consistently detect the pulse wave even when there exists strong fluctuation. Two pairs of ultrasonic transducers are placed across the flow and transmit two waves of different frequencies simultaneously. These waves are received at the opposite side and are added together in an electric circuit to generate some predominant peaks at the leading edge of the pulse. Setting the threshold to one of the predominant peaks, detection of the pulse wave is much less affected by fluctuation of an amplitude of the received signal.
A flowmeter using this method was constructed and tested. It has been proved that this method is effective to consistently measure propagating time in the disturbed flow compared with the conventional method using a single wave.