E-Zine June 2009
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In Doppler ultrasonic flowmeters, ultrasonic energy is transmitted into the pipe. Some of this energy is reflected by particles, bubbles, and eddies in the flow stream. The frequency difference between the transmitted energy and its reflected ultrasonic signal is used to determine the flow in the pipe.
At no flow conditions, the frequencies of the transmitted energy and its reflected signal are the same. However, under flowing conditions, the frequency of the reflected signal will be different than that of the transmitted energy. This is due to the Doppler effect that relates how sound is perceived when reflected from objects in motion. For example, the horn of a moving car has a higher pitch moving towards a listener than it does moving away from the listener. The frequency difference between the transmitted energy and the reflected ultrasonic signal increases linearly as flow increases. The transmitter processes signals representing the transmitted energy and its reflections to determine the flow rate where the fluid velocity is proportional to the difference in their frequencies. The following equation expresses this relationship.
Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters