Transit time ultrasonic flow meters measure the difference in time from when an ultrasonic signal is transmitted from the first transducer until it crosses the pipe and is received by the second transducer. A comparison is made of upstream and downstream measurements.
A Doppler ultrasonic flow meter uses a transducer to emit an ultrasonic beam into the stream flowing through the pipe. For the flow meter to operate, there must be solid particles or air bubbles in the stream to reflect the ultrasonic beam.
