The EUSONIC flowmeters work on the Transit time ultrasonic principals. This put simply, involves sending and receiving ultrasonic pulses from a pair of sensors and examining the time difference in the signal. This is a more direct method of determining flow than some other systems and thus provides a more reliable measurement.
Ultrasonic transducers are mounted on the external surface of the pipe and are used to generate ultrasonic pulses which are passed through the pipe wall. The flowing liquid within causes time differences, frequency variations and phase shifts in the ultrasonic signals which are then evaluated by the flowmeter to produce an accurate flow measurement.