There are three distinct
ways of measuring the time shift. These are direct measurement, conversion to a phase
change and conversion to a frequency change. The third of these options is particularly
attractive, as it obviates the need to measure the speed of sound in the measured fluid
as required by the first two methods. A scheme applying this third option is shown in
Figure 16.14. This also multiplexes the transmitting and receiving functions, so that
only one ultrasonic element is needed in each transducer. The forward and backward
transit times across the pipe, Tf and Tb, are given by: