Cross-correlation flowmeters use two such sensors placed a known distance apart in
the fluid-carrying pipe and cross-correlation techniques are applied to the two output
signals from these sensors. This procedure compares one signal with progressively
time-shifted versions of the other signal until the best match is obtained between the
two waveforms. If the distance between the sensors is divided by this time shift,
a measurement of the flow velocity is obtained. A digital processor is an essential
requirement to calculate the cross-correlation function, and therefore the instrument
must be properly described as an intelligent one.