9. Cross-Correlation Ultrasonic Meter

The cross-correlation meter employs two transverse acoustic signals separated by a short distance (figure 11-2b). Under no-flow or laminar-flow conditions, the two signals received are identical to those transmitted. When turbulent flow occurs, the movement of an eddy through a beam causes a change in the acoustic signal which has a unique signature. This particular eddy will cause an identical change in the second acoustic signal, and the eddy can be tracked as it moves down-stream. An electronic signal processor is used to compare the two received signals. When two identical signals are found, the time and distance (between the acoustic transmitters) information is used to compute velocity. In general, cross-correlation meters measure the average velocity of all the eddies crossing one pipe diameter. If no eddies are present in the flow, the meter can track sediment or bubbles. However, if the flowing fluid is homogenous and has no eddies (laminar flow), this type of meter will not work. Like the single-path transit-time meter, this meter measures an incorrectly weighted mean velocity. Therefore, the measurement is susceptible to an inaccuracy associated with variations in velocity profiles.