Abstract: Apparatus for sensing pressure pulses developed by the collapse of bubbles in a liquid flow system to enable measurement of the rate of erosion caused to material by cavitation. A sonic transducer, immersed in flowing liquid, generates voltages in response to pressure pulses developed by the collapse of bubbles in the liquid. The voltage representing each bubble collapse is transmitted to electronic equipment. The electronic equipment measures the magnitude of each voltage signal and counts the number of voltage signals representing the number of bubble collapses sensed by the sonic transducer thereby providing data from which the rate of erosion can be calculated.