The development of prognostic capabilities that predict the condition and remaining service life for key industrial systems has the potential to significantly impact performance and the economics of operation for both current and next generation plants. This paper describes an on‐line real‐time ultrasonic meter that can be used to monitor both fouling and cleaning in reverse osmosis filters. It provides a measure for the degree of fouling. A suit of ultrasonic transducers is mounted to operate through the filter‐housing wall on a pilot‐scale service water system. A “Degree of Fouling” index is given during both fouling and cleaning for the filters during operation for processing of saline solutions (simulated sea and brackish waters) and solids contamination. The fouling index is transmitted to a central computer where it is integrated in a system level prognostic algorithm. © 2003 American Institute of Physics