A new instrument based on a constant frequency pulsed phasehyphen;lockedhyphen;loop (CFPPLL) concept has been developed to accurately measure the ultrasonic wave velocity in liquids and changes in ultrasonic wave velocity in solids and liquids. An analysis of the system shows that it is immune to many of the frequencyhyphen;dependent effects that plague other techniques including the constant phase shifts of reflectors placed in the path of the ultrasonic wave. Measurements of the sound velocity in ultrapure water are used to confirm the analysis. The results are in excellent agreement with values from the literature, and establish that the CFPPLL provides a reliable, accurate way to measure velocities, as well as for monitoring small changes in velocity without the sensitivity to frequencyhyphen;dependent phase shifts common to other measurement systems. The estimated sensitivity to phase changes is better than a few parts in 107.
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