Automatic ultrasonic measurement of sound velocity in liquids by a phase-locked loop method

This paper describes the workings of a new ultrasonic technique for measuring sound velocity in liquids which relies on a phase-locked loop method to automatically eliminate dependence on temperature. The sound velocity v is easily found from the Main VCO frequency f multiplied with the coefficient as the ratio of a known sound velocity v _s and its frequency f_s. The validity and usefulness for the system was verified experimentally using NaCl solutions as test-liquid specimens. The compensated frequency f_C is gained from the output of the compensatory VCO (Comp VCO) which is automatically eliminates the strong temperature dependence of sound velocity in liquids. The Comp VCO is constructed identically to the Main VCO. The sound velocity for eliminating the temperature dependence was easily obtained with v=v_sf_C/f_s . The system with compensation circuit was applied to NaCl solutions of varying concentrations and was shown to yield 0.0005% accurate velocity determination, independent of temperature variations of ±1.3°C at approximately 24.9°C