Ultrasonic measurement of sound velocity in liquids by a phase-locked loop

This paper describes a new ultrasonic technique for measuring the velocity of sound in liquids which relies on a phase-locked loop method to automatically eliminate dependence of the result on temperature. An empirical equation using NaCl solutions as test liquids within a concentration range of 0.0-5.0% and a temperature range of 15-45 °C was obtained by analyzing the various sound velocities by the least squares method. An empirical equation was derived to design a compensatory voltage-controlled oscillator (Comp VCO) which would automatically eliminate the strong temperature dependence of the measured sound velocity in liquids. The sound velocity after eliminating temperature dependence was easily obtained from the output frequency of the Comp VCO by multiplying the coefficient. A system with a Comp VCO circuit was applied to NaCl solutions of varying concentrations and was shown to yield velocities accurate to within 0.024% independent of temperature variations of ±1.3 °C at approximately 24.9 °C.