Simple solution-concentration sensor with temperature compensation using ultrasonic waves

This paper describes how a simple new ultrasonic solution-concentration sensor works when using a phase difference method with temperature compensation. This technique relies on sound velocity changes due to variations in solution concentration. An empirical equation of the sound velocity for both the sugar and NaCl solutions, which covered a concentration range of 0.0-20.0% and a temperature range of 15-45°C, was obtained by analyzing the measurements of various sound velocities through the least squares method. It has become clear that quite the same temperature-compensation factor can be used for both solutions in terms of dependence on temperature in their equations. An empirical equation was applied to design a temperature compensation circuit. The sensor system with temperature compensation was shown to yield a 0.01% accuracy of concentration determination, independent of the temperature variation of ±1.0°C around 25.4°C for the NaCl solution and of ±0.9°C around 25.2°C for the sugar solution