Ultrasonic liquid-concentration sensor with temperature compensation

This paper describes the workings of an innovative ultrasonic liquid-concentration sensor using a phase-locked loop method with temperature compensation. This sensor is constructed with an extremely simple circuit system which exclusively treats continuous sinusoidal waves. Its validity and usefulness was verified experimentally using NaCl solutions as test liquid-specimens. Temperature-compensation circuit system is constructed with a voltage converter circuit which consists of a thermistor, a voltage-linearized circuit and a quartz crystal Colpitts oscillator. The design method was derived by analyzing the compensation circuit, and a temperature-compensation factor for NaCl solutions was derived from the equation for ultrasonic velocity in sea water. A compensation circuit which has specific value of 3.38 kHz//spl deg/C over a range of 25.0/spl deg/C/spl plusmn/2.0/spl deg/C was designed. This value was consistent with experimental value of 3.36 kHz//spl deg/C. We added it to the sensor system for compensating circuit. This sensor system with compensation circuit was shown to yield accurate values independent of a temperature variation of /spl plusmn/1.3/spl deg/C around 25.0/spl deg/C.