Contactless Liquid Sensing Technique Using a Quartz Oscillator

In this paper, a contactless sensor for the measurement of liquid concentration using a quartz oscillator is reported. An out-of-plane electric field distribution from a planar sensing capacitor (SC) is used for the noncontact sensor measurement of liquid concentration on a spacer. This sensor measures the changes in the capacitance of the SC by the changes in the frequency of the quartz oscillator. Because the capacitance is dependent on the relative permittivity and conductivity of the liquid sample on the spacer, it is possible to measure the concentration of the sample. The response is very stable because, in this sensor, the SC is incorporated into the quartz oscillator. Because the SC is in contact with the liquid sample instead of the quartz crystal resonator, the sensor has a high quality factor (approximately 30000). Moreover, this sensor can be used with a general integrated circuit (IC) and an electronic component at a low cost. The sensor is used for the noncontact measurements of liquid concentration.