Highly Sensitive Detection of Water Content in Ethanol Based on a Resistive-Capacitive Frequency Modulation Method

The accurate detection of water content in ethanol is crucial for various industrial processes, such as fuel production and chemical manufacturing. However, existing methods face challenges in terms of sensitivity, stability, and repeatability. This paper presents a new method for detecting the water content in ethanol based on a resonant sensor with a quartz crystal (QC) in series with a interdigital electrode (IDE), called QC-IDE circuit. Theoretical analysis indicates that variations in the equivalent capacitance and resistance of the IDE can influence the response frequency of the QC-IDE circuit. The QC-IDE circuit features two key frequency modulation ranges: one occurs when the IDE resistance Rs > 50 kΩ and the capacitance Cs < 50 pF, the other occurs when the resistance 1 kΩ < Rs < 10 kΩ and the capacitance Cs < 20 pF. The QC-IDE resonator sensor was prepared by adjusting the characterization parameters of IDE , such as electrode spacing and linewidth, and the water content in ethanol was tested in the range of 0% to 100% volume (vol.). Through comprehensive evaluation, the optimal electrode parameter configuration for the QC-IDE circuit was determined. Experimental results demonstrate that the developed sensor achieved a response sensitivity of up to 29.36 Hz/(% vol.) and exhibited good linearity of low water content , stability, and repeatability. These findings suggest that the proposed QC-IDE sensor holds significant potential for applications in ethanol water content detection.