Miniaturized Methanol Concentration Sensor Developed using a Quartz Resonator Connected in Series to an Interdigital Capacitor

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This paper reports the development of a monolithic methanol concentration sensor for fuel cell applications. An interdigital capacitor and a resonator are fabricated on a single crystal quartz substrate. Methanol solution is introduced into a microchannel on the capacitor. The methanol concentration can be measured by measuring the frequency shift of the resonator because the shift shows a monotonic increase with a change in the relative permittivity of the capacitor connected in series to the resonator. The sensitivity is 0.1 wt %/Hz. We also found that the smallest resonator diameter that achieves a resolution of 0.1 wt % is 1.5 mm. Because the structure and manufacturing process are simple, we expect the proposed sensor to be useful for various kinds of liquid sensing applications.