Abstract: The design of a wireless, passive LC resonant sensor based on classic circuit theory, and its application for quick, non-destructive measured of moisture content of packaged tea were described in this paper. The proposed LC resonant sensor is an inductive-capacitive resonant circuit, which is made of a single planar capacitor and spiral inductor printed on a humidity-sensitive insulating medium substrate. To measure the moisture content of packaged tea, the LC resonant sensor is embedded inside the tea package. As the variation of the humidity inside the tea package, the humidity-sensitive insulating medium substrate absorbs the water vapor, and the planar capacitor’s capacitance and the sensor’s resonant frequency were changed, which is remotely detected through a coil connected to the instrument for measuring moisture content outside the tea package. Therefore, the moisture content of packaged tea can be indirectly determined by measuring the change in sensor’s resonant frequency. The experiment results showed that the proposed method is reasonable, and is comprised with the GB method, no significant deviation is found between the measuring results. The difficult technological problem in how to quick, accurate and non-destructive measuring moisture content of packaged tea is solved.

Key words: tea, moisture content, non-destructive measurement, capacitive sensor, resonant circuit