High-resolution humidity measurements with surface transverse wave based resonant devices. Applications to wireless remote sensing

Results from an experimental study on the humidity sorptive properties of plasma polymer-coated surface transverse wave (STW) resonators, for possible applications in high-resolution humidity sensors, are presented. We used 767- and 998-MHz STW resonators covered with hexamethyldisiloxane (HMDSO) polymer films, obtained in a glow discharge process. The STW resonator frequency shift versus the sorbed water vapor mass was measured at controlled relative humidity (RH) and temperature. An average sensitivity of 1.4 ppm/%change in RH was obtained which was found to be 4–10 times higher compared to 14-MHz bulk crystal resonators coated with the same polymer. HMDSO coated and uncoated STW resonators featuring high-Q, low-loss, excellent temperature stability and low aging are implemented in a system for wireless transmission of humidity and temperature data over a 1-GHz radio channel.