A resonant sensor for relative humidity measurements based on a polymer-coated quartz crystal

Due to the growing demand for humidity control systems, there is a need for small, cheap, accurate and reliable sensors for air relative humidity (RH) measurements. In this paper, a resonant air relative humidity sensor is presented. The sensor is based on a quartz crystal coated on both surfaces with a hygroscopic polymer. Due to the adsorption of water molecules, the polymer mass increases accordingly with the surrounding air humidity, causing the decrease of the device resonance frequency. By measuring the sensor resonance frequency, an accurate RH measurement can be obtained. Experimental results are shown, comparing the fabricated sensor behavior with a commercially available RH sensor.