Humidity Sensor Using Surface Acoustic Wave Delay Line with Hygroscopic Dielectric Film

Abstract

A surface acoustic wave (SAW) sensor for measurement of humidity in ambient atmosphere is presented. SAW technology is increasingly being used in sensors. The majority of the work reported on SAW sensors to date has studied the effect of mass loading. In this paper, we propose a new SAW humidity sensor that uses the changes in the film permittivity rather than the mass. A 30 MHz dual SAW delay line fabricated on 128° YX LiNbO₃ was coated with cellulose acetate. This hygroscopic polymer coating caused a SAW velocity change of 0.36 m/s per 10% relative humidity. The response was linear in the range from 10% to 80% in relative humidity.