Humidity sensor using surface acoustic waves propagating along polymer/LiNbO3 structures

A surface acoustic wave (SAW) sensor for measurement of humidity in ambient atmosphere is presented. The majority of SAW sensors utilize the effect of mass loading. In this paper, we propose a new SAW humidity sensor that is based on the changes in the film conductivity rather than the mass. The phase velocity and propagation loss were computed for the SAW propagating on a piezoelectric (128° YX LiNbO
₃
) substrate, which is coated with a hygroscopic conductive polymer. In experiments, a 30 MHz dual SAW delay line fabricated on 128° YX LiNbO
₃
was coated with the sodium salt of polystyrene sulfonate. This hygroscopic polymer coating caused a SAW velocity change of 20 m/s per 10% relative humidity. The response was quite linear in the range from 40 to 80% RH