A THICK-FILM RESONANT SENSOR FOR HUMIDITY MEASUREMENTS

The paper describes the implementation and performance of a resonant h umidity sensor relying on the piezoelectric properties of PZT-based th ick films on an alumina diaphragm and the reversible hygroscopic behav iour of a cellulose acetate coating on the substrate. A shift of about 100 Hz of the resonance frequency f0 = 56.675 Hz (22-degrees-C, 35% R H) is obtained by changing the relative humidity from 10 to 95%, with a linear response over the whole range. In a time of 0.5 min the senso r changes its output when the relative humidity is varied from 10 to 4 0%. The dynamics of interaction between the water molecules and the or ganic film are discussed and related to the prompt response of the sen sor to changes of environmental conditions. It is suggested that the j oint exploitation of resonant techniques and thick-film technology can be a powerful strategy for manufacturing other inherently digital sen sors of chemical species and biosensors.