A THICK-FILM RESONANT SENSOR FOR HUMIDITY MEASUREMENTS

The paper describes the implementation and performance of a resonant humidity sensor relying on the piezoelectric properties of PZT-based thick films on an alumina diaphragm and the reversible hygroscopic behaviour 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% RH) 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 sensor changes its output when the relative humidity is varied from 10 to 40%. The dynamics of interaction between the water molecules and the organic film are discussed and related to the prompt response of the sensor to changes of environmental conditions. It is suggested that the joint exploitation of resonant techniques and thick-film technology can be a powerful strategy for manufacturing other inherently digital sensors of chemical species and biosensors.