A Physical Understanding of Solid State Humidity Sensors

In an effort to produce a humidity sensor suitable for automotive applications, an investigation was made of solid state sensors which change electrical impedance as a function of humidity. Physical models are presented which describe operation of the sensors. Measured impedance characteristics of humidity sensors are explained in terms of the following mechanisms: (1) physical adsorption of water vapor, (2) dissociation of water molecules, (3) electrical conduction by means of a Grotthuss chain reaction, (4) electrostatic fields associated with surface-layer complexes, and (5) electrolytic conduction inside capillary pores.