Thick-film sensors for agricultural applications

Abstract

A relative-humidity sensor and two types of soil moisture sensors (capacitance and thermal-conductivity), designed for mass production at low unit costs, were developed by the use of thick-film, hybrid, microelectronic technology and tested in the laboratory. The relative-humidity sensor operates over the range from zero to 100% r.h. and is not damaged by immersion in water. It consists of a platinum-gold electrode structure that is screened and fired on an alumina substrate and then coated with a thin layer of CaSO 4 · 1 2 H 2 O (plaster of Paris). The capacitance soil-moisture sensor operated best in the range from zero to 10% moisture content by weight when tested in Graded Ottawa Sand; the sensor consists of two coplanar, thick-film electrodes with interlocking fingers that are covered with a thin overlayer of glass. The thermal conductivity soil-moisture sensor operated best in the range from zero to 10% moisture content by weight when tested in Graded Ottawa Sand but will work up to saturation at approximately 23%. It consists of a thick-film heater and a sensing thermistor that are screened and fired on opposite sides of an alumina substrate. Both heater and thermistor are covered with a thin layer of overglaze and, if necessary, can be further coated with epoxy resin.