Determination of the humidity of soil by monitoring the conductivity with indium tin oxide glass electrodes

A number of approaches to the determination of water content of soil use a dielectric permittivity measurement, which is accurate but involves expensive and complex devices. We explore the use of indium tin oxide, a wide-band gap semiconductor with high conductivity, long-term stability, and low cost, to build a cheap sensor of low humidity values for automatic irrigation systems. The electrical resistance determined by an ac impedance method discounts the effects of electrode contacts and provides a reliable measure of the water content, even when the chemical conditions of water vary widely. The reason for this is that, at low values of humidity, the conductivity is controlled by a surface conduction mechanism that is largely insensitive to the electrolytic properties of added water. The results appear promising for applications that require the detection of very low levels of humidity in soil. (C) 2002 American Institute of Physics.