A Digital Hygrometer for Trace Moisture Measurement

A microcontroller-based digital hygrometer system using a low-cost moisture sensor in the range of 3.7-100 ppm has been developed. The sensor is of capacitive type and consists of nanoporous thin film of alumina (γ - Al₂O₃) dip coated in between two parallel gold electrodes onto an alumina substrate. The behavior of the sensor has been modeled with an object to develop a signal processing circuit to convert capacitance change into frequency. The detection electronics circuit is based on a relaxation oscillator whose output is suitable for interfacing with a digital device. The sensor has been characterized with the circuit and then output frequency is calibrated in terms of ppm. The accuracy of the digital hygrometer when compared with the commercial dew point meter is found to be ±1 PPM.