Study of Long Term Drift of Aluminum Oxide Thin Film Capacitive Moisture Sensor

Drift is an important phenomenon of capacitive humidity sensor, the most vital device in the commercial dew point meter. Due to drift in the output, most of the meters for critical applications require costly and time-consuming recalibration. Modeling of drift phenomenon for RH humidity sensors was reported in various literature. But to the best of our knowledge, there is no work reported so far about the drift of the sensors utilized for trace level moisture detection. This paper presents the mathematical model, the electrical equivalent circuits, and long-term drift analysis of the aged capacitive sensors for trace moisture sensing. The sensors were fabricated using porous alumina oxide thin film for detecting moisture in the range of 3 to 100 ppm. To study the long-term drift, experiments were performed under different operating conditions over a period of nearly one year. The response behaviors of the sensors were simulated with the help of the model. Comparing the model response with the experimental results, the proposed model can be used for drift analysis for an aged sensor, determination of hydration level and moisture condensation inside the pores. Experimental results showed that when a sensor was kept in drying agent, a maximum average drift of only 1.50% was observed but when a similar sensor was kept in open high humidity atmosphere, the maximum long-term drift was found to be ~11.20%.