Impedance-based moisture sensor design and test for gas-phase biofilter applications

A low-cost method that determines moisture content in biofilter media, e.g., woodchips and composts, is under development. Moisture content plays a critical role in determining biofilter performance; however, few reliable moisture sensors are available in this case, primarily due to the large and varied particle size of biofilter media. The moisture sensor presented in this study employed impedance as a predictor of moisture content. It took advantage of a 100 kHz sinewave signal as the input source and generated DC voltage outputs. In two samples of woodchips and one of compost, impedances were evaluated within a representative volume of 30 cm × 30 cm × 15 cm and were compared to a reference. The sensor readings changed with moisture contents, and the correlation curve was linearized using the Box-Cox transformation. To validate the sensor measurement, the impedance of media was directly measured using an impedance analyzer, and the results were used to calculate the ideal sensor output. Analysis of covariance (ANCOVA) was used to compare the directly measured result to the predicted ideal sensor reading. No significant difference was observed between the two transformed datasets in terms of slope and intercept for all three media. The role of temperature (22°C, 27°C, and 32°C) in affecting impedance measurement was examined, but the results showed that the influence was minor. Compaction of media, a common phenomenon in biofilter applications, was artificially carried out, and its effect on sensor measurement was evaluated. The results showed that the sensor reading increased with time for about two days and then stabilized; when a second compacting weight was added, a slight but significant increase was observed. This suggests that a sensor re-calibration might be necessary in a long-term application.