Adjusting Temperature and Salinity Effects on Single Capacitance Sensors

Abstract

Several newly developed capacitance sensors have simplified real-time determination of soil water content. Previous work has shown that salinity and temperature can affect these sensors, but relatively little has been done to correct these effects. The objectives of this study were to evaluate the effect of media temperature and salinity on the apparent water content measured with a single capacitance sensor (SCS), and to mitigate this effect using a temperature dependent scaled voltage technique under laboratory conditions. A column study was conducted containing two media: pure deionized water and quartz sand under varying water contents (0.05 to 0.30 cm 3 cm −3) and salinity (0 to 80 mmol L −1). Media temperature was varied between 5 and 45 °C using an incubator. The SCS probes and thermocouples were placed in the middle of the columns and were logged at an interval of 1 minute. There was strong negative correlation between sensor reading and temperature of deionized water with a rate of −0.779 mV °C −1. Rates of SCS apparent output were 0.454 and 0.535 mV °C −1 for air in heating and cooling cycles, respectively. A similar positive correlation with temperature was observed in sand at different water contents. The SCS probe was less sensitive to temperature as salinity and water content increased. Using a temperature-corrected voltage calibration model, the effect of temperature was reduced by 98%. An analytical model for salinity correction was able to minimize the error as low as ± 2% over the salinity level tested.