Temperature Dependant Scaled Voltage to Improve the Performance of Single Capacitance Sensors

Abstract

Several newly developed capacitance sensors have simplified the real-time determination of soilwater content. Previous work has shown that salinity and temperature can affect some capacitance sensors, butrelatively little has been done to correct these effects. The objectives of this study were to: 1) evaluate the effectof medium temperature and salinity on the apparent water content measured with a single capacitance sensor(SCS), 2) evaluate the effectiveness of temperature corrected scaled voltage technique to mitigate thetemperature and salinity effects under laboratory conditions. A column study was conducted using 1-litercolumns containing two media: pure Deionized water and quartz sand under varying water contents andsalinity. Media temperature was varied between 5 and 45 oC using an incubator. Single capacitance sensors andthermocouples were placed in the middle of the column and were logged at every minute. There was strongnegative correlation (R2 = 0.9??) between sensor reading in deionized water and its temperature with a rate of -0.7789 mV C-1. Rates of SCS apparent outputs were 0.4535 and 0.5347 mV oC-1 for air in heating and coolingcycles, respectively. A similar positive temperature effects were observed in soil samples at different watercontents (0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30 cm3 cm-3). Effect of temperature was less at high salinity andhigh water content level than at low salinity and water content. Results of the study showed an exponentialincrease in SCS readings due to salt concentration increase in both media. Using a temperature dependant scaledvoltage calibration model we were able to mitigate the effect of temperature by significantly reducing thetemperature effect by as high as 98%. Our experimental data clearly indicates that ignoring media temperatureto measure water content with this sensor may lead to errors, particularly at low water content.