Calibration of Capacitance Sensors for Compacted Silt in Non-Isothermal Applications

This study investigates how temperature affects the response of capacitance sensors used to measure the dielectric permittivity of unsaturated soils. The dielectric permittivity is strongly correlated with the volumetric water content, an important variable in understanding water flow processes in unsaturated soils. Although capacitance sensors have been used widely in laboratory and field applications, they have only recently been used to characterize thermally induced water flow in unsaturated soils during geothermal heat exchange processes. To date, no studies have characterized the effects of temperature on capacitance sensor output for dense, compacted soils. This paper describes a calibration methodology that was used to isolate the effects of temperature on the response of capacitance probes in compacted soils having different initial conditions (i.e., compaction water content and dry density). It was observed that changes in temperature of 26°C can lead to an increase in the measured dielectric permittivity of up to 24 % with no changes in volumetric water content of the soil. In addition to defining a soil-specific calibration equation to relate volumetric water content and dielectric permittivity under ambient temperature conditions, a correction equation was proposed for temperature effects. The capacitance sensor's response to changing temperatures was observed to be sensitive to both the initial volumetric water content and the initial density of the soil, and parameters were defined for both of these terms in the correction equation.