Calibration of TEROS 10 and TEROS 12 electromagnetic soil moisture sensors

Abstract

The TEROS 10 and TEROS 12 are widely used capacitance-based sensors for measuring volumetric water content (${\theta }_v$), but few systematic studies of the measurement volume and accuracy of these sensors have been published. The objectives of this study were to (1) calibrate and validate these sensors in mineral soils, (2) determine the sensing volume of each sensor, and (3) evaluate the temperature sensitivity of the sensors. Both sensors were calibrated in the laboratory using columns of packed soil at multiple moisture levels from air-dry to near saturation. Sensors were validated using additional laboratory and field measurements. The sensing volume was determined by quantifying the response of raw sensor outputs while increasing the level of oven-dry sand, moist sand (0.100 cm³ cm⁻³), or water around the sensor in the radial and axial directions. Temperature sensitivity was tested in controlled conditions over a range of temperatures from 2°C to 40°C using encapsulated sensors in packed soil columns at constant ${\theta }_v$. Linear calibration models resulted in mean absolute error ≤0.030 cm³ cm⁻³ for both sensors during laboratory and field validations. In moist sand, the sensing volume contributing 95% of the maximum sensor response was 439 cm³ for the TEROS 10 and 423 cm³ for the TEROS 12. Both sensors exhibited changes in ${\theta }_v$ of ≤0.02 cm³ cm⁻³ when subjected to a temperature change from 2°C to 40°C. For the soils considered here, both sensors demonstrated accuracy that is likely sufficient for a variety of agricultural and hydrological applications.