Effects of electrical conductivity on uncoated- and coated-rods of eight electromagnetic soil water content sensors

Abstract

Soil water content stands as a pivotal parameter across numerous research disciplines and engineering applications. Electromagnetic (EM) sensors have gained widespread usage in monitoring soil water content. While commonly employed sensors are suitable for typical agronomic- or landscape-soil environments, they may exhibit significant deviations under conditions of high-salinity or -electrical conductivity (EC). We investigated sensing limits under the influence of increasing EC and the resulting accuracy of dielectric permittivity (ε) determinations using commercial soil water content sensors. Tests were conducted using eight commercially available EM sensors (CS655, HydraProbe II, TDR-305N, TDR-310N, TDR-315N, TEROS 12, TEROS ONE, and WET150). For these uncoated rod sensors, most were accurate within ±3 % relative permittivity errors in saline solutions within the range of 0‒4 dS m−1 while TDR-305N, TEROS ONE and WET150 maintained an accuracy of 5 % for EC values up to 9.24 dS m−1. We explored extending the working range of some sensors to high-saline environments by adding an insulating coating on sensor rods. Since the coated probe not only perceives the characteristics of the immersed medium but also interacts with the coating material itself, a calibration model was evaluated for determining the actual ε of the evaluated medium. The strong correlation between a four-point calibration and the model demonstrates the effectiveness of calibrating coated EM sensors for ε determination using only a limited number of calibration materials. The findings presented here could serve as a valuable resource when selecting EM sensors for use in high-saline environments. Results also shed light on the potential to extend the sensing range through the integration of insulating coatings.