Sampling volume evaluation of seven electromagnetic soil moisture sensors

Abstract

The sampling volume of electromagnetic (EM) soil moisture sensors is of significant interest due to the diverse applications for which they are utilized. However, this volume can vary considerably based on multiple factors, including the number, spacing, and length of sensor rods, as well as the operational frequency. The sensor's volume of influence is dictated by electric field lines or ‘fringing fields’ that develop between pairs of powered electrodes. This study involved the development of an accurate sensor positioning system employed to characterize the EM sensor's electrical fringing field in 3-directional - x, y, and z space. Sensor outputs were recorded at mm increments beginning with the sensor positioned 20 mm above the air-water interface followed by incremental immersion into a water bath until the maximum sensor output was achieved. For each measurement cycle, the sensor was repositioned to test signal output approaching the air-water interface in −x, +x, −y, +y, −z, and +z orientations. The sensor sampling volume was determined based on the normalized 99% output value, which was assumed to represent the sensor position relative to an air-water interface where the sensor fringing field just begins to ‘sense’ or to become impacted by the air phase, thereby reducing the signal amplitude. Our results vary somewhat from other studies using a similar evaluation approach, leading us to conclude that EM-sensor sampling volume determination is both complex and imprecise as a result of the global sensor market's varied circuits and electrode configurations. EM soil moisture sensors evaluated in this study exhibited sampling volumes of 386 cm3 (TDR-315), 66 cm3 (TDR-310H), 398 cm3 (CS655), 45 cm3 (EC-5), 40 cm3 (HydraProbe II), 148 cm3 (WET150), and 49 cm3 (TEROS 12), which were smaller than manufacturer reported volumes.