Evaluation of a Multi-Rod Probe Performance for Accurate Measurements of Soil Water Content

Abstract

Dielectric sensors operating in time and frequency domain can be used to determine soil moisture content. There is still a need for developing new sensors for determination of soil water content, in order to further improve measurement accuracy, lower the price or adapt the equipment for special applications. For example, many of the existing soil moisture probes do not allow accurate measurement of dielectric properties in a small volume because of their construction. The paper presents the evaluation of a seven-rod probe for an accurate determination of soil water content in a small sample volume. Firstly, digital simulations for sensitivity zone of the tested probe were performed. Next, the prototype probe was tested for two soils with various texture and moisture content in the range from air dry to near saturation. The values of dielectric permittivity (frequency domain) and apparent dielectric permittivity (time domain) were calculated from the measured $S_{11}$ parameters. The obtained data was compared with the reference Topp's equation. It was concluded that the tested probe is able to measure soil moisture in a small volume (about 8 cm3) in the range of frequencies from 20 MHz to 200 MHz.