A Seven-Rod Dielectric Sensor for Determination of Soil Moisture in Well-Defined Sample Volumes

Abstract

This paper presents a novel seven-rod sensor used for time-domain reflectometry (TDR) and frequency-domain reflectometry (FDR) measurements of soil water content in a well-defined sample volume. The probe directly measures the complex dielectric permittivity spectrum and for this purpose requires three calibration media: air, water, and ethanol. Firstly, electromagnetic simulations were used to study the influence of the diameter of a container on the sensitivity zone of the probe with respect to the measured calibration media and isopropanol as a verification liquid. Next, the probe was tested in three soils—sandy loam and two silt loams—with six water contents from air-dry to saturation. The conversion from S 11 parameters to complex dielectric permittivity from vector network analyzer (VNA) measurements was obtained using an open-ended liquid procedure. The simulation and measurement results for the real part of the isopropanol dielectric permittivity obtained from four containers with different diameters were in good agreement with literature data up to 200 MHz. The real part of the dielectric permittivity was extracted and related to the moisture of the tested soil samples. Relations between the volumetric water content and the real part of the dielectric permittivity (by FDR) and apparent dielectric permittivity (by TDR) were compared with Topp’s equation. It was concluded that the best fit to Topp’s equation was observed in the case of a sandy loam. Data calculated according to the equation proposed by Malicki, Plagge, and Roth gave results closer to Topp’s calibration. The obtained results indicated that the seven-rod probe can be used to accurately measure of the dielectric permittivity spectrum in a well-defined sample volume of about 8 cm3 in the frequency range from 20 MHz to 200 MHz.

Dielectric Properties of Glass Beads with Talc as a Reference Material for Calibration and Verification of Dielectric Methods and Devices for Measuring Soil Moisture

“…The shortening the rods of the probe with respect to the solution from [21] enabled us to extend the measurement frequency range. The simulation of the ε values of isopropanol obtained from the short seven-rod probe with the 15 mm central rod agreed with data obtained from the literature [25] up to 500 MHz (Figure 2), which was also confirmed by the low value of root-mean-square error (0.024) between the simulated ε value and the known reference permittivity of isopropanol.…”

“…The probe was manufactured at the Laboratory of Dielectric Spectroscopy of the Institute of Agrophysics PAS, Lublin, Poland (Figure 1a,b). The probe used in this work (a short seven-rod probe) was a modified version of the probe described in [21]. The construction of the seven-rod probe Materials 2020, 13,1968 4 of 12 enabled us to measure soil moisture in a well-defined volume.…”

“…The construction of the seven-rod probe Materials 2020, 13,1968 4 of 12 enabled us to measure soil moisture in a well-defined volume. Because the operating frequency range of the probe shown in [21] was 200 MHz, in order to extend its operational frequency range, another construction of probes with shorter central and outer rods was proposed. Probes with 25 and 30 mm long central and outer rods and 15 mm and 20 mm long central and outer rods, respectively, were tested by the numerical simulations using the finite element method (FEM) in the Ansys HFSS software [22].…”

“…The simulated probes were surrounded by tested material in the form of a cylinder of 70 mm in height and 15 mm in diameter filled with calibration media (air, water, and ethanol) and isopropanol as a verification liquid. More details about the simulation space and the applied model were presented in [21]. The extraction of complex permittivity spectrum from the simulated reflection coefficient (S 11 ) was performed using the following bilinear equation:…”

“…The value of the ε of isopropanol (verification liquid) obtained from Equation (4) was based on the simulation data. Next, the obtained ε of isopropanol obtained from the two probes with different dimensions were compared with the data shown in [21] for a probe with 40 mm long inner rod and with the data of isopropanol found in the literature [25].…”