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

Justyna Szerement,Andrzej Wilczek,Agnieszka Szypłowska,Marcin Kafarski,Arkadiusz Lewandowski,Wojciech Skierucha,Aleksandra Woszczyk

doi:10.3390/s19071646

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 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.

Highlights

There is a pressing need for an indirect, rapid, and accurate moisture content measurement method in porous media [1]
Dielectric techniques rely on the determination of the relationship between dielectric permittivity and soil water content
The aim of the study was to construct, electromagnetically simulate, and test a seven-rod probe of a simplified coaxial geometry consisting of a signal rod surrounded by six ground ones

Summary

Introduction

There is a pressing need for an indirect, rapid, and accurate moisture content measurement method in porous media [1]. The direct laboratory methods currently used are often expensive, time-consuming, destructive, and labor-intensive [2]. Precise irrigation scheduling is not possible using direct methods in some cases; for example, when trying to apply laboratory methods to measure moisture in a well-defined small volume (inter alia in soil moisture observation conducted near the root system) [3,4]. The development of dielectric measurement techniques in the time and frequency domain in a broad frequency range became very useful for the fast and nondestructive in situ determination of the soil volumetric water content (θ) [5]. Dielectric techniques rely on the determination of the relationship between dielectric permittivity and soil water content.

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Conclusion