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

Abstract

This paper presents dielectric measurements of talc, glass beads, and their mixtures under different moisture and salinity levels. The measurements were conducted using a prototype seven-rod probe (15 mm long central rod) connected to a single port of vector network analyzer. The samples were moistened with distilled water and KCl solutions in order to obtain six different moisture content levels. The complex dielectric permittivity was determined from vector network analyzer reflection-coefficient measurements based on the open-water-liquid calibration procedure. Next, the fitting of volumetric water content–real part of dielectric permittivity calibration curves was performed for each material at selected frequencies, and the obtained relations were compared with well-known calibration equations. Additionally, a salinity index for the tested materials was calculated. It was concluded that pure talc is not an optimal material for the calibration and verification of dielectric methods. The calibration curves obtained for glass beads and the mixtures of glass beads with talc gave results close to well-known reference calibration functions. Additionally, the addition of talc caused the data points to be less scattered. Moreover, the values of the salinity index for the tested materials were in a good agreement with literature data for sand. The obtained results indicated that glass beads with the addition of talc can be used as a reference material for the calibration and verification of dielectric methods and devices for soil moisture measurement.

Shielded open-circuited probe for in-situ measurements of soil permittivity in Very high frequency (VHF) and Ultra high frequency (UHF) bands

“…In contrast to the approaches (Szerement et al, 2019(Szerement et al, , 2020Woszczyk et al, 2020Woszczyk et al, , 2019Wagner, 2011;Wilczek et al, Figure 4. Refractive index (RI) and normalized attenuation coefficient (NAC) measured using: proposed method (solid lines), cylindrical coaxial cell (points).…”

“…Moreover, measuring the CP spectrum in a wide frequency range was not formulated (Wilczek et al, 2016). In further studies (Szerement et al, 2019(Szerement et al, , 2020, the authors, based on the approaches of Wagner (2011) The main idea of our work is to modify the proposed constructions of non-precision coaxial probes (Szerement et al, 2019(Szerement et al, , 2020Wagner, 2011;Wilczek et al, 2016)…”

“…Recently, a number of broadband open-ended coaxial sensors have been proposed, which are either completely inserts into the soil (Lin at al., 2017; Majcher et al, 2020; Szerement et al, 2019, 2020; Szyplowska et al, 2015; Wilczek et al, 2016; Woszczyk et al, 2020, 2019), or it touches of soil surface without penetration in it (Demontoux et al, 2017; Guihard et al, 2020, 2019; Mavrovic et al, 2018, 2020, 2021). The CP measuring method based on an open-ended flange coaxial probe (Demontoux et al, 2017; Komarov et al, 2016) is primarily used for homogeneous liquid and semi-liquid materials for which a good contact between the medium and flange is provided.…”

“…The main limitation of existing shielded open-ended coaxial-waveguide probes spectroscopic methods of measuring CP is the incapability of their application directly in the soil due to the design of shielded open-circuited (Lin et al, 2017) or closed cylindrical (Baker-Jarvis et al, 2004) coaxial cells that prevent natural fluxes of moisture and heat in the soil through the measuring area of probes. Probe in fork-shaped configuration, which is consisting of one central and several shielding rods, is the most suitable for measuring ultrabroad band spectrum of soil permittivity (Heimovaara, 1994, 1996; Lin at al., 2017; Majcher et al, 2020; Szerement et al, 2020, 2019; Szyplowska et al, 2015; Wagner, 2011; Woszczyk et al, 2020, 2019; Xu et al, 2012). The use of a large number of densely placed shielding rods (six and more) makes it possible to minimize the wavefield distortion of the main Transverse electromagnetic (TEM) mode.…”

Assessing Effects of Salinity on the Performance of a Low-Cost Wireless Soil Water Sensor

“…Similarly, the other three soil types had a correlation between observed and measured water contents of R 2 ≥ 0.65 and RMSE, ranging from 0.05 to 0.09 ( Table 4 ). However, the individual fitting for each electrical conductivity solution had a correlation above 0.95, which implied that using the calibration equation depends on each salinity level, instead of a single composite equation for all salinity levels [ 38 ]. Thus, we can conclude that soil-specific calibration is necessary for low-cost soil moisture sensors at each salinity level.…”