Elements of design and a field application of a TDR-based soil moisture and electrical conductivity monitoring system are described with detailed presentation of the time delay units with a resolution of 10 ps. Other issues discussed include the temperature correction of the applied time delay units, battery supply characteristics and the measurement results from one of the installed ground measurement stations in the Polesie National Park in Poland.
Mechanical details as well as electrical models of FDR (frequency domain reflectometry) sensors for the measurement of the complex dielectric permittivity of porous materials are presented. The sensors are formed from two stainless steel parallel waveguides of various lengths. Using the data from VNA (vector network analyzer) with the connected FDR sensor and selected models of the applied sensor it was possible obtain the frequency spectrum of dielectric permittivity from 10 to 500 MHz of reference liquids and soil samples of various moisture and salinity. The performance of the analyzed sensors were compared with TDR (time domain reflectometry) ones of similar mechanical construction.
The purpose of this study is to determine the temperature influence on the soil bulk dielectric permittivity, ϵb, calculated from the measurement of the electromagnetic‐wave velocity of propagation along the parallel waveguide in a TDR probe, i.e., a probe working in time domain reflectometry technique. The experimental evidence shows that the existing models do not completely describe the temperature effect. However, it has been confirmed that the observed temperature effect is the result of two competing phenomena: ϵb increases with temperature following the release of bound water from soil solid particles, and ϵb decreases with temperature increase following the temperature effect of free water molecules. It has been found that there is a soil type–characteristic moisture value, θeq, named the equilibrium water content, at which both competing phenomena compensate each other. The equilibrium water content, θeq, is correlated with the soil specific surface area. Based on knowledge of θeq, a temperature‐correction formula is presented that adjusts the TDR soil‐moisture measurements at various temperatures to the corresponding value at 25°C. This decreases the absolute measurement error of soil moisture, θTDR, by the factor of 2 as compared to the uncorrected values.
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