ReviewTime-domain reflectometry method and its application for measuring moisture content in porous materials: A review
Section snippets
Historical overview
The application of time-domain reflectometry (TDR) in engineering and natural sciences has a relatively long history. Already in 1930s it became a recognized technique in cable testing. The physical principal was quite simple. It is well known from the electromagnetic wave theory that if any change of impedance appears along the transmission line of the waves there is a partial or total reflection on it. In a cable such a reflection means a fault. If the time between launching the waves into
TDR devices – fundamentals
A device based on the TDR principle (see e.g. [1], in more details [45]) launches electromagnetic waves and then measures the amplitudes of the reflected waves together with the time intervals between launching the waves and detecting the reflections. The fundamental element in any TDR equipment used for the determination of moisture content in porous materials is a metallic cable tester. This usually consists of four main components: a step-pulse generator, a coaxial cable, a sampler and an
TDR data interpretation
There are two basic methods of the trace analysis. The first one is based just on the identification of the times of reflections at the head and the end of the probe. In this way, the apparent permittivity of the probe (not dependent on frequency) can be determined. This method is usually called travel time analysis or time-domain analysis. The other one assumes that the model of dielectric dispersion and relaxation in the probe is known and tries to identify the unknown model parameters using
Determination of moisture content in porous materials using the measured relative permittivity
There are three basic approaches to the determination of moisture content from measured relative permittivity. The first possibility is utilization of empirical conversion functions generalized for a certain class of materials. The second is application of dielectric mixing models, which assumes knowledge of the relative permittivities of the material matrix, water, air and other parameters, that cannot be measured directly but have to be determined by empirical calibration of the model. The
Concluding remarks
The time-domain reflectometry method can be considered as a very prospective technique for measuring moisture content in porous media. In contrast to most other methods commonly used for that purpose, it does not require calibration for every material, in general. The TDR probe calibration can be done in advance all once for every single probe. Another advantage of the TDR method is that it is well applicable for the materials with higher salt content, where an application of methods such as
Acknowledgement
This research was supported by the Ministry of Education, Youth and Sports of Czech Republic, under Project No. MSM: 6840770031.
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2022, CatenaCitation Excerpt :Owing to signal deterioration with cable length, practical distances from TDR device to the probes are typically limited to 20 to 30 m (Jones et al., 2002). Longer cable lengths may provide unreliable readings where soil salinity and clay content are low (Černý, 2009; Robinson et al., 2003), thus increasing the measurement error of soil erosion and θv. This strongly restricts the monitoring area.