Elsevier

Measurement

Volume 42, Issue 3, April 2009, Pages 329-336
Measurement

Review
Time-domain reflectometry method and its application for measuring moisture content in porous materials: A review

https://doi.org/10.1016/j.measurement.2008.08.011Get rights and content

Abstract

The time-domain reflectometry (TDR) method is based on the analysis of dielectrics’ behavior in time-varying electric field which can be utilized in the determination of permittivity and electrical conductivity of a wide class of materials. Measurement of moisture content in porous materials belongs to its most frequent applications; after several decades of use TDR method became one of the most recognized techniques in this field. In this review, a short historical overview of the TDR method applications is introduced, the function of a typical TDR device is described and the main TDR data interpretation techniques are surveyed first. Then, the methods for determination of moisture content in porous media from the measured relative permittivity are analyzed and prospective ways of their application are identified. Finally, some recommendations for further developments in TDR measurement of moisture content in porous materials are formulated.

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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