Use Of Time Domain Reflectometry Research Articles

Dielectric relaxation study of 1-pentanol–ethanol binary mixtures at 10 MHz–50GHz frequency range using TDR technique

ABSTRACT The dielectric measurements have been executed by making the use of time domain reflectometry (TDR) technique for 1-pentanol and ethanol in the pure form and for their binary liquid mixture in the frequency interim of 10 MHz–50 GHz at varying temperatures, respectively. The complex permittivity spectra studied show Debye behaviour and plotted as the function of frequency. The temperature-dependent dielectric constant and relaxation time values were estimated. The Kirkwood correlation factor (g eff) has also been studied to determine the alignment of dipole moments in mixture. The thermodynamic parameters and temperature-dependent behaviour of relaxation time, i.e. Arrhenius plot, have been studied.

A New TDR-Based Sensing Cable for Improving Performance of Bridge Scour Monitoring

The use of time domain reflectometry (TDR) for real-time monitoring of bridge scour process has gone through several stages of development. The recently-proposed concept of bundled TDR sensing cable, in which two sets of insulated steel strands are twisted around and connected to a central coaxial cable to form a compact sensing cable, is a major change that has several advantages including the bottom-up sensing mechanism. Nevertheless, there is big room for improving its measurement sensitivity and signal to noise ratio (SNR). Changes in waveguide configuration also need to be made to avoid the adverse effect of insulation abrasion observed in field implementation. This study evaluated three new conductor and insulator configurations for constructing the sensing waveguide, including a balanced two-conductor waveguide (Type I), an unbalanced three-conductor waveguide with insulation coating on the middle conductor (Type II) and an unbalanced three-conductor with insulation coating on the two outer conductors (Type III). In all cases, the spacing between the two sets of steel strands (i.e., the waveguide conductors) was especially enlarged by replacing some steel strands with non-conductor wires to increases measurement sensitivity and avoid shorted conditions due to insulation abrasion. Experimental results show that Type III has the best performance on all counts. A new improved TDR sensing cable was hence proposed based on Type III configuration. Its performance was further evaluated by a full-scale experiment to take into consideration the long range of measurement in most field conditions. Detailed discussions on improvements of measurement sensitivity and SNR, limitation of sensing range, and mitigating the adverse effect of insulation abrasion are presented.

TDR application for moisture content estimation in agri-food materials

Cereals and legumes are invaluable resources, as they represent the raw materials of many foods and beverages. Cereals are also largely used as livestock feeds, thus indirectly influencing the quality of dairy products and meat. Because of their key role in human nutrition and well-being, the safety and quality of these agri-food materials are extremely important topics in food science. In this regard, moisture content is one of the ultimate factors influencing the quality, safety and price of the final food product; hence, it is crucial to monitor water content of materials in the food production line. Starting from these considerations, this paper describes the use of time domain reflectometry (TDR) for both in-line and off-line moisture content sensing of agrifood materials. In particular, after a brief description of the basic principles of TDR, two representative application cases for moisture content measurements of agri-food materials are reported and commented on.

Characterization of Early Stage Concrete by Non-destructive Testing

To non-destructively assess the behaviors of early stage concrete, seismic methods are useful tools. These methods also aid in learning about curing concrete for use in construction decisions. This paper discusses how freezing concrete causes a major change in the engineering properties. In the experimental procedure, ultrasonic tests were conducted on curing concrete subjected to different freezing process. The experimental procedures involved performing ultrasonic tests on concrete frozen in various ways. The results showed that there exists linear correlation between low strain seismic wave velocity and concrete strength under normal curing conditions. However, these relationships do not hold if the concrete is subjected to freezing process. To correct for the frozen effect, the bulk modulus needed to be adjusted. With the correction, a linear relationship with water content was found. Procedures to correct the effects of freezing are proposed, which include the use of Time Domain Reflectometry to measure the water content. Time Domain Reflectometry can be used to measure water content and help to correct the effects of freezing. This process allows an estimate for the strength of the frozen concrete. This information could be incorporated to determine the magnitude of Winter Load Increase in cold regions for government agencies.

Proof of Concept: Development of Snow Liquid Water Content Profiler Using CS650 Reflectometers at Caribou, ME, USA.

The quantity of liquid water in the snowpack defines its wetness. The temporal evolution of snow wetness’s plays a significant role in wet-snow avalanche prediction, meltwater release, and water availability estimations and assessments within a river basin. However, it remains a difficult task and a demanding issue to measure the snowpack’s liquid water content (LWC) and its temporal evolution with conventional in situ techniques. We propose an approach based on the use of time-domain reflectometry (TDR) and CS650 soil water content reflectometers to measure the snowpack’s LWC and temperature profiles. For this purpose, we created an easily-applicable, low-cost, automated, and continuous LWC profiling instrument using reflectometers at the Cooperative Remote Sensing Science and Technology Center-Snow Analysis and Field Experiment (CREST-SAFE) in Caribou, ME, USA, and tested it during the snow melt period (February–April) immediately after installation in 2014. Snow Thermal Model (SNTHERM) LWC simulations forced with CREST-SAFE meteorological data were used to evaluate the accuracy of the instrument. Results showed overall good agreement, but clearly indicated inaccuracy under wet snow conditions. For this reason, we present two (for dry and wet snow) statistical relationships between snow LWC and dielectric permittivity similar to Topp’s equation for the LWC of mineral soils. These equations were validated using CREST-SAFE in situ data from winter 2015. Results displayed high agreement when compared to LWC estimates obtained using empirical formulas developed in previous studies, and minor improvement over wet snow LWC estimates. Additionally, the equations seemed to be able to capture the snowpack state (i.e., onset of melt, medium, and maximum saturation). Lastly, field test results show advantages, such as: automated, continuous measurements, the temperature profiling of the snowpack, and the possible categorization of its state. However, future work should focus on improving the instrument’s capability to measure the snowpack’s LWC profile by properly calibrating it with in situ LWC measurements. Acceptable validation agreement indicates that the developed snow LWC, temperature, and wetness profiler offers a promising new tool for snow hydrology research.

Use of Time Domain Reflectometry to Estimate Moisture and Density of Unbound Road Materials: Laboratory Calibration and Field Investigation

Soil moisture content and dry density of unbound granular pavement materials are important properties for compaction control that influence pavement performance under cyclic loading. Under these loading conditions, increasing moisture content can accelerate significant changes in density. Time domain reflectometry (TDR) is a method for measuring the moisture content and density of soils with rod probe sensors. This paper introduces new calibration functions for TDR measurements using these rod probe sensors embedded in the soil. TDR measurements were taken in the laboratory for a typical road base material at two basically different conditions: at constant moisture content with different dry densities and at constant dry density with different moisture contents. In this study, a relationship was developed between the voltage drop occurring for the passage of an electromagnetic wave through the soil and the bulk density. The permittivity of the soil sample obtained from the travel time of TDR signals was used to calculate the volumetric moisture content. Finally, the gravimetric moisture content was obtained from the volumetric moisture content and bulk density relationship. For the validation of the calibration functions, rod probe sensors were installed in a road to obtain in situ moisture content and density under field conditions. Laboratory results indicate that the calibration functions are independent of moisture and density, and the field test shows the applicability of the method. The newly developed calibration functions allow for the monitoring of the long-term pavement performance, leading to a better understanding of the time-dependent evolution of, for example, rutting of roads.

실내모형실험을 통한 TDR 함수량계의 현장 함수비 보정

지하매설구조물인 상수도관 주변 지반의 간극수 동결로 상수도관의 변형에 영향을 미칠 수 있다. 이에 최근에는 현장 계측을 통한 포장 하부구조체의 함수비를 주기적으로 측정하기 위해서 미국 Campbell사의 TDR(time domain reflectometry) CS616 함수량계와 독일의 IMKO사 제품의 TRIME FM를 이용한 수동 함수량계 TRIME P3, T3 Probe를 사용하고 있다. 일반적으로 TDR 함수량계와 TRIME 수동 함수량계는 흙의 종류, 입도, 다짐도, 온도 등에 의해 오차가 유발될 수 있기 때문에, 현장의 시료를 사용하여 TDR 함수량계의 실내보정실험을 반드시 수행해야 한다. 즉, TDR함수량계의 매뉴얼에서는 일반적인 흙의 보정방정식을 제안하고 있지만 현장 적용시에는 반드시 보정실험을 수행하도록 권고하고 있다. 본 연구에서는 상수관 등 지하매설구조물의 주변의 다양한 채움재료를 사용하여 실내 보정 실험을 수행하였으며, 이 결과로부터 TDR함수량계인 CS616, TRIME-T3, TRIME-P3의 보정모델식을 제안하였으며, 보정식에 대한 현장검증을 수행하였다. Water content of subgrade soil in water supply systems has a large effect on performance. Many researchers lately make use of time-domain reflectometry (TDR) probes to measure the soil water content of subgrade soil from monitoring. The laboratory calibration test of TDR probe should be performed with soil field, because TDR probe can cause an error by type, gradation, density, and temperature of soil. This study shows the laboratory calibration test using TDR CS616, TDR-P3, TDR-T3. The calibration equations of TDR were then proposed. It was confirmed from the study that the data of TDR probe monitored in field could be used to estimate the freezing, unfrozen water content, and matric suction of soil.

Influência da densidade do solo na estimativa da umidade em um nitossolo vermelho distroférrico, por meio da técnica de TDR

Entre as técnicas para avaliação da umidade do solo, destaca-se a que utiliza a Reflectometria no Domínio do Tempo (TDR), a qual consiste em um método indireto, não destrutivo, rápido, exato e pouco dependente dos fatores ambientais. Porém, trabalhos desenvolvidos após as suas aplicações sugerem a influência de fatores ligados às características mineralógicas e estruturais dos solos, o que torna necessária a calibração local para o adequado funcionamento da TDR. O objetivo deste trabalho foi estudar a influência da densidade do solo (Ds) na medida da umidade volumétrica do solo (θ), utilizando-se a técnica de TDR, em um Nitossolo Vermelho distroférrico. Um modelo que expressa a relação entre a constante dielétrica do solo (Ka) e θ foi ajustado por meio de regressão polinomial múltipla, com a incorporação da Ds para a estimativa dos seus coeficientes. Os resultados permitiram concluir que, nesse solo, o modelo de calibração deve levar em conta o efeito da Ds, sem o que a qualidade das estimativas de umidade, a partir das medidas de constante dielétrica, fica comprometida. Os resultados evidenciaram também que a sensibilidade da TDR à variação da umidade do solo é elevada, tornando possível que a técnica seja utilizada com exatidão em ações que envolvam o monitoramento da variação da umidade do solo. Os resultados indicaram a necessidade de mais estudos sobre a utilização da TDR em solos tropicais.

Correction of TDR-based soil water content measurements in conductive soils

Time Domain Reflectometry (TDR) is a widespread technique for measurement of soil water content (SWC). The main assumption behind the use of Time Domain Reflectometry (TDR) is of negligible losses, therefore assuming that only the real part determines the value of the TDR-measured apparent dielectric permittivity. This assumption does not hold for soils where surfaces are conductive (clay soils) or where high concentrations of electrolyte are present in the soil solution (saline soils) because under these conditions the contribution of the imaginary part becomes important. One of the main effects of dielectric losses on the TDR measurement is overestimation of SWC. In this study we present a methodology for separating the real and the imaginary part from the measurement of the apparent dielectric permittivity. This approach allows correction of the SWC overestimation, by using the TDR-measured electrical conductivity as indicator of dielectric losses. Oven-dry gravimetric soil water content was used as an independent method for soil water content assessment. The original SWC overestimation (in respect to the oven-dry gravimetric based measurement) reached values of up to 20% of total soil saturation, after the correction the differences were reduced to a 3–5%. The methodology can be applied based on knowledge of measured permittivity and electrical conductivity only, making it readily applicable to field experiments.

Single-Ended Loop Make-Up Identification—Part I: A Method of Analyzing TDR Measurements

Loop qualification consists of determining whether a loop can support digital subscriber line (DSL) services or not, and generally, the estimate of the transfer function is sufficient for such purposes. Loop qualification is one of the most important steps that service providers need to address in deploying DSL technology. In this paper, a solution to a more ambitious problem-single-ended automatic loop make-up identification, i.e., the determination of the length and the gauge of all loop sections (including bridged taps) via single-ended testing without human intervention-is proposed. Loop make-up identification will allow operators to more accurately qualify a loop for DSL service and to update and reorder telephone-company loop records, which can be accessed to support engineering, provisioning, and maintenance operations. Despite its potential importance, the possibility of achieving loop make-up identification via single-ended measurements has seldom been addressed in the literature. The use of time-domain reflectometry (TDR) measurements, which are analyzed by a novel step-by-step maximum-likelihood (ML) algorithm, is proposed here to achieve accurate loop make-up identification. In this paper, the proposed algorithm needs no a priori information, whereas in Part II of this study, a means to reduce the computation time of the identification algorithm will be provided, and the case where a statistical characterization of the loop may be available prior to testing will also be considered.

On the Use of Time-Domain Reflectometry for Full-Wave Electromagnetic Optimization of Nonuniform Waveguides

We present the application of an optimization technique in time domain for the design of cascaded waveguide structures. In this case, the traditional high-dimensional optimization problem can be decomposed into several one- or two-dimensional problem in the time domain. For this purpose, the time-domain response is divided into a series of small time intervals that have to be optimized successively utilizing time-domain reflectometry. A finite-difference time-domain simulator is used to obtain the time-domain response. The number of function calls (expensive full electromagnetic simulations) could be reduced up to 90% in comparison to a frequency-domain optimization. Numerical and measurement results are presented to validate the new technique.

Time Domain Reflectometry Development for Use in Geotechnical Engineering

AbstractThis study extends the use of time domain reflectometry (TDR) in geotechnical engineering, a technique originally developed to locate faults in transmission lines. Different elements of the TDR technique are developed, including design of TDR probes, probe installation/test methodology, and relationships between TDR measured dielectric constant and water content of soil. A coaxial probe is developed that is used for measuring the dielectric constant of soil prepared in a cylindrical cell or compaction mold. A multiple-rod field probe is developed that modifies previously developed multiple-rod probes and extends their capability for measuring the in-place dielectric constant of soil. An analytical solution is developed to determine the sampling volume and spatial bias of the TDR measurement. The solution is extended to study the effect of soil disturbance and presence of air gaps due to probe insertion. Experimental results validate the solutions. New relationships are proposed between dielectric constant and water content to eliminate some of the limitations of the existing calibration relationships. Several possible applications of the developed probes, test methodology, and calibration equations for measuring water content and density of soil are illustrated.

THE TACQ COMPUTER PROGRAM FOR AUTOMATIC TIME DOMAIN REFLECTOMETRY MEASUREMENTS: I. DESIGN AND OPERATING CHARACTERISTICS

Despite the increased use of time domain reflectometry (TDR) for measurement of soil water content and bulkelectrical conductivity (BEC), there are few public releases of software for TDR system control. The TACQ program,under development since the early 1990s on a wide variety of soils, allows control of multiplexed systems supporting up to256 TDR probes. The program is DOS-based to ease creation of low-power, embedded computer systems; and toeliminate resource conflicts and timing difficulties inherent to multi-tasking, Windows-based operating systems. So that itcan be easily used in embedded systems, the program was written to run with as little as 640 kbytes of RAM and 1 Mbyteof expanded memory, and with a variety of graphics standards and CPUs ranging from 80186 to Pentium. Embeddedcomputer systems based on the PC-104 specification have been implemented using TACQ; and the program has beenintegrated into a supervisory control and data acquisition system (SCADA) for irrigation scheduling and control. Using aparallel port, the program controls multiplexers from both Campbell Scientific (Logan, Utah), and Dynamax (Houston,Texas); and it allows reading of probes in any user-defined order if using the latter. The user has complete control overmultiplexer address assignments, interconnection of multiplexers, and probe locations on each multiplexer; includingindividual settings for probe length, window width, averaging, distance to each probe, gain, and type of data acquired(waveform, travel time, apparent permittivity, water content, relative voltages for bulk electrical conductivity, or acombination of these). Interfaces to TDR instruments including Tektronix 1502 (modified), 1502B, and 1502C cabletesters are implemented through an RS-232 port. Concurrent temperature data may also be acquired (ComputerBoardsmodels CIO-DASx). System power control is implemented through the computers own power management capabilities,and through direct control of power to the TDR instrument and video subsystem where applicable, thus allowing creationof very low-power systems. The program is stable and suitable for use in environmental measurement systems that areunattended for long periods of time. Waveform interpretation methods are discussed in the second paper in this series.

A New Model for the Response of TDR to Heterogeneous Dielectrics

The use of time domain reflectometry (TDR) techniques for measuring the moisture content of composite materials is a mature art but usually assumes homogeneity of the material in the transverse plane. As the basis of a forward solution to TDR imaging, we describe an integral equation approach to model the response of the TDR system to a lossless heterogeneous dielectric body. Then, in conjunction with a suitable dielectric model of the composite material, the TDR response to moisture content distribution may be quantified.

THE TACQ COMPUTER PROGRAM FOR AUTOMATIC TIME DOMAIN REFLECTOMETRY MEASUREMENTS: II. WAVEFORM INTERPRETATION METHODS

Despite the increased use of time domain reflectometry (TDR) for measurement of soil water content and bulkelectrical conductivity (BEC), there are few public releases of software for TDR system control. Even though graphicalinterpretation of the waveform to find pulse travel times is key to success with the method, the few published descriptionsof computer methods are incomplete. The TACQ program, under development since the early 1990s on a wide variety ofsoils, allows control of multiplexed systems supporting up to 256 TDR probes. Waveform interpretation methods are user-controlledand allow interpretation using various methods reported in the literature or methods available only in TACQ.The default methods allow automatic interpretation of waveforms from a variety of media including loose, air-dry soil,and wet clay. The present study shows that interpretation methods can have a large effect on reported water contents. Theadditive effects can result in water content change errors as large as 0.08 m 3 m 3 as the soil wets and dries, and as TDRsystem temperature changes over a season. Thus, the interpretation methods used should be reported in rigorous studiesinvolving TDR water content determination, calibration, and temperature effects. The TACQ program, anddocumentation, may be downloaded from http://www.cprl.ars.usda.gov/programs/.

Laboratory Calibration and In Situ Measurements of Moisture by Using Time-Domain Reflectometry Probes

Excessive moisture in pavement systems can cause considerable damage and can lead to early deterioration. One method for continually monitoring the moisture content of pavement systems nondestructively is the use of time-domain reflectometry (TDR) probes. Although originally developed to measure faults in electrical cables, TDR probes employ an electromagnetic wave that is transmitted along a set of metallic conducting rods (or waveguides). The velocity of the electromagnetic wave is influenced by the dielectric constant (ϵ) of the material surrounding the waveguides. The large contrast between the dielectric constants of free water and of dry soil makes this an effective nondestructive evaluation method. Soil samples with different moisture contents were prepared and the TDR output, which is a function of the dielectric properties, was compared with the measured gravimetric moisture content. Calibration equations were developed in a laboratory setting for two types of TDR probes (CS610 and CS615) embedded in the Virginia Smart Road test facility at Blacksburg, Virginia. Preliminary field data were collected for the two different probe types embedded in different pavement structures. It is shown that the two types of TDR probes yield similar data in some situations and different data in other circumstances. It appears that the composition of the pavement structure has an effect on the moisture measured in the subbase layer. Although preliminary results indicate that the use of CS615 TDR probes in pavement applications is promising, further continuous monitoring of both types of TDR probes is necessary to determine if the CS615, which can be readily connected to a data acquisition system, can be used in lieu of the CS610, which requires a time-consuming collection procedure or possible additional data collection equipment.

Measurement of relative permittivity in sandy soils using TDR, capacitance and theta probes: comparison, including the effects of bulk soil electrical conductivity

Measurement of the apparent dielectric permittivity of soils (dielectric constant) is becoming a popular way of estimating soil volumetric water content. This paper focuses on the measurement of apparent permittivity in four sandy soils using; time domain reflectometry (TDR), a surface capacitance insertion probe (SCIP) and a Theta probe. Measurement of the apparent permittivity using the SCIP and Theta probe are compared with the apparent permittivity measured using the TDR. Calibration of such instrumentation has remained relatively empirical following the engineering approximation presented by Topp et al. (Topp, G.C., Davies, J.L., Anan, A.P., 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Res. Research 16, 574–582.). The refractive index model proposed by Whalley (Whalley, W.R., 1993. Considerations on the use of time domain reflectometry (TDR) for measuring soil water content. J. Soil Sci. 44(1), 1–9.) based on that of Birchak et al. (Birchak, J.R., Gardner, C.Z.G., Hipp, J.E., Victor, J.M., 1974. High dielectric constant microwave probes for sensing soil moisture. Proc. IEEE 62(1), 93–98.) is investigated as a means of gaining some physical understanding of the relative contributions of the different dielectric components in soils. Predictions made by the model are tested against results using multiple linear regression. The predictions agree well with the observed measurements. Inter-electrode conductivity is found to contribute significantly to the apparent permittivity measured using the SCIP and to a lesser extent the TDR but not the Theta probe. Inclusion of inter-electrode conductivity in regression analysis improved results. The Theta probe was found to overestimate the apparent permittivity of the soil by ∼1.5 when compared with TDR results. It is suggested that this may be the result of compaction of the soil close to the electrodes, coupled with a strong bias in the sensitivity of the probe to the region very close to the central electrode.

Comparison of empirical and partly deterministic methods of time domain reflectometry calibration, based on a study of two tropical soils

The use of time domain reflectometry for the determination of volcanic soils' moisture content in tropical areas has shown the necessity to set up an accurate calibration. We chose to test the most representative calibration models on two tropical volcanic soils coming from two different climatic areas of the Réunion Island. The empirical model of Topp et al. [Topp, G.C., Davis, J.L., Annan, A.P., 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resources Research, 16 (3) 574–582.] was found to offer the best accuracy/simplicity trade-off for simulations with a cambisol, with an accuracy of ±2.46 dielectric constant points (or about ±2.7% for a water content close to 45%). In contrast, it gave poorer results for analysis of the andosol containing the greatest quantity of amorphous minerals (±5.74 dielectric constant points). The partly deterministic models were found to be better for this latter application, including that of Ansoult et al. [Ansoult, M., De Backer, L.W., Declercq, M., 1985. Statistical relationship between apparent dielectric constant and water content in porous media. Soil Sci. Soc. Am. J., 49, 47–50.] which gave the best accuracy (±3.84 dielectric constant points). For both soil materials studied, there was only a slight gain of accuracy by introducing a specific polynomial calibration determined in the laboratory: ±1.72 and ±3.46 dielectric points for the cambisol and the andosol, respectively. Choices of models to be used should thus be based on the type of soil to be analysed and accuracy requirements.

Examining the use of time domain reflectometry for measuring liquid water content in frozen soil : E. J. A. Spaans & J. M. Baker, Water Resources Research, 31(12), 1995, pp 2917–2925

Examining the use of time domain reflectometry for measuring liquid water content in frozen soil : E. J. A. Spaans & J. M. Baker, Water Resources Research, 31(12), 1995, pp 2917–2925

Examining the use of time domain reflectometry for measuring liquid water content in frozen soil

Time domain reflectometry (TDR) offers a unique opportunity to measure liquid water content θL in frozen soil, since the permittivity of ice is much lower than that of water. However, calibrations of TDR derived from drying unfrozen soil, where water is replaced by air, may not apply to a freezing soil, where water is replaced by ice, since the permittivity of ice is greater than that of air. We designed a gas dilatometer to calibrate TDR for θL in frozen soil. A soil sample is hermetically sealed in the gas dilatometer; subsequent soil freezing reduces total air space and hence increases pressure inside the gas dilatometer, since ice is less dense than water. The amounts of soil water frozen were computed from measured pressure change as temperature was incrementally decreased. TDR calibrations for two samples of the same soil at different total water contents had identical slopes but different intercepts, supporting our hypothesis that there exists no unique calibration of TDR for θL for frozen soil, but rather a family of calibration curves, each curve corresponding to a different total water content.