scholarly journals A New Method to Determine the Spatial Sensitivity of Time Domain Reflectometry Probes Based on Three-Dimensional Weighting Theory

Water ◽  
2020 ◽  
Vol 12 (2) ◽  
pp. 545
Author(s):  
Peng-Ju Qin ◽  
Zhang-Rong Liu ◽  
Xiao-Ling Lai ◽  
Yong-Bao Wang ◽  
Zhi-Wei Song ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Dielectric Constant ◽  
Time Domain ◽  
Three Dimensional ◽  
Time Domain Reflectometry ◽  
End Effect ◽  
Design And Optimization ◽  
Spatial Sensitivity ◽  
Dimensional Weighting ◽  
Apparent Dielectric Constant

The time domain reflectometry (TDR) method has been widely used to measure soil water content for agriculture and engineering applications. Quick design and optimization of the probe is crucial to achieving practical utilization. Generally, the two-dimensional weighting theory, calculation of the spatial sensitivity of TDR probes in the plane transverse to the direction of electromagnetic wave propagation, and relevant numerical simulation techniques can be used to solve any issues. However, it is difficult to tackle specific problems such as complex probe shape, end effect, and so forth. In order to solve these issues, a method including a three-dimensional weighting theory and the relevant numerical simulation technique was proposed and verified to confirm the feasibility of this method by means of comparing the existing experimental results and the computational values. First, a shaft probe was used to determine the impact of the shaft on the effective dielectric constant of the probe. Then, three-rod probes were calibrated by a sample with a special shape and water-level variations around the probe using the proposed method to determine the values of the apparent dielectric constant. Besides, model boundary size and end effect were also considered in the computation of dielectric constants. Results showed that compared with the experimental and computational data, the newly proposed method calculated the measurement sensitivity of the shaft probes well. In addition, it was observed that experiment dielectric constant values were slightly different from computational ones, not only using a vertical probe but also horizonal probe. Moreover, it was also found that there was a slight influence of sample shape and end effect on the apparent dielectric constant, but model boundary size has a certain impact on the values. Overall, the new method can provide benefits in the design and optimization of the probe.

Time domain reflectometry for water content and density of soils: study of soil-dependent calibration constants

2005 ◽  
Vol 42 (4) ◽  
pp. 1053-1065 ◽  
Author(s):  
V P Drnevich ◽  
A K Ashmawy ◽  
X Yu ◽  
A M Sallam
Keyword(s):  
Dielectric Constant ◽  
Water Content ◽  
Time Domain ◽  
Field Experiments ◽  
Engineering Application ◽  
Time Domain Reflectometry ◽  
Sensitivity Analyses ◽  
Dry Density ◽  
Theoretical Predictions ◽  
Apparent Dielectric Constant

The paper studies the soil-dependent calibration constants used for determining water content and density of soil using time domain reflectometry (TDR), specifically, to establish the typical soil calibration values and study the extent of the uncertainty in calibration factors on measurement accuracy. The TDR method described here makes use of a calibration equation normalized by soil dry density, which involves two soil-dependent constants, a and b. Both a and b have physical significance, with the value of a related to the apparent dielectric constant of the dry density – normalized dry soil solids and the value of b related to the apparent dielectric constant of the pore fluid. From theoretical predictions, typical values of a are around 1.0, and typical values of b are around 9. Practically, the constants a and b are obtained through calibration tests performed in conjunction with standard compaction tests. Experimental study shows that calibration constants fall within the ranges from theoretical predictions. Tests on five soil mixtures provided average values of a = 0.945 and b = 8.76, while 11 clean sands resulted in average values of a = 1.0 and b = 8.5. The study also shows that there are no significant effects of compaction energy on the measured values of a and b. Sensitivity analyses indicate that variations in a and b both cause variations in TDR-determined water content and density, but the variations are typically within acceptable limits for engineering application purpose. Results from TDR tests on simulated field experiments are consistent with the sensitivity analyses.Key words: time domain reflectometry, TDR, calibration constants, water content, dry density, sensitivity.

scholarly journals Measurement of soil water content using the combined time-domain reflectometry – thermal conductivity probe

1987 ◽  
Vol 24 (1) ◽  
pp. 160-163 ◽  
Author(s):  
T. H. W. Baker ◽  
L. E. Goodrich
Keyword(s):  
Thermal Conductivity ◽  
Dielectric Constant ◽  
Water Content ◽  
Soil Water ◽  
Soil Water Content ◽  
Time Domain ◽  
Empirical Relation ◽  
Time Domain Reflectometry ◽  
Apparent Dielectric Constant ◽  
Water Contents

A two-pronged metal probe measures the thermal conductivity and apparent dielectric constant of soils in the laboratory and in the field. One prong acts as a transient line heat source probe in measuring thermal conductivity. The apparent dielectric constant of the soil is determined by the time-domain reflectometry (TDR) technique, using both prongs as a parallel transmission line. Volumetric water content is determined from the apparent dielectric constant, making use of an empirical relation valid for most soils. For volumetric water contents above about 8%, the apparent dielectric constant shows a strong dependence on water content and relatively small changes can be measured; sensitivity increases with water content. For volumetric water contents less than 8%, a soil-dependent empirical relation between water content and thermal conductivity has been developed that is most sensitive at lower water contents. The combined probe provides a means of monitoring the water content of soils over a wide range of values, in the field and in the laboratory. Key words: soil water content, time-domain reflectometry, thermal conductivity.

Impact of density on the hydraulic properties of peat and the time domain reflectometry (TDR) moisture calibration curve

2005 ◽  
Vol 42 (1) ◽  
pp. 279-286 ◽  
Author(s):  
Anushka Shibchurn ◽  
Paul J Van Geel ◽  
Paula L Kennedy
Keyword(s):  
Dielectric Constant ◽  
Soil Moisture ◽  
Water Content ◽  
Time Domain ◽  
Calibration Curve ◽  
Hydraulic Properties ◽  
Time Domain Reflectometry ◽  
Organic Soils ◽  
Dry Density ◽  
Water Contents

The hydraulic properties of a peat used in a commercial peat biofilter were evaluated to determine their relationship with density and to establish a time domain reflectometry (TDR) calibration curve for water content as a function of the measured dielectric constant. The peat studied was a milled Sphagnum peat with a high organic content (99%). The dry densities evaluated in this study ranged from 90 to 180 kg/m3. The saturated hydraulic conductivity (Ks) decreased with an increase in dry density (ρdry) and was found to follow a log-linear relationship (Ks = 0.2462 exp(–0.0438ρdry), correlation coefficient R2 = 0.9789). As expected, the soil moisture curve was impacted by density, with a higher density resulting in higher water contents for a given suction. The data were fit to the van Genuchten relationship. A TDR calibration curve was generated at five different densities. A comparison of the curves indicates that the water content as a function of dielectric constant was not dependent on density because of the significantly larger dielectric constant (Ka) of water compared with those of peat solids and air-filled voids. The TDR calibration curve for the peat evaluated in this study (volumetric water content Θv = 0.2667 ln(Ka) – 0.1405, R2 = 0.9564) predicted higher water contents for a given dielectric constant compared with those from similar calibration curves for peat published in the literature. The data were compared with those from six other studies and indicated that the TDR calibration varied for different organic soils. The density-dependent hydraulic parameters and TDR calibration curve are important parameters needed to study the hydraulics of peat biofilters.Key words: peat, TDR, time domain reflectometry, density, hydraulics, soil moisture.

scholarly journals Time domain reflectrometry measurements using a movable obstacle for the determination of dielectric profiles

2008 ◽  
Vol 6 ◽  
pp. 1-4
Author(s):  
B. Will ◽  
M. Gerding ◽  
S. Schultz ◽  
B. Schiek
Keyword(s):  
Dielectric Constant ◽  
Water Content ◽  
Time Domain ◽  
Measurement Techniques ◽  
Measurement Data ◽  
Time Domain Reflectometry ◽  
Dielectric Measurement ◽  
Effective Dielectric Constant ◽  
Pulse Delay

Abstract. Microwave techniques for the measurement of the permittivity of soils including the water content of soils and other materials, especially TDR (time domain reflectometry), have become accepted as routine measurement techniques. This summary deals with an advanced use of the TDR principle for the determination of the water content of soil along a probe. The basis of the advanced TDR technique is a waveguide, which is inserted into the soil for obtaining measurements of the effective soil permittivity, from which the water content is estimated, and an obstacle, which can mechanically be moved along the probe and which acts as a reference reflection for the TDR system with an exactly known position. Based on the known mechanical position of the reference reflection, the measured electrical position can be used as a measure for the effective dielectric constant of the environment. Thus, it is possible to determine the effective dielectric constant with a spatial resolution given by the step size of the obstacle displacement. A conventional industrial TDR-system, operating in the baseband, is used for the signal generation and for the evaluation of the pulse delay time of the obstacle reflection. Thus, a cost effective method for the acquisition of the dielectric measurement data is available.

scholarly journals Influência da Densidade na Estimativa da Umidade Volumétrica em um Latossolo Vermelho-Amarelo (Influence of Density in Estimation of Volumetric Moisture an Oxisol)

2013 ◽  
Vol 5 (5) ◽  
pp. 1056 ◽  
Author(s):  
Carlos Alexandre Barros de Almeida ◽  
Antonio Celso Dantas Antonino ◽  
Rejane Magalhaes de Mendonça Pimentel ◽  
Carlos Alberto Brayner de Oliveira Lira ◽  
José Romualdo de Sousa Lima
Keyword(s):  
Dielectric Constant ◽  
Soil Moisture ◽  
Bulk Density ◽  
Time Domain ◽  
Time Domain Reflectometry ◽  
Calibration Equation ◽  
Moisture Sensor ◽  
Soil Moisture Sensor ◽  
Volumetric Soil Moisture ◽  
Use Of Time

A estimativa da umidade volumétrica do solo pode ser realizada por vários métodos, entre eles destaca-se o uso da Reflectometria no Domínio do Tempo (TDR). Este tem como uso padrão, uma equação que relaciona a constante dielétrica do meio com a umidade sugerida pelo manual do fabricante. Este estudo objetivou avaliar a medição a umidade volumétrica do solo pelo sensor CS616. Na sua realização foi feita a calibração deste sensor em laboratório, para quatro camadas em um Latossolo Vermelho-Amarelo que apresentam densidades diferentes. Foram utilizados cinco métodos diferentes, três consagrados pela literatura e outros dois sugeridos por esse estudo. Os resultados permitiram concluir que nesse solo há uma grande disparidade entre os resultados encontrados durante a calibração do sensor e que a densidade do solo é um parâmetro importante nas medições de umidade do solo.Palavras-chave: reflectometria no domínio do tempo, medição direta da água no solo, equação de calibração Influence of Density in Estimation of Volumetric Moisture an Oxisol ABSTRACTThe estimation of volumetric soil moisture can be accomplished by various methods, among them stands out the use of Time Domain Reflectometry (TDR). This standard is to use an equation that relates the dielectric constant of the medium with humidity suggested by the manufacturer's manual. This study aimed to evaluate the measured volumetric soil moisture sensor for the CS616. In its realization was made to calibrate this sensor in the laboratory for four layers in an Latossolo Vermelho-Amarelo which have different densities. Was used five different methods, the literature established three and two others suggested by this study. The results showed that this soil there is great disparity between the results obtained during calibration of the sensor and the bulk density is an important parameter in measurements of soil moisture.Keywords: time domain reflectometry, direct measurement of soil water, calibration equation

Unfrozen water content in saline soils: results using time-domain reflectometry

1985 ◽  
Vol 22 (1) ◽  
pp. 95-101 ◽  
Author(s):  
D. E. Patterson ◽  
M. W. Smith
Keyword(s):  
Water Content ◽  
Time Domain ◽  
Time Domain Reflectometry ◽  
Frozen Soil ◽  
Probe Design ◽  
Unfrozen Water ◽  
Use Of Time ◽  
Unfrozen Water Content ◽  
Pore Water Salinity ◽  
Apparent Dielectric Constant

The use of time-domain reflectometry (TDR) for determining the phase composition of saline permafrost from measurement of the apparent dielectric constant, Ka, is examined.Combined TDR–dilatometry experiments were performed to assess whether the TDR method could be used on frozen soil samples with high pore water salinity. In general, unfrozen water content determinations by TDR were within ±0.025 cm3∙cm−3 of those obtained by dilatometry, with no marked influence due to salinity. A novel probe design for use on saline core samples shows promise as a means for determining unfrozen water contents in the field.

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