Time Domain Reflectometry Instruments Research Articles

Using 5TE Sensors for Monitoring Moisture Conditions in Green Parks.

The ground surface and subsurface of green parks in arid and desert areas may be subjected to desiccation as a result of weather and hot temperatures. It is not wise to wait until plants are turning pale and yellow before watering is resumed. Given the scarcity of water in typical desert zones, we recommend full control of irrigation water. This study presents a method of recycling irrigation water using 5TE sensors, employing time-domain reflectometry (TDR) technology. A trial test section was constructed along the coast of the eastern province of Saudi Arabia. Water recycling involves using clay-sand liners placed below the top agricultural soils to intercept excess water and direct it towards a collection tank, and then it is pumped out to a major water supply tank. The main properties of soils and clay-sand liners normally taken into account include moisture content, density, and hydraulic conductivity. An assessment of geotechnical properties of clay-sand mixtures containing 20% clay content was conducted. The profiles of moisture and temperature changes were monitored using 5TE sensors and data loggers. The 5TE sensors provided continuous measurements at varying temperatures and watering cycles. Twenty-nine watering cycles were conducted over a six-month period. An additional section was considered with a liner consisting of the same clay but enhanced with bentonite as one-third of the clay content. The volumetric water content was found to vary from 0.150 to 0.565 following changing weather and direct watering cycles. The results indicated that the use of a TDR instrumentation is a cost-effective and time-saving technique to construct a system for saving irrigation water.

TDR-based Multiple Leak Detection System using an S-parameter Transmission Line Model for Long-Distance Pipelines

Leaks in water distribution systems should be detected to avoid economic, environmental, and social problems. Existing Bayesian Inference based time-domainreflectometry (TDR) methods for leak detection have a limitation for real applications due to the lengthy time in building sample data. As the pipeline distance becomes longer and multiple leaks must be considered in long distance pipelines, the computational time for building training data gets larger. This paper proposes a scattering-parameter-based forward model to relieve computational burden of the existing TDR methods. It was shown that the proposed model outperformed the existing RLGC-based forward model in terms of computational time. The proposed model that is combined with Bayesian inference and TDR signal modeling is validated with an experimental pipeline, leak detectors, transmission line, and TDR instrument for leak detection. In summary, the proposed method is promising for leak detection in long pipelines as well as multiple leaks.

Recent advances in the TDR-based leak detection system for pipeline inspection

Abstract In this paper, the most recent advances in the time-domain reflectometry (TDR)-based system for leak-localization in underground pipes are described in detail. More specifically, a new design of sensing element and the use of a new connection modality are proposed. Thanks to these new features, the practical implementation of the system becomes much quicker and its use more effective. Additionally, the present work also describes all the practical aspects and technical details (from installation to functional tests), related to the practical implementation of the system. Finally, to assess the possibility of further increasing the cost-effectiveness of the TDR-based leak localization system, experimental tests were carried out by comparatively using two TDR instruments, differing in specifications and costs, to identify the position of a leak.

New improvement of the combined optical fiber transducer for landslide monitoring

Abstract. Landslide monitoring is important in predicting the behavior of landslides, thereby ensuring environmental, life, and property safety. On the basis of our previous studies, we conducted the double shear test by using a third-generation optical fiber transducer that uses expandable polystyrene (EPS) as base material. However, the third-generation transducer has poor performance when cohesive force is present between the grout and capillary stainless steel pipe of the transducer. Thus, the fourth-generation optical fiber transducer was invented. Similar to the third-generation transducer, the fourth-generation transducer also used EPS as its base material. Single shear test was conducted on the fourth-generation transducer after being grouted with cement mortar (1 : 1 mix ratio). The micro-bend loss mechanism of the optical fiber was considered, and the optical time domain reflectometry instrument was used. The fact that the loss sequence of optical fibers subjected to loading is different at various locations is found. The relationship of the loading-point displacement vs. optical fiber sliding distance and optical loss were measured. Results show that the maximum initial measurement precision of the newly proposed device is 1 mm, the corresponding sliding distance is 21 mm, and the dynamic range is 0–20 mm. The fourth-generation transducer can measure the movement direction of loadings, thus making this transducer applicable for landslide monitoring.

A Reevaluation of Time Domain Reflectometry Propagation Time Determination in Soils

Time domain reflectometry (TDR) is an established method for the determination of apparent dielectric permittivity and water content in soils. Using current waveform interpretation procedures, signal attenuation and variation in dielectric media properties along the transmission line can significantly increase sampling error in estimating the time, t2, at which the pulse arrives at the end of the probe. Additionally, manual adjustment of waveform analysis parameters is frequently required in current software to accommodate changes in media properties when processing large time series of TDR measurements. Our objectives were to reevaluate conventional propagation time analysis and difficulties with these methods, introduce the AWIGF (adaptive waveform interpretation with Gaussian filtering) algorithm that circumvents these problems, and compare interpretation methods using waveforms obtained with different TDR instruments and under widely varying media properties. The AWIGF algorithm filters signal noise using Gaussian kernels with an adaptively estimated standard deviation based on the maximum gradient of the reflection at the termination of the probe. Two fitted parameters are required to scale the smoothing level for a given step pulse generator. Additionally, the maximum second derivative is used to evaluate t2. The AWIGF‐determined t2 was compared with TACQ, a standard waveform interpretation algorithm. The strategies of AWIGF permitted the determination of t2 without parameter adjustment when the loss characteristics of the medium changed, such as with an increase in soil water content and bulk electrical conductivity. Using the new method, the sampling error of t2 was <0.06 ns across a wide range of medium properties and less than or equal to that obtained with TACQ. In strongly attenuated waveforms, the water content sampling error determined with AWIGF was 0.005 m3 m−3 compared with 0.038 m3 m−3 obtained using TACQ.

Comparison of Resistivity and Time Domain Reflectometry Sensors for Assessing Moisture Content in Bioreactor Landfills

Abstract Instruments that can monitor the in situ moisture content of landfilled waste would be of great benefit to landfill operators, especially those operating their facilities as bioreactor landills (wet landfills). Two instrumentation technologies for measuring in situ moisture content (resistivity sensors and time domain reflectometry (TDR)) in landfills were examined and compared. Resistivity and TDR instruments were installed in close proximity at multiple locations in a leachate recirculation well field at a bioreactor landfill in Florida, and resulting moisture content measurements were assessed. The resistivity instruments were less expensive, easier to install, and found more reliable over time than TDR sensors. Both technologies predicted transient moisture changes in the landfill. The magnitudes of the moisture content values obtained using the two technologies were much higher compared to values estimated using mass balance, suggesting that accurate calibration to field conditions is difficult.

TDR Moisture Estimation for Granular Materials: An Application in Agro-Food Industrial Monitoring

Time-domain reflectometry (TDR) is a well-established technique for continuous monitoring of the moisture levels and dielectric properties of materials; typically, it is widely used in soil science applications. Despite the high flexibility, elevated accuracy, and the low cost of TDR instrumentation, the state of the art is actually rather lacking in specific applications for the quality monitoring of agro-industrial processes and agro-food products. Therefore, the possibility of real-time control of moisture level plays a crucial role in terms of optimization, quality preservation, and energy saving in several industrial applications. In this regard, the development of suitable models relating the dielectric properties of granular materials to their moisture content is a still an open issue. To fill this gap, in this paper, the authors study the feasibility of adopting the TDR technique for such a purpose. The proposed goal is successfully reached through a comparative analysis among different calibration procedures and through experimental measurements on various granular materials. Finally, the adopted methodology is assessed through a rigorous metrological characterization, thus allowing the evaluation of the associated measurement uncertainty and repeatability.

A TDR Method for Real-Time Monitoring of Liquids

Time-domain-reflectometry (TDR) measurements, which were originally used to locate and diagnose faults in transmission lines, have been widely applied in geology and soil science for accurate and flexible measurements of soil moisture and water content. Furthermore, the most attractive advantages of TDR rely on the possible determination of the spatial location and nature of various objects, both in real time and with a nondestructive approach. This makes the TDR technique an appealing candidate for a variety of environmental and industrial applications. Although the TDR instruments are commonly used to date, particularly for the aforementioned purposes, the state of the art is rather lacking in liquid-monitoring applications. This paper describes how the suitable combination of TDR detecting functionalities can lead to a simultaneous monitoring of quantitative and qualitative properties of liquid samples. In fact, the proposed TDR method allows, in one shot, the measurement of liquid levels, the determination of multiple interfaces in layered media, and the evaluation of dielectric properties such as dielectric permittivity or electrical conductivity. Some applications to real cases are proposed, which are referred to petrol-chemical mixtures or water-based liquids, thus validating the approach on a wide range of materials.

Simultaneous measurement of dielectric properties and levels of liquids using a TDR method

Several monitoring methods, in the recent years, involved microwave/radio frequency (RF) reflectometry techniques, particularly in remote sensing, environmental and industrial application fields. The main advantages using reflectometry are due to its relevant performance characteristics, in terms of high flexibility, high sensitivity and large application conditions. Furthermore, the time domain reflectometry (TDR) presents some attractive advantages, related to the possible determination of the spatial location and nature of various objects, in real-time and with a non-destructive approach. This makes the TDR technique an appealing candidate for a variety of environmental and industrial applications. Although TDR instruments are commonly used today, particularly for the above mentioned purposes, the state of the art is rather lacking in liquid monitoring applications. In this paper we demonstrate that a suitable combination of TDR detecting functionalities, in terms of spatial localization, dielectric properties estimation and coaxial probe performance characteristics, leads to an enhanced quantitative and qualitative monitoring method. Really, in one shot, the analysis of TDR data allows the measurement of liquid levels, the determination of multiple interfaces in layered media, as well as the evaluation of dielectric properties, such as dielectric permittivity or electrical conductivity. Moreover, some applications to real cases are proposed, referred to petrol–chemical mixtures or water-based liquids, thus validating the approach on a wide range of materials.

CALIBRATION OF A TDR INSTRUMENT FOR SIMULTANEOUS MEASUREMENTS OF SOIL WATER AND SOIL ELECTRICAL CONDUCTIVITY

Time domain reflectometry (TDR) can be used to simultaneously measure in-situ water content and electrical conductivity (ECTDR). The method used to determine soil electrical conductivity from TDR waveforms requires measurement of the reflection voltages from a special part of the waveform. However, not all TDR instruments have this ability. The purpose of our research was to extend the ability of the MP917 Moisture Point instrument (ESI Environmental Sensors, Inc.) such that bulk soil electrical conductivity could be obtained. Our method involved use of resistors of known resistance to establish a relationship between arbitrary voltage and the reflection coefficient. As we did not have a standard of comparison (another TDR system that could measure both soil moisture content and EC), we used two probes of different lengths (150 and 220 mm). If our method worked, then the two probes should calculate the same EC values. Using our method, we achieved the following: determination of geometric constants for solution EC values between 0.7 and 25.7 dS m-1, ability to determine reasonably accurate KCl solution EC values for each of the two probes, and ability to determine sand ECTDR that had a 1:1 agreement between the 150 and 220 mm probes for moisture contents less than 0.17 m3 m-3 for non-saline and saline conditions, but not for saline conditions near saturation (0.28 to 0.35 m3 m-3). For such conditions, the long probe produced ECTDR values that were 1.4 times that of the short probe. Although the ECTDR values obtained from the MP917 could not be confirmed with an independent measure, there appears to be sufficient response to a range of saline conditions that the relative values of ECTDR provide suitable accuracy such that the usefulness of the MP917 can be extended to relative measurements of EC.

A time domain transmission sensor with TDR performance characteristics

Time domain reflectometry (TDR) has become a standard method for determining water content in soils. However, the cost of instrumentation and the level of user-ability required often limit its application, especially for agricultural and urban soil water management. The objective of this paper is to report on a new, lower cost time domain transmission (TDT) sensor and to demonstrate that it has the performance characteristics of commonly used TDRs. We approximate the sampling volume of the TDT sensor and compare it to that of TDR. We demonstrate that the sensor accurately determines permittivity from travel time measurements by comparing the TDT sensor with two TDR instruments, and a network analyzer providing measurements of frequency-dependent permittivity. Both TDRs and the TDT operated within ±3 permittivity units of each other across the permittivity range of 9–80. The reported rise time of the TDT sensor is 180 ps, which suggests a frequency bandwidth and upper frequency limit similar to TDR. We determined the maximum passable frequencies of all three instruments in non-relaxing media and demonstrate that they are in fact quite similar with the average maximum passable frequencies of the two TDR systems being 1.64 and 1.45 GHz, and the average maximum passable frequency of the TDT being 1.23 GHz. In addition to having accuracy and frequency characteristics similar to TDR, the TDT sensor offers the advantage of having the pulse generating and sampling electronics mounted in the head of the probe which reduces attenuation and overcomes the constraint that prevents TDR being used with long coaxial cables and multiplexers connecting the instrument to TDR probes.

The Early Development of TDR for Soil Measurements

The early development of time-domain reflectometry (TDR) for measuring water content in soils began somewhat by chance and continued almost in spite of the lack of support for the work from the sponsoring organizations. Following their first meeting, mutually supportive relationships quickly developed among the authors to sustain the research and development (R&D) program through the early difficult stages. The consistency across varied soils in the early findings buoyed up the authors' enthusiasm, but there was considerable reluctance by others to accept that one relative permittivity vs. water content relationship would apply as widely as claimed. The development of the first TDR instrument specifically for soil measurement highlighted a number of difficulties that may occur during the translation of scientific concepts and results to the instrumentation stage. An important method of dissemination of TDR results was the personal demonstration of the technique at conferences and workshops. These direct personal contacts often assisted others to pursue diverse applications, such as multiplexing for better spatial coverage, estimating liquid water in frozen soil, electrical conductivity, and solute movement. By 1987, TDR in soil had come of age with the presentation of five TDR papers from four nations at the Utah State Centennial Symposium. These experiences are offered as inspiration for others working on development research or perhaps as a warning of possible difficulties. In spite of which, this proved to be one of the most exciting and rewarding projects for the authors.

The Early Development of TDR for Soil Measurements

The early development of time‐domain reflectometry (TDR) for measuring water content in soils began somewhat by chance and continued almost in spite of the lack of support for the work from the sponsoring organizations. Following their first meeting, mutually supportive relationships quickly developed among the authors to sustain the research and development (R&D) program through the early difficult stages. The consistency across varied soils in the early findings buoyed up the authors' enthusiasm, but there was considerable reluctance by others to accept that one relative permittivity vs. water content relationship would apply as widely as claimed. The development of the first TDR instrument specifically for soil measurement highlighted a number of difficulties that may occur during the translation of scientific concepts and results to the instrumentation stage. An important method of dissemination of TDR results was the personal demonstration of the technique at conferences and workshops. These direct personal contacts often assisted others to pursue diverse applications, such as multiplexing for better spatial coverage, estimating liquid water in frozen soil, electrical conductivity, and solute movement. By 1987, TDR in soil had come of age with the presentation of five TDR papers from four nations at the Utah State Centennial Symposium. These experiences are offered as inspiration for others working on development research or perhaps as a warning of possible difficulties. In spite of which, this proved to be one of the most exciting and rewarding projects for the authors.

Time domain reflectometry liquid level sensors

The technique of time domain reflectometry (TDR) has been used by electrical engineers for testing the characteristics of transmission lines and diagnosing faults. Although TDR instruments for testing cable are commonly used today, few instrumentation engineers are aware that the technique can measure liquid levels and fluid interfaces. In this article, I review the theory and principles of time domain reflectometry and present the application of this technology as a liquid level sensor.

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 soil metric potential sensor based on time domain reflectometry

We developed and tested a new sensor based on time domain reflectometry (TDR) to measure soil matric potential (h). The TDR‐matric (TM) sensor is constructed of porous disks having different known maximum pore sizes and stacked within a coaxial cage. The constant relationship between water content (θ) and h of the TM porous matrix is initially calibrated and subsequently used to infer matric potential of the surrounding soil, similar to existing porous heat dissipation and electrical resistance sensors. The θ of the sensor's porous matrix in hydraulic equilibrium with the surrounding soil is measured by TDR travel time analysis. Prototype sensors were constructed using porous ceramic and plastic disks having maximum pore diameters between 120 μm(2.5 kPa) and about 0.6 μm (0.5 MPa). Calibration tests to evaluate sensor θ‐h relationships were completed in a pressure chamber apparatus using four soils. These results and those from sensors installed in soil lysimeters in the presence of growing plants showed consistent θ‐h relationships and synchronized responses of soils and TM sensors to changing water status. Pairing standard TDR probes with the new TM sensors facilitates in situ determination of soil θ(h) relationships, using conventional TDR instrumentation. The sensor design accommodates construction of media‐ or application‐specific sensors using combinations of disks having different pore sizes. There is a trade‐off between the TM sensor's matric potential range and its sensitivity to changes in the surrounding soil. Additionally, a mismatch between the pore size distributions of the TM sensor and the soil (mostly relevant to coarse‐textured soils) can lead to hydraulic decoupling of these and other porous sensors.

Time domain reflectometry: a seminal technique for measuring mass and energy in soil

Soil water exerts a strong influence on the transfer and storage of solutes, heat, air, and even water itself, within the soil profile. Soil water also dominates the mass and energy balance of the soil–atmosphere interface. Over the last decade or so, the development and continuing refinement of the time-domain reflectometry (TDR) technique for in situ, nondestructive measurement of water, ionic solutes and air has revolutionized the study and management of the transfer and storage of mass and energy within the soil profile. TDR-measured water content has been applied successfully to water balance studies ranging from the km scales of small watersheds to the mm scale of the root–soil interface. TDR-measured ionic solute status, which applies to the same sample volume as the water content measurement, has been used successfully on soil column, field plot and whole field scales for in situ determination of solute transport parameters, such as pore water velocity and dispersivity. TDR-measurement of air-filled porosity in space and time has given new insights into the mechanisms controlling aeration and gaseous exchange in the crop root zone. The combined water content – solute mass measurement capability of TDR has made this technique a very powerful tool for characterizing solute leaching characteristics, as well as for evaluating solute transport theories and solute transport models. The portability of TDR instrumentation coupled with the simplicity and flexibility of TDR soil probes has allowed the separation of water and solute content measurement error from soil variability, resulting in the capability for determining the mechanisms behind the spatial and temporal variability in field-based soil water content distributions and solute leaching patterns. The usefulness and power of the TDR technique for characterizing mass and energy in soil is increasing rapidly through continuing improvements in operating range, probe design, multiplexing and automated data collection.

A two-probe method for measuring water content of thin forest floor litter layers using time domain reflectometry

Few methods exist that allow non-destructive in situ measurement of the water content of forest floor litter layers (Oa, Oe, and Oi horizons). Continuous non-destructive measurement is needed in studies of ecosystem processes because of the relationship between physical structure of the litter and the biological and chemical processes that take place therein. We developed a method using time domain reflectometry (TDR) to monitor water content in a coniferous forest floor litter layer. Litter and mineral soil horizons were reconstructed in test beds in which TDR probes were placed and measurements taken using a range of litter and mineral soil water contents. Two probes are necessary when litter thickness is less than the spatial sensitivity (6 to 8 cm) of the TDR probes; one probe placed in the mineral soil and another one at the interface of the litter and mineral soil. Using this arrangement of TDR probes and simple mathematical relationships, the volumetric water content of forest litter can be estimated continuously. When the results of the two-probe method are compared to volumetric water content of forest litter obtained by gravimetric means there is a strong positive linear relationship between the two measured values of litter water content ( r 2 = 0.93). The two-probe method, however, underestimates litter water at low water contents and overestimates it at high water contents. This error has at least three components: (1) TDR instrument error, (2) errors in estimating volumetric water content from gravimetric data, and (3) using a TDR calibration curve not specific for high organic matter litter layer material. Calibrating the instrument for this specific condition should be improve the overall estimate of the litter layer water content.

Point specific measurement and monitoring of soil water content with an emphasis on TDR

During the past decade, a number of developments have improved the possibilities for real-time, in situ monitoring of soil water content, providing potential for much improved testing and validation of soil water models Brief reviews of the principles and procedures are presented for the gravimetric and gamma ray attenuation methods. A summary of the developments of time-domain reflectometry (TDR) demonstrates its high capability for continuous monitoring of soil water content. Portable dielectric probes (PDP) and capacitance instruments also use the high dielectric constant of water as an indicator of soil water content. Commercially available TDR instruments offer a choice of options that allow this method to be fitted to a variety of specific measurement requirements. The ease of use and portability of TDR instruments and the flexibility of design for the TDR probes has made possible the rapid measurement of surface and near surface water contents on a field scale. Multiplexers and data loggers now allow the efficient monitoring of water content to record both spatial and temporal patterns not previously detectable. Fine resolution and analyses within the electric field of a TDR probe has allowed us to measure and record the diurnal patterns of water uptake from, and its release to, relatively dry sod adjacent to growing corn roots Water distribution calculations show that the water measured as moving in and out of roots daily is sufficient to hydrate and stabilize the sheaths of soil that surround the root growing in dry soil. Key words: Soil water content, TDR, root uptake, root exudation, field measurement, water balance

Errors in Converting Time Domain Reflectometry Measurements of Propagation Velocity to Estimates of Soil Water Content

AbstractThe determination of soil water content by time domain reflectometry (TDR) involves the measurement of the propagation velocity of a radio frequency (RF) pulse in soil and the conversion of this measurement to an estimate of soil water content. The objective of this study was to quantify errors in this conversion. A comprehensive error analysis can only be carried out if a well‐defined measurement procedure is followed. We define and recommend such a procedure and identify the physical origin and the magnitude of each of the error sources in the procedure. Total measurement errors are calculated as the root mean square sum of each of the individual errors. We express propagation velocity measurements in terms of time intervals, that is, the travel time of an RF pulse in soil (T) relative to that in air (Tair). Our measurement and error calculation procedure is based on an analysis of our own and published data for nonclay soils, which reveals that in all cases, there is a linear relation between T/Tair and volumetric water content (θv) with a mutually independent slope and intercept. A theoretical explanation for this observation is presented. We show that the dominant measurement error source is the transition time of the TDR reflection, which in turn is a function of θv. Absolute measurement errors range from 0.015 to 0.028 m3 m‐3 if the T/Tair vs. θv intercept is known and increase to 0.023 to 0.034 m3 m‐3 if a nominal value (1.55) is used. We recommend that a T/Tair vs. θv slope value of 0.1193 (5% less than theory) be used for measurements in nonclay agricultural soils. This leads to little loss in accuracy. Measurement resolution is primarily a function of the time base error of the TDR instrument and can be as good as 0.0018 m3 m‐3.