Use Of Time Domain Reflectometry Research Articles

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.

Evaporation from bare soil in a temperate humid climate—measurement using micro-lysimeters and time domain reflectometry

In humid areas soil evaporation is an important factor in relation to soil tillage and timing of irrigation early in the growing season. Direct measurements of evaporation from a loamy sand were made using micro-lysimeters. The accuracy of daily measurement was about ± 0.5 mm H 2O day −1. The micro-lysimeter method was not valid in periods with high precipitation. The use of time domain reflectometry (TDR) for measuring soil water content was investigated using a manual interpretation of the trace. The precision of changes in soil water content calculated from daily measurements with TDR was about 1.3 mm H 2O, when using probes of 50 cm length. However, improved precision may be obtained by the use of an automatic interpretation of the trace. Estimates of daily evaporation from bare soil calculated from the water balance equation and measurements of soil water content with TDR were compared with measurements with micro-lysimeters. The TDR technique was suitable for estimating bare soil evaporation when the soil water content was integrated over a 0–50 cm soil profile and drainage had ceased at the lower depths of the profile. Evaporation during a 13 day drying period in spring, just after the soil had been fully rewetted, was about 26 mm. During a 23 day drying period later in the season the evaporation from the bare soil was about 30 mm. In both periods the accumulated evaporation was rather high and equivalent to about 65% and 50% of the accumulated potential evapotranspiration in the first and second drying period, respectively, even though the soil water content in the 0–50 cm profile was well below field capacity at the beginning of the second drying period.

Water content measurement in forest soils and decayed wood using time domain reflectometry

The use of time domain reflectometry to measure moisture content in forest soils and woody debris was evaluated. Calibrations were developed on undisturbed soil cores from four forest stands and on point samples from decayed logs. An algorithm for interpreting irregularly shaped traces generated by the reflectometer was also developed. Two different calibration equations were needed to estimate volumetric moisture content at the four sites, but commonly implicated soil characteristics (organic matter content, bulk density, and soil texture) could not fully account for the differences between calibrations. The calibrations differed from previously published calibrations for mineral and organic soils. Estimation of moisture content in decayed wood was possible with a single significant regression. The standard errors of estimate for volumetric water content were less than 0.02 m3 m−3 for the soil calibrations and just over 0.06 m3 m−3 for the decayed wood calibration. We found we could reliably interpret most traces from field samples using an automated algorithm, but had to use a modified algorithm for one of the sites. This study suggests a need to calibrate time domain reflectometry measurements for individual forest sites and advises caution when using systems that have preprogrammed calibration and trace analysis routines.

Use of Time-Domain Reflectometry (TDR) to measure the water-content of sandy soils

We investigated the potential sources of error when using time domain reflectometry (TDR) to measure the water content of sandy soils and evaluated the technique as a means of measuring evaporation from columns of soil and changes in soil water storage beneath crops. Inaccurate depth location of the transmission lines or the development of a hole at the tip of the transmission lines introduced an error about 10 times larger than the errors associated with hardware and software. Calibration in two sandy soils gave a curve of similar shape to that found by others except for values of dielectric constant < 6 when measured values of water content were less than those expected. Daily evaporation from soil columns measured by weighing and with TDR showed large differences between the two techniques (up to 32%) but compensating errors over time allowed cumulative evaporation to be estimated with TDR to within 6.6% of that determined by weighing over a 162 h period. Under field conditions, the agreement between TDR and neutron probe measures of changes in soil water storage in the upper 0.3 m was good and generally within 10% over both 14 day and longer periods.

Potential use of time domain reflectometry for measuring water content in rock : Hokett, S L; Chapman, J B; Russell, C E J Hydrol V138, N1/2, Sept 1992, P89–96

Potential use of time domain reflectometry for measuring water content in rock : Hokett, S L; Chapman, J B; Russell, C E J Hydrol V138, N1/2, Sept 1992, P89–96

The structure of water determined by microwave dielectric study on water mixtures with glucose, polysaccharides, and L-ascorbic acid

Dielectric relaxation measurements over an extremely wide frequency region from 1 MHz to 20 GHz were performed on water mixtures with glucose, polysaccharides, and L-xylo ascorbic acid by the use of time domain reflectometry. For mixtures of polysaccharides bigger than maltotriose, two relaxation peaks were definitely observed. The high frequency relaxation is the water relaxation and the low frequency one is due to orientation of polysaccharide molecules. In the case of glucose, only one relaxation peak could be observed. It is shown that a hexagonal cluster in the lattice of ice can be replaced easily by the glucose molecule, where the lattice is distorted slightly, but stabilized by several hydrogen bonds between the glucose molecule and the lattice. Although the cluster can be replaced by the L-ascorbic acid molecule too, the lattice cannot be kept stable. Its water mixture shows two relaxation peaks clearly. It is suggested that water has a structure of the distorted lattice of ice. Fluctuation of the lattice breaks the hydrogen bonds and the lattice is decomposed. Orientation of the water molecules released gives rise to the relaxation concerned.

Potential use of time domain reflectometry for measuring water content in rock

Quantifying water movement through bedrock materials requires a technique which can accurately measure water content at precise locations within the medium. Time Domain Reflectometry (TDR) is widely used to measure the water content of soil and has a measurement sensitivity which is tightly confined to the area immediately surrounding the probes. This study applies TDR to the measurement of water content in rocks. The technique was evaluated using sandstone and welded tuff in a series of laboratory experiments. A block of each rock type was instrumented with a series of evenly spaced TDR probes. Each block was first saturated with water and then allowed to air dry. The water content was determined gravimetrically using the TDR over a period of several weeks. The results indicate that the TDR technique can be used to determine the water content of rock with an accuracy comparable with that reported for the technique used in soils.

The time-domain reflectometry method for measuring soil water content and salinity

This paper discusses the physical principles and use of time-domain reflectometry as a new tool for studying water and solute transport in unsaturated soils. In-situ measurements of water content and bulk soil electrical conductivity are shown to give results that are comparable with those obtained by conventional non-destructive techniques. An equation is presented that relates the bulk soil electrical conductivity to the soil solution electrical conductivity. Also derived are constraints that water content and electrical conductivity place on the use of time domain reflectometry sensors.

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

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.

Utilisation de la réflectométrie en domaine temporel pour l'étude des transferts d'humidité en milieu poreux perméable

The use of time domain reflectometry allows instantaneous highly accurate location of humidities in a cell filled with a porous medium. The principles and process of the method will be briefly surveyed. Then, reference will be given as to its performances when small quantities of water seep through the porous medium. This technique is likely to be increasingly used in a laboratory for further study of the kinetics of unsaturated flow as well as in a natural medium.

The use of time domain reflectometry for the measurement of unfrozen water content in frozen soils

This brief paper reports upon experiments to determine volumetric unfrozen water content, using time domain reflectometry to measure the dielectric property. Laboratory work with unfrozen soils has shown that the dielectric property varies in a consistent way with water content, and that this relationship is largely unaffected by soil type, bulk density and soluble salts (Topp et al., 1979). The technique has now been applied to frozen soils and the dielectric property determined for a variety of soils over a range of freezing temperatures. Using results from ice/water mixtures, the dielectric values have been converted to unfrozen water contents. Comparison with values from dilatometer experiments indicates promise for this new technique.

Nanosecond Pulse Circuit Board Interconnections

The difficulties involved in using mating connectors to interconnect nanosecond pulse circuit boards are discussed. The effects of such connectors on pulse characteristics may be predicted by the techniques presented in this article. Experimental verification of predicted performance is presented in the form of photographs through the use of time domain reflectometry. The connector used to illustrate these techniques is a miniature printed circuit board type. An equivalent circuit for this connector is arrived at through an experimental procedure. Additional experimental justification of this circuit is presented. This equivalent circuit is used to derive an expression for the waveform of the pulse reflected from a connector contact pair. The risetime degradation of the pulse transmitted through the connector contact is evaluated by use of the equivalent circuit. The expression for the waveform coupled into a set of contacts by a pulse driving an adjacent pair is derived with the aid of the equivalent circuit. These expressions are presented and illustrated in the text. The mathematical derivations are presented in a set of appendices. Experimental verification of the analysis in the form of photographs of pulse reflection and pulse crosscoupling waveforms is offered. Methods of improving performance of the connector with regard to pulse risetime degradation, pulse reflection, and crosscoupling are demonstrated. Various arrangements of contact connectors and their relative merits are discussed.