Time Domain Reflectometry Technology Research Articles

Electromagnetic Wave Propagation Model for Differentiation of Geotechnical Disturbances Along Buried Cables

Abstract The lumped-circuit, hybrid, finite-difference/FFT (fast Fourier transform) model described herein simulates effects of multiple discontinuities in a coaxial cable at variable distances on a time domain reflectometry (TDR) signature. This model is verified by comparison with measured reflections from perfectly defined, multiple deformities. Accurate simulation is an important component of TDR technology as it allows improved calculation of the amount of shearing at multiple locations along special cables grouted in a rock or soil mass. This paper is arranged in four sections. The introduction or background section presents the need for the model, nature of discontinuities, and the uses of long TDR cables. Design of the hybrid computer model is presented in the following section: Modeling of Coaxial Cable Response. Calculated and field-measured signatures are compared and methods of calibration are introduced in the section entitled: Calibration of Model and Comparison with Field Data. The paper closes with a summary and conclusions.

Comparison of calculated and measured volumetric water content at four field sites

Terrestrial field dissipation studies are currently required as part of new active ingredient submission packages in many countries. Results from field dissipation studies are used to determine the fate of pesticides under field conditions which includes evaluation of compound degradation, mobility and other routes of dissipation. Time domain reflectometry (TDR) instrumentation has been helpful for determining if water movement through the soil profile was correlated with or was sufficient to cause compound mobility. A simple method is presented for estimating soil water flux from the soil profile using TDR technology. Additionally the question has been raised regarding how useful simple calculations of volumetric water content (VWC) can be for this purpose as well. A simple method of calculating VWC and profile flux are presented and compared to measurements using TDR and TDR based flux calculations. Results from this comparison indicate that calculated VWC estimates compare favorably with those based on actual TDR measurements. Calculated patterns of flux at a 120 cm depth based on TDR and the calculated VWC method correlate well, although the flux quantities can vary. Occurrence of flux from both methods were very similar, but quantities of flux were greater with the calculated VWC method compared to flux quantities based on the TDR calculation method. Additionally, because the calculated VWC method is computationally simple, the technique should also be useful for estimation of groundwater recharge at a regional scale using tools such as GIS.

Liquid water content of wood tissue at temperatures below 0°C

Time domain reflectometry (TDR) offers an opportunity to measure the liquid water content of otherwise frozen plant material. We applied TDR technology to the examination of freezing in three types of wood represented by Robinia pseudoacacia L. (ring porous), Populus trichocarpa Torr. & A. Gray (diffuse porous), and Pinus contorta Dougl. ex Loud. and Larix occidentalis Nutt. (conifer wood). Gravimetric analysis revealed similar water contents of all wood types during the summer. In contrast, winter data showed that R. pseudoacacia wood exhibited a lower total (liquid and ice) water content (0.250 m3·m-3) than that of Populus trichocarpa (0.600 m3·m-3) or of the two conifer species' wood (0.510 m3·m-3). Additionally,R. pseudoacacia wood contained more air by volume during the winter than all other wood types (air-filled porosity 0.34 m3·m-3 compared with 0.12-0.22 for all other species). At all temperatures below 0°C, R. pseudoacacia wood contained less liquid water than the other wood types, as revealed by TDR measures. The TDR analysis further demonstrated that more than 25% of the water in wood of all species was liquid even at temperatures of -15°C. This liquid water is likely found within the cell wall and is potentially transportable at temperatures well below 0°C.

Liquid water content of wood tissue at temperatures below 0°C

Time domain reflectometry (TDR) offers an opportunity to measure the liquid water content of otherwise frozen plant material. We applied TDR technology to the examination of freezing in three types of wood represented by Robinia pseudoacacia L. (ring porous), Populus trichocarpa Torr. & A. Gray (diffuse porous), and Pinus contorta Dougl. ex Loud. and Larix occidentalis Nutt. (conifer wood). Gravimetric analysis revealed similar water contents of all wood types during the summer. In contrast, winter data showed that R. pseudoacacia wood exhibited a lower total (liquid and ice) water content (0.250 m3·m-3) than that of Populus trichocarpa (0.600 m3·m-3) or of the two conifer species' wood (0.510 m3·m-3). Additionally,R. pseudoacacia wood contained more air by volume during the winter than all other wood types (air-filled porosity 0.34 m3·m-3 compared with 0.12-0.22 for all other species). At all temperatures below 0°C, R. pseudoacacia wood contained less liquid water than the other wood types, as revealed b...

Practical considerations for using a TDR cable tester

Time domain reflectometry (TDR) technology can be used to measure soil water content, but due to an incomplete understanding of this technology, some scientists are still reluctant to adopt it. The objectives of this paper are to discuss the basic principles of TDR to measure soil water content, to present the equations to convert TDR screen readings to soil water content values, and to describe a custom built TDR system. Commercial cable testers operate under the assumption the dielectric constant of the medium is known. For soils applications the dielectric constant varies with soil water content and is unknown. The equations which govern the TDR technology presented in logical order allow the user to understand the relationships between wave velocity, dielectric constant, and soil water content. The custom-built TDR system described here uses a commercially available cable tester; a balun transformer; shielded, two-wire transmission cable; and wave guides constructed from stainless steel welding rod. Several of these systems have been successfully used for six years.