HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text Free Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (16) Citing Articles via Google Scholar Google Scholar Articles by Castiglione, P. Articles by Shouse, P. J. Search for Related Content PubMed Articles by Castiglione, P. Articles by Shouse, P. J. Agricola Articles by Castiglione, P. Articles by Shouse, P. J. Related Collections Soil Physics

The Effect of Ohmic Cable Losses on Time-Domain Reflectometry Measurements of Electrical Conductivity

George E. Brown Jr. Salinity Laboratory USDA-ARS, 450 W. Big Springs Rd., Riverside, CA 92507

View larger version (14K): [in a new window]   Fig. 1. Time domain reflectometry waveforms recorded with a 20-cm probe. The reflection coefficient is defined as the ratio of the incident to the reflected voltage. Because of the ohmic dissipation, the signal undergoes attenuations when propagating through the conductive ( = 0.68 dS m-1) CaCl2 solution.

View larger version (9K): [in a new window]   Fig. 2. Schematic representation of the time domain reflectometry setup. The sample dielectric of length d is placed at the end of a coaxial cable of length L and characteristic impedance Z0, and is excited with a step voltage produced by the tunnel diode. From the reflected signal, recorded in the time domain by a sampling scope and processed with a personal computer, it is possible to obtain the dielectric properties of the sample (, ).

View larger version (18K): [in a new window]   Fig. 3. Dependence of the measured reflection coefficients () on the sample resistance (Rs) and length (L) of an RG 58A/U coaxial cable ( = 0.0175). A = 1 for L = 0 m, while for L = 10 m A = 1.419. Note that no reflection occurs for Rs = 50 .

View larger version (18K): [in a new window]   Fig. 4. Expected relative errors in Rs measurements when neglecting the cable losses. Q = Rs,a/Rs, with Rs,a given in Eq. [22]. The relative error becomes negligible as |Rs| approaches Z0 = 50 .

View larger version (37K): [in a new window]   Fig. 5. Waveforms recorded in electrolytic solutions with the 20-cm probe connected to (a) the 4-m long coaxial cable and to both (b) the 4- and the 16-m cables. In both cases the reflection coefficients reach a flat plateau after few reflections. The long-time reflection coefficients to be used in the Eq. [32] through [34] is measured at about 600 ns.

View larger version (15K): [in a new window]   Fig. 6. Sample conductance calculated with the lossless cable model (Eq. [32]). The hypothesis is reasonable when using the 4-m long cable, whereas it is unrealistic for the 16-m cable, as the relative data fail to align on a straight line.

View larger version (20K): [in a new window]   Fig. 7. Sample conductance calculated by scaling the reflection coefficients with respect to the waveform measured in (a) air, and to both (b) the waveform in air and with the probe short-circuited.

View larger version (15K): [in a new window]   Fig. 8. G1 is the sample conductance assuming cable and sample as resistors in series; the solution conductivity is independently measured with a standard meter. The limitations of the model are only evident in the low conductivity range.

View larger version (11K): [in a new window]   Fig. 9. Rcable predicted by the series resistance model (Eq. [23]) for the RG 58A/U cable. When the sample conductance G is not too small (>1 dS), Rcable appears to be constant and the data from the calibration appears to be aligned on a straight line.