DESIGN OF TRIPLE-WIRE TIME-DOMAIN REFLECTOMETRY PROBES IN PRACTICE AND THEORY

Time domain reflectometry (TDR) is accepted as a valuable tool for measuring soil water content and bulk soil electrical conductivity. The accuracy of TDR measurements depends on the quality and type of probes, as well as on the length of cable used. The objective of this study was to test the effects of different triple-wire TDR probe dimensions and cable lengths on the measurements. Additional measurements were done in order to test the performance of small triple-wire probes in soils with a wide range of water contents. Measurements in air and in water showed that the position of the first reflection from the connection between cable and probe is influenced by the dielectric medium. This problem was solved by using the zero of the cable tester as a time reference, and calibrating the TDR probes before measurements. The major effect of increasing cable lengths is that the rise time of the TDR voltage pulse increases, spreading each reflection across a larger time interval, which influences the accuracy of the wave form analysis and causes a possible underestimation of the apparent dielectric permittivity in dry soils. As a result, it is not possible to use short probes with long cable lengths. Smaller spacing of the wires results in steeper reflections from the end of the probe. Small triple-wire probes are convenient in laboratory applications. Large triple-wire probes are convenient for automated field applications in which long lengths of cable are required.