Technical Note:<br>Practical considerations on the use of down-sized time-domain reflectometry (TDR) probes

Abstract

Nine time-domain reflectometry (TDR) probes, 2 to 10 cm long, were evaluated by comparing their measurement accuracy of TDR-pulse travel time in a sand and sandy loam soil, and electrical conductivity in NaCl solutions. TDR probes tTDR varied erratically with the predicted pulse travel time tg (from volumetric soil-water content) for the probes tTDR varied linearly with tg and followed the 1:1 line. TDR could not measure tTDR Lmin and the lowest allowable soil-water content qmin that the probe can accurately measure govern this lowest pulse travel time tmin. The mean absolute deviation between tTDR and tg was 77 ps for the 2.3 cm long probe and 1.39 ps for all probes ≥2.5 cm in length. All probes ≥2.5 cm in length measured electrical conductivity of salt solutions sTDR that compared well with the electrical conductivity measured by a conductivity meter sm. The length of the probes did not exert any noticeable influence on the accuracy of electrical conductivity measurement. Keywords: TDR probe, pulse travel time, dielectric constant, electrical conductivity