Technical Note:
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 <2.5 cm in length generated trough-haped TDR waveforms with rounded corners at the points of the pulse reflection from the probe ends. The sharpness of the pulse reflection on the waveforms increased with both the increasing probe length and soil-water content. The transition time for the propagation of TDR pulse at the probe entrance increased as the soil dried up. The increased transition time caused a rightward movement of the first peak of the waveform at the probe entrance. Because of such peak movement, TDR-support software algorithm determined travel path of TDR pulse through the probe that was smaller than the actual travel path. TDR-measured pulse travel time t_TDR varied erratically with the predicted pulse travel time t_g (from volumetric soil-water content) for the probes <2.5 cm in length. But, for all probes ³2.5 cm in length, t_TDR varied linearly with t_g and followed the 1:1 line. TDR could not measure t_TDR <300 ps accurately. A minimum probe length L_min and the lowest allowable soil-water content q_min that the probe can accurately measure govern this lowest pulse travel time t_min. The mean absolute deviation between t_TDR and t_g 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 s_TDR that compared well with the electrical conductivity measured by a conductivity meter s_m. 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