Experimental evaluation of the measurement errors of soil water content due to interference of two adjacent TDR probes

摘要

The objective of this experimental study was to determine the measurement errors caused by the interference effects when two TDR probes operate nearby in various distance apart for different soil types and soil water contents. The results will help to guide the design of an experimental apparatus which aims to study the coupled heat and moisture transfer in soils at high temperatures of higher than 40 degrees C with the highest possible spatial resolution. The interference effect between two adjacent time domain reflectometry (TDR) probes has been evaluated and analyzed experimentally. It was observed that the distance between the probes, soil texture, and the water content of the testing soil is the key parameters which affect the interference and TDR probe capability of measurement. In this study, three types of soils were examined, namely FSJ#1 (silty clay), Matilda (loamy sand), and Ottawa sand C-190 (quartz sand). Three different volumetric water contents were evaluated for each soil. The interference effect was noticeable in all soil samples when the distances between the probes were 1 cm. For the samples with higher moisture content, the effect of interference on the TDRs' electromagnetic waveform signals is more prominent. Interference was found in Matilda samples with degrees of saturations of 0.5 and higher even though the distances between the probes were greater than 1 cm; nevertheless, the determination of volumetric water content was not affected by the interference, because the interference only caused the two TDRs' electromagnetic waveform signals to shift by the same length. No interference was observed in cases of FSJ#1 and Ottawa sand when the distances between the probes were greater than 1 cm. From the results, it was found that the sensitivity zone of our TDR equipment is a radius of about 10 mm surrounding the central needle of the TDR probe. In case of FSJ#1, when interference occurs, the measurement error of water content significantly increases from 9.3% to 15.5%, corresponding to the degree of saturation from 0.25 to 1.0.