Experimental and numerical studies on the sample area and skin effect of the three-rod time domain reflectometry probe

Abstract

The sample area and skin effect of the widely used three-rod time domain reflectometry (TDR) probe is not fully understood. The seepage analysis software was adopted to compute the electrostatic potential field surrounding the TDR probe and a quantitative assessment of the sample area and skin effect for probes that varied in design with respect to rod diameter (D) and rod spacing (L) was conducted. The effectiveness of the numerical method was demonstrated by experiments designed to determine the extent of the sample area for the three-rod probe with a rod diameter of 6 mm and a rod spacing of 30 mm. The main findings from the experimental and numerical studies are as follows. The electrical conductivity of the test medium has a negligible effect on the sample area of the three-rod probe when it is limited within 150 ms/m. The sample area (S_f = 90%) for the three-rod probe can be represented as an ellipse with a semi-major axis length equal to L + D/2. The magnitude of the sample area is predominantly determined by the rod spacing for the dimension ranges under consideration (D = 4–8 mm; L = 15–60 mm), and it can be predicted by a quadratic equation. The extent of the skin effect (S_f = 50%/S_f = 90%) for the three-rod probe is dependent on the ratio of the rod spacing to the rod diameter (L/D) and the quantitative skin effect can be also evaluated by a quadratic equation. On the basis of these findings, a design procedure is proposed to determine the optimal rod diameter (D) and rod spacing (L) for a three-rod TDR probe.

Keywords:

• three-rod TDR probe
• sample area
• skin effect
• numerical analysis
• design

Acknowledgements

The authors would like to thank the National High Technology Research and Development Program of China (Project No. 2012AA062601) for support. The authors also thank Shunyu Wang and Jinshu Huang from MOE Key Laboratory of Soft Soils and Geoenvironmental Engineering in Zhejiang University for their help manufacturing the experimental equipment.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National High-Tech Research and Development Program of China (863 Plan) [grant number 2012AA062601].