Abstract

The article examines the fringe effects of a two-rod capacitor inserted in a medium like a soil, due to its finite height h and presence of leads to measurement unit. The effects modify the coefficient g of conversion from probe capacitance to medium permittivity. The change of g from its expression $g_{th}$ without effects is determined in function of probe and lead dimensions through numerical simulations and laboratory experiments. Simulations give the rise of $g/g_{th}$ with the ratio h/D, with D the spacing of probe electrode, from about 1.09 at $h/D=4$ to $g/g_{th}=1.6$ at $h/D=0.5.$ Other ratio $\Phi /D,$ with $\Phi$ electrode diameter, presents a smaller influence. Experiments using different probes immersed in a liquid at different heights examine influences of leads and protective tubes around them. Contribution of leads, equivalent to a capacitor in parallel, is made negligible by keeping the ratio ${\alpha }_l={\Phi }_l/\Phi ,$ where ${\Phi }_l$ is lead diameter, at about few percent. Choice of metallic tubes for practical reasons is not detrimental provided they are not electrically connected. Due to operating at high frequency, tubes with same voltage as the probe, in spite of acting like guards, introduce strong bias.

摘要

本文研究了插入土壤等介质中的双棒电容器的边缘效应，这是由于其有限的高度h和存在测量单元的引线。这些效应修改了从探针电容到介质介电常数的转换系数g。g从其表达式的变化$G_{吨H}$通过数值模拟和实验室实验，根据探头和引线尺寸的功能确定无影响。模拟使$G/G_{吨H}$与比率h / D，与D探针电极的间距，从大约 1.09 在$H/D=4$到$G/G_{吨H}=1.6$在$H/D=0.5.$其他比例$\Phi /D,$和$\Phi$电极直径，影响较小。使用浸入不同高度液体中的不同探头进行的实验检查了引线和周围保护管的影响。引线的贡献，相当于并联一个电容器，通过保持比率可以忽略不计${\alpha }_l={\Phi }_l/\Phi ,$在哪里${\Phi }_l$是引线直径，约为百分之几。出于实际原因选择金属管并不有害，只要它们没有电连接。由于工作在高频下，与探头电压相同的管子，尽管起到保护作用，但会引入强偏置。