Study of fringe effects of a two-rod capacitor embedded in a medium in order to deduce its permittivity
Résumé
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 Φ/D, with Φ 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 α l = Φ l /Φ, where Φ 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.
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