Influence of variable plate separation on fringing electric fields in parallel-plate capacitors

Measurement of bulk physical properties in dielectric materials is typically performed using a parallel-plate capacitor configuration, where a dielectric material is placed between two conducting plates and the complex permittivity is found by measuring capacitance and conductance between the electrodes. Fringing electric fields exist between any two parallel conducting plates of finite length and the additional capacitance these fields add is not easily accounted for. It is important to account for edge effects in many capacitive sensing applications in order to ensure accurate results. Unfortunately, these edge effects are not easily quantifiable and are subject to change with varying sensor geometries. A quantitative relationship between fringing field strength and sensor geometry will improve the accuracy of many capacitive sensing applications. This work presents finite element simulations showing the influence of variable plate separation on fringing field effects in a system designed to measure the dielectric properties of a fluid. A quantitative relationship is established that relates the sensor response to the plate distance and fluid position. This relationship can be applied to experimental data in order to compensate for any edge effects present in the system.