Interdigitated Sensors: A Design Principle for Accurately Measuring the Permittivity of Industrial Oils

An accurate measurement of the relative permittivity, ε
_r
, of industrial oils using interdigitated dielectrometry sensors (IDS) is challenging due to the parasitic capacitance caused by the IDS spatial structure. In this paper, the investigated IDS structure consists of a substrate, a ground plane, an insulating layer below the electrodes and a solder mask above the electrodes having a periodicity of λ. This structure was simulated to determine the influence on the expected values of capacitance C, sensitivity S, and deviation D (accuracy) for typical values of ε
_r
for industrial oils. It is shown that for particular structure ratios and values of ε
_r
, the parasitic capacitance can be compensated in order to provide an accurate measurement of ε
_r
. A standard rapid and cost efficient printed circuit board (PCB) technology is used to manufacture an IDS based on the simulations to verify the expected high accuracy in practice. Experiments conducted with this IDS structure on both new and degraded oils showed a high accuracy ≈99.5%, verifying the simulations. These results have the potential to reduce the number of IDS design iterations in order to carry out highly accurate and precise measurements. This may go beyond the in-situ monitoring of oil degradation, such as a number of related sensing applications employing interdigitated sensor platforms.