Interdigitated Sensors: The Next Generation “Sensing Permittivity and Conductivity of Oils - Unaffected by Temperature”

Interdigitated dielectrometry sensors (IDS) are used as capacitive sensors to measure the permittivity, ε
_r
, and DC conductivity, σ
_DC
, (e.g., dissipation factor tan δ) of oils. Both variables can yield the oil's health. However, parasitic capacitances can be a source for a temperature dependence, impreciseness and inaccuracy. This paper shows a design of an IDS that uses deflector electrodes to reduce parasitic capacitance effectively. The design parameter for a next generation IDS that is highly accurate, precise, and effectively insensitive to temperature was determined using finite-element modeling. The air capacitance between 30 °C and 90 °C was stable at C
_air
= 6.27 ± 0.02 pF. Measurements of standard transformer oil have shown an accuracy of 99% for ε
_r
and σ
_DC
. The conductivity detection threshold was as low as 6 pS/m ≈ tanδ = 0.001. Additionally, C
_air
correlated to 93% with the air humidity. This IDS design can be applied in various applications where accurate permittivity and conductivity monitoring of liquids in environments with temperature fluctuations is required.