Relating low-frequency dielectric properties and refractive index of mineral oils

This study demonstrates how closely the refractive index of mineral oil measured at optical frequencies is related to the relative permittivity measured at audio frequencies. This indicates that the low-frequency relative permittivity is due to the electronic polarizability of the molecules, especially for the naphthenic/paraffinic hydrocarbon oils of low aromatic (<5%) content. Compared with relative permittivity and tan δ as a monitor of the condition of the oil, refractive index is easily, reliably, and inexpensively measured, even in the field. However, by itself it is found that the refractive index is not as sensitive as relative permittivity and tan δ for detecting changes due to aging. Accurate density measurements also give an indication of the condition of the oil