Effect of the Oxidation Products of Base Oil on the Dielectric Constant of Lubricating Oil

Oxidation of base oils leads to the formation of insoluble deposits, such as varnish and sludge, which can cause operational problems including control valve malfunction.Consequently, reliable condition monitoring of lubricating oils is of critical importance.This study aims to clarify the influence of oxidation products on the relative dielectric constant measured using a multi-functional fluid sensor (MFS).Mineral oil-based base oil samples were subjected to stepwise oxidation using a Rotating Pressure Vessel Oxidation Test (RPVOT).Oxidation products were selectively extracted from the oxidized oils by solid-phase extraction and subsequently added stepwise to fresh mineral oil-based base oil to prepare model samples with controlled oxidation product contents.The prepared oils were evaluated using the MFS and a commercially available fluid property sensor (FPS).The results showed that the relative dielectric constant of the mineral oil-based base oil increased with oxidation progression, and a clear correlation was observed between the relative dielectric constant and the total acid number.Furthermore, the addition of extracted oxidation products was identified as a contributing factor to the increase in the relative dielectric constant.Comparison with the FPS demonstrated that the MFS could detect a correlation between the relative dielectric constant and concentration of oxidation products with higher sensitivity, enabling detection from the early stages of oxidation.