A simple formula for the dielectric relaxation time—temperature—pressure relationship of lubricating oils and its application to viscosity prediction

The dielectric constant and the loss factor for several lubricating oils were both measured within the frequency range of 100 Hz–1.5 MHz. Measurements were made at atmospheric pressure with varying temperature, and at fixed temperature with varying pressure. The temperature dependence of the dielectric relaxation time could be expressed by the Vogel—Fulcher—Tammann (VFT) equation. In order to express the dielectric relaxation time as a function of temperature and pressure, a pressure‐dependent term was introduced into the characteristic temperature of the VFT equation. The experimental relaxation time could then be represented by a simple formula. Moreover, high‐pressure viscosity was calculated from the dielectric relaxation data by introducing the pressure dependence of the bulk modulus. The predicted results showed fairly good agreement with the viscosity data.