Pressure-Viscosity Coefficients of Liquid Lubricants

The Barus equation relating viscosity to pressure includes a pressure-viscosity coefficient used widely by investigators even though that coefficient is grossly inaccurate at elevated pressures. A new empirical equation was developed relating viscosity to pressure. The equation is mathematically simple and provides two coefficients that are satisfactory over a wide pressure range. These coefficients are temperature dependent and produce a pressure-viscosity relationship with a maximum error of only about 8 percent at a temperature of 37.8°C over a pressure range to 0.60 GPa for a number of fluids. One of the coefficients in this new equation directly reflects the chemical nature of the lubricants investigated. It distinguishes between paraffinic and naphthenic petroleum oils and relates to their degree of aromaticity.