Factors that Influence the Precision of Brookfield Viscometry of Automotive Lubricant Fluids

This paper summarizes and analyzes data that have led to a proposed generalized Brookfield procedure for measuring the low temperature, low shear rate viscosity of automotive lubricant fluids. The data were collected between 1974 and 1977 by ASTM Committee D-2, Research and Development Division VII, Brookfield Viscometry Task Force in cooperation with the Institute of Petroleum, the Groupement Francaise de Coordination, and the National Research Council of Canada. The sources of error in the generalized Brookfield procedure are examined in detail. A system based on the known viscosity-temperature function of Newtonian reference fluids is proposed for estimating error magnitudes from existing cooperative data.

Application of these estimated error magnitudes to engine and gear oil data produced repeatability and reproducibility estimates that are functions of viscometer rpm for non-Newtonian products. Viscometer rpm is shown to be a critical test parameter at low temperature. RPM sensitivity is largely due to the Brookfield viscometer's ability to sense the strength of the wax or wax-additive gels that exist below the cloud point. The physical and rheologic reasons for this sensitivity are outlined.

This analysis of cooperative data results in a significant advance in the understanding of the viscometry of automotive lubricant fluids at low temperature. It is published primarily as an information report. At this writing, the precision estimates and the generalized Brookfield method are being circulated for approval by the cooperating organizations.