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The current ASTM Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature (D4684-89) was designed to cool an engine oil at a rate that will maximize its tendency to form a flow limiting structure. The cooling program development work with the Pumpability Reference Oils (PRO) demonstrated that temperature control and cooling rate could greatly effect the observed viscosity of the test oil. By utilizing this method, all oils with known field problems are identified by exhibition of a yield stress greater than 35 Pa, and/or excessive viscosity at the appropriate test temperature. During the evaluation of cooling rates, it was found that small differences in test temperature resulted in large differences in observed viscosity of temperature sensitive engine oils. This paper shows the importance of closely monitoring and controlling the cooling rate as well as the final test temperature. Inaccuracy of temperature measurement was the largest contributor to measurement error in determining pumping viscosity.
Apparent Viscosity, Pumping Viscosity
Senior Rheology Specialist, Ethyl Petroleum Additives, Inc., St. Louis, MO