ASTM D3829 Predicting the Borderline Pumping Temperature of Engine Oils
For automobile engines to function properly, they require constant lubrication with engine oil. Oil must start flowing on engine start-up, especially at low temperatures, and continue flowing during engine operation. The ability of an oil to start flowing at low temperatures is determined by its critical yield stress, which increases as temperature decreases. The ability of an oil to continue flowing is determined by its viscosity, which likewise increases as the temperature decreases. Under very cold conditions, the yield stress and/or the viscosity may increase to the point that oil will not continuously flow. The borderline pumping temperature is the lowest temperature at which an adequate flow of oil can be continuously supplied to the engine. It is defined as the maximum temperature (whichever is higher) of either the critical yield stress or the critical viscosity.
This test uses a specially designed viscosity measuring apparatus which contains a temperature controlled mini-viscometer and a calibrated rotor-stator assembly, as described in the method. The sample is placed in the viscometer test cell assembly, heated for two hours and cooled slowly to the test temperature for the duration of the test. The yield stress is determined by applying force to the rotor shaft until the rotor begins to turn. The apparent viscosity is determined by multiplying the rotor speed times the test cell calibration factor. Reported is the test temperature, yield stress and viscosity.