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ASTM D91
ASTM – D91 is the Standard Test Method for Precipitation Number of Lubricating Oils is a laboratory method used to determine the amount of insoluble material present in certain lubricating oils, especially steam cylinder oils and black oils. So ASTM D91 determines the volume of insoluble precipitate formed when a lubricating oil is mixed with hexane, indicating the amount of heavy or poorly soluble components in the oil.
The method measures the quantity of naphtha-insoluble particles in a lubricating oil. These insolubles are often asphaltenes or heavy degradation products. The result is reported as the precipitation number.
