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.
- Low precipitation number → oil is highly refined and contains little insoluble material.
- High precipitation number → oil contains more heavy, insoluble compounds (less refined or contaminated oil).
The basic principle of the method uses a sample of lubricating oil (typically 10 mL) which is mixed with hexane. The mixture causes insoluble components in the oil to precipitate. The mixture is centrifuged so the precipitate settles at the bottom of the tube. The volume of sediment (mL) is read in the centrifuge tube. This volume is reported as the precipitation number.
ASTM D91 is mainly used to evaluate refining quality of lubricating oils. It detect asphaltene-like or insoluble contaminants. Other applicatuins are quality control of of steam cylinder oils and heavy lubricants or identification of oil degradation or contamination
In fully refined petroleum oils, the precipitation number should be close to zero, because they normally contain no insoluble material.