Fully refined petroleum oils normally contain no naphtha insoluble material. Semi refined or black oils frequently contain some naphtha insoluble material (sometimes referred to as asphaltenes ). This ASTM D91 test measures the amount of naphtha insoluble material in the oil. This quantity is reported as the precipitation number. The method covers the determination of the precipitation number of steam cylinder stocks and black oils, and can be used for other lubricating oils.
10 mL of the oil to be tested is needed and put into two centrifuge tubes. These tubes are than filled with heaxane to a 100ml contents. The tubes are place in a water bath at 32 °C after gently shaking the contents for 5 minutes.
In a centrifuge the tubes than are whirled until the volume of sediment in each tube remains constant for three consecutive readings. The amount of sediment in ml forms the ASTM Precipitation Number.
The values stated in the Standard Test Method for Precipitation Number of Lubricating Oilsare in acceptable SI units. The standard does not purport to address all of the safety concerns associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.