ASTM D91 standard test method for precipitation number of lubricating oils
8. Procedure
8.1 Add 10 more or less 1 mL of the oil to be tested in each of two clean, dry centrifuge tubes at room temperature. Fill each tube to the 100-mL mark with hexanes and close tightly with a softened cork (not a rubber stopper). Then invert each tube at least 20 times, allowing the liquid to drain thoroughly from the tapered tip of the tube each time. Place the tubes in a water bath at 32 to 35°C for 5 more or less 1 min. Momentarily remove the corks to relieve any pressure, and invert each tube again at least 20 times exactly as before. The success of this method depends to a large degree upon having a thoroughly homogeneous mixture which will drain quickly and completely from the tapered tip when the tube is inverted.
8.2 Balance the two centrifuge tubes or pairs of tubes with their respective trunnion cups and place them on opposite sides of the centrifuge head. Then whirl them for 10 min at a rate sufficient to produce a relative centrifugal force (rcf) between 600 and 700 at the tips of the whirling tubes (see 5.2). Repeat this operation until the volume of sediment in each tube remains constant for three consecutive readings. In general, not more than four whirlings will be required for oils having a low precipitation number.
