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This test measures the acidic constituents in oils and is reported in milligrams of potassium hydroxide per gram of sample. Oils degrade while in service, due to both acidic and basic oxidation. An increase in the acid number of a used oil is a measure of the amount of acidic substances absorbed. It is measured by determining the Total Acid Number (TAN) of the oil. The value is checked against published condemning limits. The rate of change of the TAN is more important than its absolute value. A rapid increase may be caused by excessive degradation due to oxidation, hotspots from skidding bearings, carbon seals, dirty oil ways or by top-up with a different oil.BACK