TAN (Total Acid Number) measures the amount of acid formed in the oil as a result of oxidation, contamination or degradation. An increasing acid number indicates that the oil is degrading and that the risk of corrosion is increasing.
The analysis is carried out by titration, where the acid content of the oil is neutralized with a base and the amount required is recorded. The result is expressed as mg KOH per gram of oil.
A high TAN may indicate that the oil needs to be changed or that there are underlying problems, making the analysis important for monitoring oil aging and system condition.