TAN (Total Acid Number) measures the amount of acid formed in the oil as a result of oxidation, contamination or degradation. An increasing acid count indicates that the oil is about to break down and that the risk of corrosion increases.
The analysis is carried out by titration, in which the acid content of the oil is neutralized with a base, and the amount required is recorded. The result is shown as mg KOH per gram of oil.
A high TAN may mean that the oil needs to be changed or that there are underlying problems. This makes the analysis important for monitoring the aging of the oil and the state of the system.