The ASTM D2440 test method covers the Oxidation Stability of Mineral Insulating oil. The oxidation stability test of mineral transformer oils is a method for assessing the amount of sludge and acid formed in a transformer oil when the oil is tested under prescribed conditions. Good oxidation stability is necessary to improve the service life of the oil. Oils that meet the requirements tend to minimize electrical conduction, ensure acceptable heat transfer, and preserve system life. For this test, a test specimen of mineral insulating oil is placed in an oil receptable together with a copper catalyst coil to simulate the ageing of the sample. The sample is in the bath at 110°C for 72 or 164 hours. During this time, oxygen is constantly bubbled through the sample. After this procedure, the oil is evaluated by measuring the amount of sludge and acids formed.