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What is the significance of using an acid value tester for insulating oil testing? When doing tests related to insulating oil, the application of an acid value tester is indispensable. The acid value of insulating oil indicates that the oil contains acidic substances, that is, the total value of organic acids and inorganic acids. Generally, it is expressed by the number of mg of potassium hydroxide required for acid to neutralize the acidic substances in 1g of insulating oil. New, unused transformer oil contains almost no acidic substances and its acid value is quite small. However, if the oil is stored for a long time, especially after the charging equipment is put into operation, it will inevitably come into contact with oxygen in the air, and the oil will be easily aged. In the early stage of oxidation, low molecular organic acids are mainly generated, and further oxidation generates high molecular organic acids and acid products. The presence of the above-mentioned acidic substances in the insulating oil will increase the conductivity of the oil, reduce the insulation performance of the oil, and may also cause corrosion to metals. When the operating temperature is high (above 80~C), the solid fiber paper insulation material is caused to age, thereby shortening the service life of the equipment. Generally, unused (new) transformer oil contains almost no acidic substances, and its acid value is low, with a pH value in the range of 6 to 7. The pH value is mainly used to indicate the water-soluble acid of the insulating oil. According to oil analysis of on-site investigations, simulation tests, and laboratory aging test results in my country, the acid value of transformer oil in operation is generally greater than 0.1m (pH)H/g, and oil precipitates when the pH value is equal to or less than 4.0. The possibility of sludge increases. On the contrary, the transformer oil can basically ensure that the transformer works well and reliably. When the acid value rises above 0.2mgK()}L/g or the pH value is lower than 3.8, the oil quality will deteriorate significantly and more sludge will be produced. Therefore, the pH value of the operating oil should be greater than 4.2.