ASTM D644 is the test method for monitoring the degradation acid content of in-service oil. The acid number test method is performed by diluting the sample with a mixed solvent in a reaction beaker. While mixing, potassium hydroxide is titrated to chemically react with acidic components in the sample. When the end point of the chemical reaction is reached, the amount of titrant used is reported in milligrams of potassium hydroxide equivalent per gram of sample (mg of KOH/g).