Acid Number

ASTM D974: A measure of the acidic constituents present within the lubricant. Most rust inhibitors used in turbine oils are acidic and contribute to the acid number of the new. Increases from the new oil level are monitored, and for the most part, increases reflect the presence of acidic oxidation products. Though less likely, increases in acid number could be attributed to contaminants, mixtures of products, and/or chemical transformations