The interfacial tension measures the tension at the interface between two liquid (oil and water) which do not mix and is expressed in dyne/cm.

The test is sensitive to the presence of oil decay products and soluble polar contaminants from solid insulating materials.

Good oil will have an interfacial tension of between 40 and 50 dynes/cm. Oil oxidation products lower the interfacial tension and have an affinity for both water (hydrophilic) and oil. this affinity for both substances lowers the interfacial tension. The greater the concentration of contaminants, the lower the interfacial tension, with a badly detriorated oil haveing an interfacial tension of 18 dynes/cm or less.

Test method: ASTM D971