There is a definite relationship exists between acid number and interfacial tension. An increase in acid number should normally be followed by a drop in interfacial tension. The interfacial tension test is a powerful tool for determing how an insulating oil has performed and how much life is left in the oil before maintenance is required to prevent sludge.

The interfacial tension provided an excellent back up test for the acid number.

Interfacial tension not accompanied by a corresponding increase in acid number indicates polar contamination which have not come from normal oxidation.

Although a low interfacial tension with a low acid number is an unusual situation, it does occur because of contamination such as solid insulation materials, compounds from leaky pot heads or bushings, or from a source outside the transformer.