What does interfacial tension testing reveal about transformer oil condition?

Interfacial tension (IFT) indicates the presence of polar contaminants and oxidation byproducts. As oil ages and oxidizes, acids and sludge precursors increase and IFT typically drops. Low IFT can signal advanced oxidation, higher risk of sludge formation, and deterioration of paper insulation environment. Because IFT is sensitive to aging chemistry, it is used together with acidity, color, dissipation factor, and DGA to decide whether oil treatment, reclamation, or replacement is needed.