This test is the most important physical determination that is made to insulating oils. Covers measurement under non-equilibrium interfacial tension, which maintains an insulating fluid Waterfront. The interfacial tension is a measure of the attractive forces between the molecules of two immiscible fluids. The result is expressed in dynes / cm or millinewton / meter (mN / m). This test is an excellent way to detect soluble polar contaminants and oxidation products in insulating oils.