Measuring interfacial tension of insulating liquids against water according to ASTM D971

ASTM D 971 is a standard issued by the American standardization organization ASTM International for testing the aging condition of transformer oils by measuring the interfacial tension against water.

The du Nouy ring and the Wilhelmy plate methods are the two most common techniques used to determine surface tension at a liquid-air interface, or interfacial tension at a liquid-liquid interface: Lecomte du Nouy is a French Physicist who developed in the late 1800's a measuring method of surface tension by means of a precision balance with hanging probe assembly, a platinum ring with defined geometrical dimensions and a precision lift mechanism to vertically move sample liquid in a glass beaker. The ring hanging from the balance hook is first immersed into the liquid and then carefully pulled up, by lowering the sample cup, through the surface of liquid. The gravimetric assembly is recording the torque while pulling through the surface or the interface. The surface tension or interfacial tension is the maximum force needed to detach the ring from the aqueous liquid surface.