How is transformer oil interfacial tension tested?

Transformer oil interfacial tension is tested using a tensiometer, typically following the ASTM D971 standard. The procedure involves placing a small amount of transformer oil in a test vessel, then introducing a water layer. The interfacial tension is measured by determining the force required to detach a thin wire or plate from the oil-water interface. This measurement helps assess the oil’s purity and its ability to separate from contaminants, which is crucial for maintaining transformer performance.