Interfacial Tension Testing

Interfacial tension (IFT) testing helps evaluate the overall condition of transformer insulating fluid by detecting oxidation byproducts, polar contaminants, and other decomposition products. These may include acids, intermediate oxidation compounds, varnishes, or contamination from other fluids. As these compounds accumulate, they weaken the interaction between the oil and water, causing the IFT value to decrease. A declining result can indicate oil deterioration and an increased potential for sludge formation, which may restrict heat transfer and reduce cooling efficiency.

To perform interfacial tension testing, the fluid being evaluated is placed on top of a layer of distilled water. Because fluids such as mineral oil and ester have a lower density than water, they remain above the water and form a stable interface between the two layers. Fluids in better condition generally create stronger interfaces, while contaminants in the oil destabilize and weaken the boundary.

A platinum ring is pulled from the distilled water layer into the oil, and the force required to break the interface is measured. This force is reported in millinewtons per meter (mN/m) or dynes per centimeter (dynes/cm). For reference, one dyne is a unit of force equal to approximately 0.000002247 pounds. A higher force generally indicates a stronger interface and fluid in better condition, while a lower result may indicate the presence of contaminants or deterioration products.