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Transformer oil testing

Measurement of interfacial tension for ensuring the quality of transformers

Transformer oil surrounds and insulates the coil windings in high-voltage transformers. During operation, the oil oxidizes and the oxidation products accumulate in the transformer oil. As a result, the electrical resistance of the oil falls and this can lead to a voltage flashover. The possible consequences are functional failure and even explosive fires. To prevent this, electricity suppliers throughout the world use our tensiometers for ensuring the quality of the oil.


During the ageing process, the interfacial tension between transformer oil and water reduces. The interfacial tension therefore becomes a quality criterion: if it is below a predefined limit, the oil must be changed. Quality assurance for transformer oil is often implemented in accordance with the ASTM D-971 standard. Our tensiometers measure according to this standard using automatic or semi-automatic measuring procedures.


The ASTM D-971 standard specifies that measurements are to be carried out using the Du-Noüy ring method. With this method, a ring suspended from a force sensor is moved from the heavy, aqueous phase to the lighter oil phase. Our tensiometer software calculates the interfacial tension from the tension force of the liquid lamella which forms below the ring.


On the one hand, the measurements ensure that the transformer operates reliably. On the other hand, expensive premature replacement can be prevented due to the fact that the usability of the oil is determined.

Discuss your application with our specialists

Our application experts are curious about your individual task in interfacial analysis. Contact us for a conversation and benefit from their experience.

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