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Accurately Monitoring Interfacial Tension of Transformer Oil Quality

How the Attension Sigma 702ET Quickly and Easily Monitors the Interfacial Tension and Quality of Transformer Oil

Transformers account for nearly 60% of the cost of high voltage substations [1], and the time to repair some transformers is over one year [2]. Given the long downtime for repairs, t is imperative to accurately monitor the health of the transformer. Over time, transformer insulator oil degrades due to mechanical, electrical, and chemical stresses, and its quality is an excellent indicator of a transformer’s condition [3]. For years, measuring the interfacial tension (IFT) between the transformer oil and water according to the ASTM D971 standard [4] has been effectively used to monitor oil and transformer health. This is made easier with the Attension Sigma 702ET Transformer Oil Analyzer, the only tensiometer on the market dedicated to the quality control of transformer oils.

In September 2018, a new transformer quality control standard based on IFT measurements was released, IEC 62961 [5], and it will likely replace the previous ASTM standard. The new IEC standard is more convenient and practical than the ASTM standard. All new Attension Sigma 702ET tensiometers are designed to perform IFT measurements according to both ASTM D971 and IEC 62961, and older instruments can have their software updated remotely to add IEC 62961 capability.

Interfacial tension and oil quality

At the interface formed between two immiscible liquids, the molecular cohesive and adhesive forces cause a “film” to form which is analogous to an elastic sheet with an associated tension force. This property is called the interfacial tension (IFT) and it is very sensitive to impurities in either liquid. As transformer oil degrades over time or as contaminants are introduced, the IFT between it and pure water will drop, thus indicating deteriorating health of the transformer. In fact, IFT measurements are an excellent way to test the quality of many commercially relevant liquids such as jet fuels [6].

How is IEC 62961 an improvement over ASTM D971?

The IEC 62961 standard makes improvements in testing transformer oil IFT in four key areas:

The time to measure IFT after forming the oil-water interface is longer

The ASTM standard requires the IFT to be measured within 60 s of forming the interface, while the IEC standard changes this time to 180 +/- 30 s. IFT is dynamic as impurities adsorb to the interface over time. Generally, IFT changes fastest immediately after forming the interface and gradually reaches an equilibrium. Waiting longer to make the measurement when the IFT is less dynamic decreases variability in the measurements.

The acceptable water surface tension is less strict

In the ASTM standard, the surface tension of the water with air must be between 71-73 mN/m. Since water is easily affected by impurities, this required strict cleanliness and very pure water. The IEC standard calls for water surface tension to be between 70-73 mN/m which is easier to achieve. Additionally, the IEC standard does not require that water surface tension is measured before each transformer oil test, instead of requiring only a daily verification.

Automated testing equipment is encouraged

The Du Nouy ring method is sensitive to the speed and smoothness with which it is lifted through the interface. Using the automated Sigma 702ET removes any bias in the measurement caused by human operation and allows for multiple measurements to be done in sequence.

Instructions for cleaning the materials is clarified

Since the IFT measurement is a measurement of impurities in the transformer oil, it is important to have a properly cleaned system. The IEC standard provides clear instructions for cleaning the vessel and Du Nouy ring.


  • [1] A. Jahromi, R. Piercy, S. Cress, J. Service, W. Fan. An approach to power transformer assest management using health index. IEEE Electrical Insulation Magazine 25 (2009) 20-34.
  • [2] M. Wang, A.J. Vandermaar, K.D. Srivastave. Review of condition assessment of power transformers in service. IEEE Electrical Insulation Magazine 18 (2002) 12-25.
  • [3] M.M. Islam, G. Lee, S.N. Hettiwatte. A review of condition monitoring technique and diagnostic tests for lifetime estimation of power transformers.  Electrical Engineering 100 (2018) 581-605.
  • [4] ASTM, “Standard Test Method for Interfacial Tension of Oil Against Water by the Ring Method,” D971 – 12, 2012.
  • [5] IEC, “Insulating liquids – Test methods for the determination of interfacial tension of insulating liquids – Determination with the ring method,” 62961:2018, 2018.
  • [6] W.F. Taylor. Coordinating Support of Fuels and Lubricant Research and Development (R&D) 2, Delivery Order 0001: Water Separation Methods Study (No. TR-632). Coordinating Research Council Inc., Alpharetta, GA. 2004.

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