Unlocked by Ergon: Insulating liquids testing – lesson #10: Oil Colour and Interfacial Tension (IFT)

Key focus

To understand the principles, methods, limitations, and diagnostic value of oil colour and interfacial tension testing, and how these tests support practical assessment of insulating liquid condition.

Lesson description

This lesson explains the theory and practical significance of two important and often underestimated oil quality indicators: oil colour and interfacial tension (IFT). It describes how colour changes reflect ageing, contamination, oxidation products, suspended materials, and possible operational abnormalities, and why colour is often the simplest first visual indicator that something in the oil has started to change.

The lesson also examines the principles of IFT measurement, the main test methods, and the practical difficulties that affect accuracy and repeatability. It explains why IFT is highly sensitive to polar contaminants, ageing by-products, and traces of foreign materials, and why it may provide early warning of oil deterioration even before other parameters become critical. Special emphasis is placed on why IFT is more useful for some oil types, especially inhibited mineral oils, and less representative for others such as certain ester fluids.

The session highlights the diagnostic relationship between colour, IFT, oxidation stability, sludge tendency, inhibitor depletion, and oil treatment effectiveness. Historical development of both tests is also presented to show how their practical role evolved in transformer maintenance and oil evaluation.

Learning outcomes

• Explain what oil colour and interfacial tension represent and why they are important in insulating liquid diagnostics.
• Identify the main causes of oil darkening and reduced IFT in service-aged transformer oils.
• Understand the principles, methods, and limitations of colour and IFT measurements.
• Recognise why IFT has different diagnostic value for different insulating liquid types.
• Use colour and IFT results together with other oil parameters to support maintenance and treatment decisions.

Content of this lesson is fully vendor agnostic but it is sponsored by Ergon!