Insulating oils

Kinematic viscosity and interfacial tension of transformer oils


Transformers are among the most important elements in energy supply. Here, transformer oils are used for insulating and cooling. However, unusual strains, like for example, transient overvoltage, overheating in emergency operation, or disruption to cooling, lead to accelerated deterioration of the oils and a reduced life-span. With the oxidation and deterioration of oils which are heavily stressed or are in use for a number of years, insoluble and polar components build up. By measuring the interfacial tension this build-up in the oil can be detected. This helps to classify the deterioration and remaining life-span of the oil. In the lab report, the interfacial tension is indicated in mN/m. If their value compared either to fresh oil or to the previous sample has decreased significantly, the oil has deteriorated, which means that regeneration or an oil change is necessary. Due to their compact design and intuitive operation using the separate Command remote control, the LAUDA Scientific ring tensiometers TD 1 C and TD 3 are best suited for mobile use on location as well.

In Arctic regions, the temperature in a deactivated transformer can sink to -40 °C. Here, the viscosity increases dramatically, sometimes accompanied by paraffin flocculating. This changes the insulating properties with the risk of damage to the transformer. As such, the viscosity at very low temperatures also needs to be measured when formulating the oils. LAUDA Scientific provides tailored configurations for this purpose.

Recommended standards

  • ASTM D 971
    Standard Test Method for Interfacial Tension of Oil Against Water by the Ring Method
  • DIN EN 14370
    Surface active agents – Determination of surface tension

Recommended equipment

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Automatic tensiometer – Ring method according to Du Noüy

  •  TD 4

Manual tensiometer – Ring method according to du Noüy

  • TC 1

each with:

  • PTT Peltier thermostating unit
  • Measuring ring (2-legged)
  • Density measurement set
  • Calibration weight
  • Printer and/or data transfer software for PC (TD 4 only)

Request consulting

Applications Lubricants, oils and fuels

Lubricants and fuels

Viscosity index and kinematic viscosities of SAE/ISO motor oils


Recovery rate in tertiary oil production

Oil transport

Pumpability of crude oils (record of viscosity temperature characteristics)

Waxes, resins, silicone and polyols

Absolute, kinematic, and dynamic viscosities of highly viscous liquids