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Specialist Materials for Industry and Science

Fluids Comparison

Fluids Comparison

When choosing a transformer fluid it is important to evaluate all the options, to get the best fit with the minimal of risk for your application. In the past mineral oil has been the main fluid specified for transformers, but in the last 30 years two further options have been made available in the shape of synthetic and natural esters which bring many benefits to the users and specifiers.

The main risks associated with using mineral oil are flammability, high environmental impact and poor moisture tolerance. Ester based fluids provide the solution to these risks, while still offering excellent dielectric strength and cooling properties.

The table below shows a comparison of the properties of three insulating oils, synthetic ester (MIDEL 7131), natural ester (MIDEL eN) and mineral oil. It shows that our two esters are K-class, meaning they have much higher fire points than mineral oil. In addition, both esters have high moisture saturation points and are readily biodegradable.

As a high performance fluid MIDEL 7131 is recommended for the full range of transformer applications. It is perfectly suited to innovative compact designs of the type used on rolling stock and in wind turbines, which require higher temperature stability from the dielectric fluid. It also benefits from having the highest possible fire safety classification (K3).

MIDEL 7131 is also suitable for both breathing and sealed applications due to excellent oxidation stability, whereas MIDEL eN, as with all natural esters, is only recommended for use in sealed distribution and power transformers. 

 MIDEL 7131MIDEL eNMineral Oil
Fire Safety Class K3 K2 O1
Readily / Fully Biodegradable
Breakdown Voltage kV >75 >75 >70
Moisture Saturation ppm at Amb Temperature 2700 1100 55
Kinematic Viscosity at 40°C 29 37 12
Density at 20°C 0.97 0.92 0.88
Expansion Coefficient 0.00075 0.00074 0.00075
Permittivity 3.2 3.1 2.2
Pour Point °C -56 -31 <-50
High Temperature Excellent Good Poor

All properties quoted in this table are typical values and do not constitute a specification.

For further information please visit our Documents or Contact Us today.


  • Apiezon
  • Metrosil
  • Midel
  • Wolfmet

Products of M&I Materials Ltd.