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Specialist Materials for Industry and Science



Q: Who is responsible for manufacturing Midel ester-based dielectric transformer fluids?
A: Midel ester-based dielectric transformer fluids are manufactured by M&I Materials Limited.

Q: Who are M&I Materials Limited?
A: M&I Materials Limited is a strong global business that specialises in materials for industry and science. Please see

Q: What are M&I Materials Limited’s environmental credentials?
A: M&I Materials Limited takes its responsibility to the environment very seriously and has achieved certification by Lloyd's Register Quality Assurance under the Environmental Management Standard ISO 14001.

Q: I am interested in learning more about Midel, what should I do next?
A: Please contact our expert sales team and they will endeavour to answer all of your questions. Tel: +44 (0)161 864 5422 or contact us 

Q: Can I order Midel online?
A: At present Midel is not available to buy online.

Q: What are the typical lead times for Midel?
A: Normally ex-stock. We always endeavour to satisfy customer requirements.

Q: What are the nett weights of the containers used to transport MIDEL 7131 and MIDEL eN?
Please see MIDEL 7131 Packaging and MIDEL eN Packaging.

Q: Does M&I Materials provide guidance about the storage and handling of MIDEL 7131?
A: Yes, please see the Storage and Handling section of the MIDEL 7131 Technical Information Pack

Q: Does M&I Materials provide guidance about the storage and handling of MIDEL eN?
A: Yes, please see the Storage and Handling sections of the MIDEL eN 1204 Technical Information Pack and MIDEL eN 1215 Technical Information Pack.

Q: What is Midel?
A: Midel is the M&I Materials Limited brand name for the ester based dielectric fluids for transformers.

Q: What is an Ester?
A: In chemical terms an ester is derived from the reaction of an alcohol with acid. Esters also occur in nature in the form of seed oils. Please visit About Esters for more information.

Q: What are the differences between synthetic and natural esters?
A: Synthetic esters are manufactured by reacting selected acids and alcohols to tailor their properties to the application. Natural esters are derived directly from seed oils, their properties are dependent on the base oil used. The main operational difference is that synthetic esters are far more oxygen stable than natural esters and are therefore suitable for use in free-breathing applications, while natural esters are not.

Q: Are ester transformer fluids a proven technology?
A: Synthetic esters have been in use in transformers for over 30 years and have a proven track record in demanding applications such as traction transformers. Natural esters are a more recent development and have been available commercially since the late 1990's. M&I Materials produces both types of ester - Synthetic ester MIDEL 7131; Natural ester MIDEL eN.

Q: Is Midel suitable for all transformer applications?
A: Our synthetic ester, MIDEL 7131 has a long and successful track record in the following applications:-

  • distribution transformers in urban areas and buildings
  • power transformers up to 238 kV
  • traction transformers
  • rectifier transformers
  • pole-type transformers
  • tapchangers
  • thyristor cooling in power electronics
  • retrofilling

Our natural ester - MIDEL eN has been manufactured from sustainable sources and is most suited to sealed applications where the thermal demands on the fluid are not as great as could be placed upon a synthetic ester.

Q: Why should I use esters instead of traditional mineral oil transformer fluids?
A: Esters have a high fire point (>300°C) and are readily biodegradable. They offer enhanced fire safety whilst being more environmentally friendly. Esters are also far more moisture tolerant than mineral oil, improving cellulose lifetime and allowing extended operation in demanding ambient conditions.

Q: Is there technical information available about MIDEL 7131 and the natural esters MIDEL eN 1204 and 1215?
A: Please visit our Documents

Q: What are the thermal properties of MIDEL 7131 and MIDEL eN?
A: Please visit MIDEL 7131 Thermal Properties or MIDEL eN Thermal Properties

Q: How does MIDEL 7131 and MIDEL eN compare to mineral oil?
A: Please see Fluids Comparison

Q: Are MIDEL 7131 and MIDEL eN compatible with all materials?
A: Please see MIDEL 7131 Materials Compatibility and MIDEL eN Materials Compatibility.

Q: Do MIDEL 7131 and MIDEL eN contain corrosive sulphur?
A: Recently the transformer industry has reported the failure of multiple large units due to the presence of corrosive sulphur. MIDEL 7131 and MIDEL eN have been tested by independent laboratories to ASTM D1275 B, Standard Test Method for Corrosive Sulphur in Electrical Insulating Oils and IEC 62535, Insulating liquids - Test method for detection of potentially corrosive sulphur in used and unused insulating oil. They were found to be Non-Corrosive.

Q: Are MIDEL 7131 and MIDEL eN RoHS compliant?
A: MIDEL 7131 and MIDEL eN are RoHS compliant; they do not contain the flame retardants PBB and PBDE, nor are they formulated using lead, cadmium, mercury or hexavalent chromium. The concentrations of these heavy metal contents are all below 1ppm.

Q: Can DGA and furan analysis be performed on MIDEL 7131 and MIDEL eN?
A: Yes DGA can be performed on both MIDEL 7131 and MIDEL eN. A full explanation of this is given in the Cigre Technical Brochure 443 (2010) and in various papers written by the University of Manchester.

The key indicator gasses such as Actylene (arcing) and Hydrogen (PD) are still relevant to Midel esters. For diagnosing common faults the standard methods (Rogers' Ratio, Duval Triangle) are good guides. Care should be taken in borderline cases as the fault diagnosis boundaries are slightly different for esters. For MIDEL 7131 the total amount of gas will be lower for the same fault level, when compared to mineral oil, and as with other natural esters, MIDEL eN will produce more ethane. For more information please contact

Q: Are there safety data sheets available for MIDEL 7131 and MIDEL eN?
A: Please see MIDEL 7131 Safety Data Sheet,  MIDEL eN 1204 Safety Data Sheet and MIDEL eN 1215 Safety Data Sheet.

Q: Why are MIDEL 7131 and MIDEL eN thought to be fire safe?
A: Both Midel fluids have a high fire point and 100% fire safety record, see MIDEL 7131 Fire Safety and MIDEL eN Fire Safety.

Q: What is K-class classification?
A: K-class is defined by IEC 61039. In order to achieve K-class classification a fluid must have a fire point in excess of 300°C.

Q: Which independent agencies have verified the safety of MIDEL 7131 synthetic ester?
A: The safety of MIDEL 7131 fluid has been verified by:-

  • Underwriters Laboratory - UL® classified EOUV dielectric medium
  • Factory Mutual (FM) Global – FM Approved Less Flammable Transformer Fluid
  • SGS BASEEFA, UK - ATEX Certification
  • Allianz
  • TUV Nord

Q: Which independent agencies have verified the safety of MIDEL eN natural ester?
A: The safety of MIDEL eN fluid has been verified by:-

  • Underwriters Laboratory - UL® classified EOUV dielectric medium
  • Factory Mutual (FM) Global – FM Approved Less Flammable Transformer Fluid
  • TUV Nord

Q: Are there reduced requirements for fire protection when using Midel fluids?
A: FM Global® (Factory Mutual) allow transformers to be sited closer together and without fire walls. They state that there should be no need for further fire protection if an approved transformer and fluid are used. Please see FM Global.

Q: What is the relevance of EN 45545 for traction transformers?
A: European Standard EN 45545 requires that only K Class fluids can be used on rolling stock.

Q: Why are MIDEL 7131 and MIDEL eN environmentally friendly?
A: They are both readily biodegradable, fully biodegradable and classified as non-water hazardous.

Q: What is meant by readily biodegradable?
A: In order to be classified as readily biodegradable a substance must pass a strict test to OECD or similar methods. A substance which is readily biodegradable is expected to degrade quickly and completely in the environment. Please see the Greater Environmental Protection information in the MIDEL 7131 Technical Information Pack for more information.

Q: What is meant by fully biodegradable?
A: : IEC 61039 classifies insulating fluids as fully biodegradable when they reach at least 70% degradation in 28 days, using either the OECD 301C or OECD 301F test methods. Please see the Greater Environmental Protection information in the MIDEL 7131 Technical Information Pack for more information.

Q: What certification have MIDEL 7131 and MIDEL eN got to demonstrate they are environmentally friendly?
A: MIDEL 7131 and MIDEL eN clearly pass the OECD 301 series test for ready biodegradability, and are classified as non-water hazardous (nwg) by the German Federal Department of the Environment, UBA (Umweltbundesamt). They also meet the Fully Biodegradable standard of IEC 61039.

Q: Can Midel transformers be installed without fluid containment?
A: This depends greatly on the local government in the area that you are installing the transformer. The German Federal Department of the Environment , UBA (Umweltbundesamt) has identified both Midel esters as being non-water hazardous, which brings reduced requirements for containment.

Q: What is meant by the term moisture tolerant?
A: Midel can absorb far more water than mineral oil before the dielectric characteristics are compromised.

Q: How does moisture tolerance extend cellulose lifetime?
A: Reducing water content in the paper slows down the rate of depolymerisation. See Moisture Tolerance

Q: Why is moisture tolerance an advantage in retrofilling?
A: It can bring transformers back into service which were previously too wet for mineral oil. Theory suggests that drying of very wet transformers can be speeded up with the use of ester fluids due to their ability to absorb more water.

Q: Is Midel suitable for use in high temperature compact transformers?
A: Our synthetic ester MIDEL 7131 has been used for many yars in compact traction transformers operating with high temperature insulation such as NOMEX®. MIDEL 7131 is also specified for use in compact wind turbine transformers which can operate with a top oil temperature of 120°C.

Our natural ester MIDEL eN is only suitable in sealed transformer systems where the thermal conditions are less demanding. For more information please contact 

Q: What is the maximum operating temperature for MIDEL 7131?
A: In a sealed system MIDEL 7131 can operate long term at temperatures of up to 130°C (top oil) and 150°C (winding hot spot). Temperatures above this maybe possible depending on the application, please contact

Q: What is meant by the terms ‘Tan Delta’ or ‘Dielectric Dissipation Factor’
The ‘Tan Delta’ or ‘Dielectric Dissipation Factor’ of an insulating material is the tangent of the loss angle, which is the angle by which the phase difference between applied voltage and the resulting current deviates from 90°. A high value indicates slightly more heat loss in the dielectric material.

Q: What is the acceptable measurement for Tan Delta?
For new synthetic ester fluid supplied from the factory the acceptable limit according to IEC 61099 is <0.03 at 90°C. For new natural ester fluid supplied from the factory the acceptable limit according to IEC 62770 (2013) is <0.05 at 90°C. When synthetic ester fluid is in service IEC 61203 advises a limit of ≤0.01 at ambient temperatures. Users should be aware that a slightly higher value of tan delta does not affect the fluid’s ability to provide a safe dielectric medium. For monitoring fluid condition in Midel fluids M&I Materials recommends checking breakdown voltage, water content and neutralisation value as relevant measures to ensure good continued operation of equipment.

Q: What are the typically Tan Delta values for MIDEL 7131 and MIDEL eN
Before MIDEL 7131 and MIDEL eN are shipped from M&I Materials all batches are tested and the tan delta values are very low (typically <0.008).

Q: Why am I getting a different Tan Delta value than M&I Materials when I test MIDEL 7131 or MIDEL eN?
Tan Delta is a very sensitive measurement and the results can be affected by minute amounts of contamination with other fluids. Equipment previously used for mineral oil measurements and not properly decontaminated is likely to give high values for MIDEL 7131 in particular. M&I Materials offer a guide to measuring tan delta in esters, which is available on request.

Q: Why is the Tan Delta of Midel fluids higher than mineral oil?
Midel fluids are based on esters, which have a different molecular structure to mineral oil. This difference makes esters more polar than mineral oil, leading to a higher tan delta value. The fact that the fluids have a slightly higher tan delta does not compromise their ability to provide excellent dielectric strength.

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