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Protecting the heart of your electrical systems 

Fluid testing for oil-filled transformers assures the oil is performing how it should. Insights on oil health, contamination, and the detection of possible faults help drive informed decisions that maximize transformer life and support safety measures.

A transformer’s protective insulating oil can degrade from use, wear, or contamination, making it more prone to arcing and corona discharge or less effective at dissipating heat. These situations can cause a catastrophic failure, possibly resulting in an explosion, fire, or even life-threatening damage.

Failure also has a financial impact, resulting in significant delays in replacing equipment. Regular testing allows historical trend analysis to help guide your transformer maintenance decisions. 

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The ALS difference

Guided by science and innovation and grounded in a values-driven culture, we are committed to building sustainable future for everyone. Safer, smarter, healthier, together. 

Our network of laboratories and experience give ALS a distinct advantage of efficiency gains through our process and innovation. Providing customers, a variety of options to meet them where they are. Metrics help to drive our commitments of efficiency which are measured by turnaround time. Efficiency gains support our ability to offer competitive pricing for all customers regardless of size along with a robust ISO accredited quality system which ensures our results are accurate. 

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Transformer oil tests and what they measure

Transformer fluids can be Mineral Oil (Paraffin and Naphtha based), Silicone Oil, and Natural or Synthetic Esters. It is common to use some different methods for testing because of the elemental differences.

Care should be taken when sampling transformer fluid and should be performed by a trained operator. Sampling for Dissolved Gas Analysis (DGA) requires a special sampling syringe, which ALS can help provide.

  • Color (D1500)

    An observation of the darkening of the oil by comparing it to previous samples of oil from the same transformer.

  • Dielectric breakdown voltage (D1816)

    Measures the voltage at which the oil fails electrically, which is indicative of the amount of contaminant (usually moisture) in the oil .

  • Interfacial tension (D971)

    Measures the presence of soluble contaminants and oxidation products in transformer oil. A decreasing value indicates an increase in contaminants and/or oxidation products within the oil.

  • Water Content (moisture) (D1533)

    Measured in parts per million (ppm) using the weight of moisture divided by the weight of oil. Moisture content in oil lowers the insulating dielectric strength.

  • Dissipation factor (Power Factor) (D924)

    Indicates the dielectric losses of the oil, or energy that is dissipated as heat. Useful for measuring changes within the insulating oil resulting from contamination or deterioration.

  • Acid number (D974/D664)

    New transformer oils contain practically no acids. The acidity test measures the content of acids formed by oxidation and contaminates.

  • Oxidation inhibitor content (D2668)

    Added to some oils to help protect the paper insulation from oxidation.

  • Resistivity (D1169)

    Measures the electrical insulating properties of transformer oil.

  • Dissolved gas analysis (D3612)

    Identifies various gas levels that are present in transformer insulating oil. Different gasses dissolved in the oil indicate various types of thermal and electrical stress occurring within the transformer.


    Corona discharge Low energy discharges create methane and hydrogen and smaller quantities of ethylene and ethane.
    Arcing Large amounts of hydrogen or acetylene or minor quantities of ethylene and methane can be produced.
    Overheated Oil Overheating oil will produce methane and ethylene (300ºF) or methane and hydrogen (1,112ºF). Traces of acetylene might be created if the unit has electrical contacts or if the problem is severe.
  • Polychlorinated biphenyls (PCB) content (D4059)

    Used for measuring PCB content in electrical equipment.

  • Furanic Compounds (D5837)

    Used as an indirect measurement of the condition of the paper insulation by measuring the amount of Furanic compounds within the oil. Furanic compounds are an intermediate breakdown product from overheating of paper in an anaerobic environment.

  • Visual examination (D1524)

    This test method for visual examination is applicable to mineral oils of petroleum origin that have been used in transformers. It is used to determine if the sample has been taken correctly, with the appropriate amount of flushing.

Related topics & resources

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Increase transformer lifespan

Properly maintained transformers should not have to be replaced for 50 years or more


Routine fluids analysis

The key to transformer maintenance

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Calculate your ROI

Find out the true cost-benefits of a properly implemented Transformer Oil Analysis program