Moisture in Transformer Liquids: What You Need to Know

What is it?

Moisture content in transformer liquids is the amount of water in a sample on a weight-to-weight basis (mg/kg), also known as parts per million (ppm).

How is it measured?

At AVO Diagnostic Services, we use the Karl Fischer titration method described in ASTM D1533 for precise analysis.

Why does moisture in transformer liquid matter?

Excess moisture can: 

• Reduce the dielectric strength of the insulating system  
• Accelerate paper aging  
• Increase the risk of bubble generation  
• Promote free water formation during rapid temperature drops  
• Raise the acid content in the liquid 

How does moisture get in?

• During manufacturing, commissioning, or repairs
• Through leaks
• As a byproduct of natural aging

Visual inspections can be misleading

Below are examples of samples received in the lab. While dissolved water may pass visual inspection, emulsified and free water often fail.

Why might my results show low moisture but fail the dielectric breakdown test? 

• At AVO labs, we measure moisture directly from the syringe to accurately reflect the conditions inside the transformer.
• The dielectric breakdown sample, however, is taken from a bottle. This sample can absorb moisture from the air or be affected by improper sampling, potentially showing a higher moisture content than what is actually inside the transformer.