How to assess transformer condition using liquids testing

[...]insulating liquid testing is a key method for assessing a transformer's condition and identifying incipient faults before they become critical. Generally, glass or aluminum bottles or metal cans are the preferred options. Because insulating liquids can degrade in sunlight (photodegradation) leading to the synthesis of hydrogen or an increase in acid number, the containers, sleeves and/or packaging should be light-proof to protect the sample from sunlight. [...]a trend of several data points needs to be established, which will inform the asset manager if the gassing is stable, becoming more intense or is progressing from one fault type to another. [...]different insulating liquids have different levels of affinity for water. [...]it is important to know which oil is being tested as the difference is particularly marked between mineral oils and ester liquids.