There are five tests that transformer oil is subject to before it starts flowing through a circuit. Depending on the type of transformer, there’s a lot more testing and measurements.
Usually, these tests measure things like viscosity, dielectric strength, and resistivity. In some cases, they also check for water content and the presence of dissolved gases such as hydrogen sulfide or carbon dioxide (which may give gas bubbles). Too many unsaturated hydrocarbons in the form of cracked paraffin can cause problems with electrical sparks inside the oil.
After completing each test, engineers will often graph out results to show what changes need to be made in order to bring everything back within range limits.
Originally published on quora @sequoia-global