What does a PPM test for transformer oil measure?

“PPM” in transformer oil testing usually refers to concentration measured in parts per million, most commonly moisture content (water) in ppm, and sometimes contamination such as PCBs or specific inhibitors. Moisture ppm is critical because even small increases can sharply reduce dielectric strength and accelerate paper aging. Depending on the lab program, ppm can also be used for dissolved gases, metals, or sulfur compounds, but moisture is the most common meaning.