How is oil sampling performed on hermetically sealed transformers?

Sampling is done through the dedicated sampling valve, using clean dry containers and controlled flow to avoid air ingress and contamination. The valve area is cleaned, a small amount is flushed to waste, then the sample is collected midstream with minimal turbulence. For DGA, bottles are filled to eliminate headspace, sealed immediately, and labeled with temperature, load, and location. Operators avoid opening the tank because hermetically sealed units rely on minimal oxygen exposure to slow oxidation.