What is the correct procedure to take an oil sample from a transformer?

First, follow safety and lockout rules and ensure the sampling valve is clean and accessible. Flush the valve by draining a small volume to remove stagnant oil and debris. Use clean, dry, correct containers for the intended tests, such as sealed DGA bottles or moisture bottles. Avoid splashing and air bubbles, fill to the specified level, cap immediately, label with date, unit ID, temperature, and sampling point. Keep samples protected from heat and sunlight and ship promptly with chain of custody for reliable lab results.