Q: How should transformer oil be sampled for DGA, and what are the most common sampling mistakes?
A: DGA results are only as reliable as the sampling procedure. Contaminated or aerated samples can produce misleading results.
Sampling location:
• Always sample from the bottom valve (sediment and heavier gases collect there).
• Never sample from the top oil fill port.
|
Item |
Specification |
Purpose |
|
Syringe |
20–50 mL glass syringe, Luer lock |
Air-tight sample, no air entrainment |
|
Sampling bottle |
500 mL amber glass, Teflon-lined cap |
Headspace-free storage and transport |
|
Tubing |
PTFE (Teflon) - NOT PVC or rubber |
PVC absorbs hydrocarbons; rubber releases them |
|
Cooling box |
Insulated, < 10°C |
Slows microbial gas generation |
Step-by-step sampling:
1. Open bottom valve, let 1–2 L of oil flow to waste.
2. Connect PTFE tubing to the sampling valve.
3. Flush tubing with oil for 10–15 seconds.
4. Insert syringe needle into flow stream; withdraw 20 mL slowly - avoid bubbles.
5. Tap syringe to displace micro-bubbles; eject 1–2 mL; seal with Luer cap.
6. Wrap in light-proof bag (UV generates H₂ in oil).
Critical mistakes:
|
Mistake |
Consequence |
|
Sampling from top air vent |
Gas concentrations NOT representative |
|
Stopping waste oil flow too early |
Draws stagnant oxidized oil from valve neck |
|
Using rubber or PVC tubing |
False C₂H₆, CH₄ peaks (leaching) |
|
Leaving air bubble in syringe |
H₂ and O₂ unreliable (air contamination) |
|
Exposing sample to direct sunlight |
UV generates H₂ → false PD indication |
|
Delaying > 48 hours without refrigeration |
Biological activity generates H₂, CH₄, CO₂ |
Transport: Deliver to lab within 48 hours at 4–10°C.