The Most Common Sampling Questions, Answered By The Lab
Have questions about transformer oil testing or your test results? We’ve compiled answers to some of the most common ones we receive, directly from our experts:
What causes bubble formation in a DGA syringe?
Common causes include:
- A syringe that is not fully sealed after sampling or during shipment
- Improper sampling methods, such as leaks in the sampling hose or collecting a DGA sample from a bottle instead of directly from the unit - This bubble should be expelled
Bubbles can also form naturally when hot oil cools after sampling. In these cases, the gas bubble is part of the sample and should still be analyzed.
How can bubbles affect a DGA sample?
Bubbles may reduce dissolved gas concentrations in the sample, potentially leading to an inaccurate interpretation of transformer condition.
Why did my diagnostic limits change from the previous sample?
IEEE Guide C57.104 has statistical limits based on age and the O2/N2 ratio of a transformer. One common issue is that O2/N2 ratio can change due to improper sampling or a potential leak in the transformer. A change in ratio will alter the limits used in diagnostics. A higher O2/N2 ratio than in previous results can indicate a leak in the unit or an improper sampling.
What causes low dielectric breakdown values?
Low dielectric breakdown (DB) is commonly associated with:
- High moisture content
- Polar contaminants in the fluid
Additional tests, such as interfacial tension, power factor, and visual inspection, can help confirm the root cause.
Why did my sample fail visual inspection for free water if moisture content was low?
Contaminants such as dirt or free water may remain in the sampling valve if it was not properly flushed before sampling. These contaminants can appear in the bottle during visual inspection.
Because moisture testing is performed using the sealed syringe sample, the measured moisture content may still be low.
Reviewing sampling procedures and ensuring clean sampling valves can help prevent this issue.
Questions about your test results?
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