Taking one tank of oil and sending it for testing twice, with two reports carrying different breakdown voltage figures, is not unusual in the oil quality testing done on site. The person sending the sample usually suspects the instrument or the method, and in most cases the cause turns out to be the sample that was sent.
What breakdown voltage actually measures
Breakdown voltage is measured with the oil in a cup, raising the voltage gradually between two electrodes until the insulation breaks down, and recording the voltage at that moment. The basis is GB/T 507, and the specification for the treated oil of the ZYD-100 is ≥65 kV. The figure reflects the insulating capacity of the oil in an electric field.
Breakdown in a field starts at the weakest point. Impurities, suspended particles and water in the oil all become weak points, and the temperature of the oil as a whole decides how these weak points behave under the field. With the same composition, a different oil temperature means a different viscosity and a different state of molecular motion, and the threshold at which the oil breaks down moves with it.
Three conditions that are easily overlooked
- Oil temperature. A sample measured while it is cold has a higher viscosity and restricted molecular motion, and the result is usually lower. The report should state the oil temperature at sampling or at the test, and two results can only be compared when the conditions are comparable.
- Trapped bubbles. Air drawn in during sampling is stretched and narrowed as the voltage rises, forming a locally weaker region than the oil itself, and the breakdown usually starts there, so whether the sample contains bubbles affects the result very directly.
- Standing time. A freshly taken sample is in a disturbed state, with particles and bubbles still moving; standing lets them distribute naturally, and only then is the measured value close to the real state in the tank.
None of these three needs extra equipment, but they decide whether the data can be compared at all. If only one of the two submissions records the sampling conditions and the oil temperature, the difference between the two figures cannot be interpreted.
| Difference between the two results | What to check first | How to handle it |
|---|---|---|
| The figures differ widely and only one report states the oil temperature | Test temperature | Take a fresh sample and retest at the same oil temperature |
| The figures are low and the line was flowing while the sample was taken | Trapped bubbles | Let the sample stand, retest and confirm whether bubbles are present |
| Both results are stable but below what GB/T 507 requires | The oil itself | Move to an analysis of the water content and the cleanliness together |
| The water content meets the requirement and the breakdown voltage still does not | Particles and the state of the oil | Review against the NAS 1638 cleanliness class |
What belongs on the request form
The information on the request form decides whether the report can be used for the decisions that follow. It should carry at least four items: the source of the sample and the point it was taken from, the oil temperature at sampling, the standing time and the storage conditions between taking the sample and sending it, and the present state of use of the oil. With these written down, the laboratory can work to a uniform basis and two reports can be compared at all.
The sampling itself has to meet the same standard. Where the sample is taken from, whether there is residual oil in the line, whether the container is clean and dry, and whether the sample has been contaminated by anything else on site, all decide what the sample that reaches the laboratory actually stands for. Once sampling has gone wrong, every test that follows is measuring a sample that does not represent the tank.
Reading the breakdown voltage together with the water content
Breakdown voltage is often discussed on its own, as though it were an independent index. In practice it is tied to the water content: the treated oil specification of the ZYD-100 is a water content ≤3 ppm (GB/T 7600), and a high water content lowers the breakdown voltage directly, the two acting on each other as cause and effect.
A low breakdown voltage is therefore better handled by first looking at the water content in the same report, rather than concluding at once that the machine is at fault. If the water content already meets the requirement and the cleanliness is normal while the breakdown voltage is still low, the state of the oil itself is the next question: the degree of oxidation, a different medium that has been mixed in, or traces left behind by an earlier treatment. The number is a clue in itself, and it does its work when read next to the other indices.
Breakdown voltage measures an electrical property, but what it measures first is the state of the sample; when the conditions are not aligned, the difference between two figures says nothing at all.
