Not every place where oil samples are handled has a laboratory next to it. Most oil treatment sites have only a few hand tools and one sampling bottle, but that does not mean there is nothing to do but wait for the sample to be sent off for testing. There are three lines of judgement that need no complicated instruments and still give a clear direction: look at the separation, look at how the oil behaves at two temperatures, and compare the breakdown voltage before and after. These three lines cannot replace a formal test report. What they do is answer, before the sample is sent away, what state this batch of oil is in, and whether the problem with this machine lies in the oil or in the machine.
The first line: separation and clarity in a transparent container
Put the oil you have sampled into a transparent sample bottle or measuring cylinder and let it stand, then look at three positions: top, middle and bottom. If the oil is clear and the bottom graduation of the container can be read through it, there is no visible free water and no suspended matter; a layer at the bottom means that water has settled out; if the whole bottle is cloudy and transmits less light, the oil contains water in an emulsified state and it will not separate on its own.
This step is particularly important for insulating oil. The objective is to bring the water content down to ≤3 ppm (GB/T 7600), and oil that contains water often gives a hint in its appearance first: once the water exceeds a certain amount, a layer forms at the bottom of a standing sample. A clear sample does not mean that the water content is within limits, only that the water has not yet reached an amount the eye can see.
The second line: how the same oil behaves at two temperatures
Viscosity changes with temperature, and the same oil flows and clings to the wall quite differently at different oil temperatures. The method is simple: split the same oil into two containers, keep one beside the machine close to the current working temperature (the temperature control range is 45–80 °C, with the upper limit pulled down to 70 °C when the water content is high) and let the other cool down, then compare the two. If the cold oil is clearly more viscous and draws a thread when it is poured, and the hot oil flows noticeably better, that is normal behaviour and there is nothing to report.
The real signal of an abnormal condition is this: the difference between the hot oil and the cold oil is much larger than expected, or there is a visible separation drift in the hot oil close to the tank. This kind of behaviour points to a change in the state of the oil itself rather than to a fault in the machine. Keep one sample at each of the two temperatures; they are useful as references when the sample is later sent for testing.
The third line: the breakdown voltage before and after
A portable breakdown voltage tester shows how the oil behaves in an electric field. The breakdown voltage required of the machine for insulating oil is ≥65 kV (GB/T 507), and that figure is tied to a corresponding electrode size and test method. The portable instrument used on site is not necessarily identical to the laboratory one, so do not compare the portable reading directly with the standard value. What to do is compare across: with the same instrument and the same sample cup, test the incoming oil first and then the treated oil, and look at the direction of the change.
There is another comparison available on site: take two samples from the same batch, let one stand and stir the other lightly to redistribute the bubbles, then test them separately. A clearly lower reading on the standing sample is often connected with water, bubbles and suspended matter. This is the other side of the same point: a breakdown voltage cannot be judged on the water content alone.
| Line of judgement | What it can show | What it cannot show |
|---|---|---|
| Separation and clarity in a transparent container | Whether there is visible free water and any sign of emulsification | The actual water content, whether it has reached ≤3 ppm |
| Comparison of the flow behaviour at two temperatures | Whether the state of the oil has changed abnormally | The numerical value of the viscosity |
| Portable breakdown voltage compared before and after | The relative direction of the change before and after treatment | Whether the standard requirement of ≥65 kV (GB/T 507) is met |
| Visible particles and the appearance of the oil | Whether the cleanliness is clearly trending worse | Whether NAS 1638 class 5 is satisfied |
What to do after the three lines have been used
- Take three samples from every batch: the incoming oil, the treated oil and a retained sample. Use the same container and the same batch of labels for all three, and write the sampling point and the time on each.
- Record the oil temperature of the day, the flow rate at the time and the running time of the machine; all three figures are needed later when the test results have to be explained.
- In the on-site check record, write the observation only and not a conclusive judgement: write that there is a layer at the bottom of approx. one finger deep, not that the water content is over the limit.
- Keep the retained sample until the test results come back; when the two sets of results do not agree, the sample itself is the starting point for the investigation.
- When the same machine processes several batches of oil in succession, retain one sample from every batch, and do not sample only when there is a doubt.
The three lines on site give a direction, not a number. With the direction right, the sample sent for testing is worth having; the numbers can only come from the instrument.
