On site it is common to bring two oil samples, one before and one after treatment, and ask why the colour has darkened and whether the oil has been damaged by the treatment. The answer starts by establishing which kind of darkening it is, because one part of it is entirely normal and another means the oil can no longer be used. The judgement does not come from guessing with the eye but from the way the sample was taken and from a description of its condition.
Reading a colour has one precondition: the sample container, the background light and the temperature of the sample have to be the same. For the same drum of oil, the conclusion can be exactly the opposite depending on whether the sample is held in a clear glass against a white wall or in a brown bottle held against the drum. A valid colour comparison fixes the same container type, the same background and the same temperature, with the two sample cups standing side by side.
Three common normal changes
The first is slight oxidation caused by heating. The treatment process heats the oil in the 45–80 °C range, and the upper limit is brought down to 70 °C when the water content is high. Oil in the heated state oxidises more readily than at ambient temperature, and although it is under vacuum protection, a long residence time and a slightly elevated local temperature will still deepen the colour slowly. This change is uniform and slow: comparing the two sample cups before and after treatment shows a difference, but there is no sediment and no burnt smell.
The second is a change in translucency after dehydration and degassing. Once the water and the dissolved gas have been removed, the refractive index of the oil changes, and oil of the same depth appears more saturated in colour. This is an optical effect and not an increase in contamination. On many sites this change is mistaken for the oil having been contaminated.
The third is the disturbance of the sediment in a dark oil. Oils that are dark to begin with, such as guideway oil and heavy-duty gear oil, settle the dark material to the bottom of the drum when left standing, and once the drum is connected to the machine the screw oil pump ZH and the circulation break that sediment up, so the whole drum appears to have stepped up in colour. After standing for a few hours and comparing the colour again, the settled layering can usually be seen again, and that is the key characteristic for the judgement.
The three colour changes that really need watching
- Black with a metallic sheen: this normally goes with metal particles such as iron or brass swarf in the oil, and it is a sign of mechanical wear in the machine or in an upstream component.
- Red or brown: the typical appearance of high-temperature cracking or local carbonisation, and it means the oil has been in an overheated state at some point for a long time.
- Darkening together with flocculent or gummy deposits: the oil has started to deteriorate and has formed insoluble products, and these particles will directly contaminate the downstream equipment.
When the machine has to be taken out of service
| Observed phenomenon | Assessment | Action |
|---|---|---|
| Uniform and slight darkening, no sediment after standing, no unusual odour | Usually slight oxidation or a change in translucency | Continue to use it and keep a reference sample before the next treatment |
| The dark oil shows layering again after it has been left standing | The sediment has been disturbed, a common phenomenon | Continue to use it and let it stand for a while before connecting it to the machine |
| Black with a metallic sheen | Possible metal particles | Stop the machine, check the pump and the upstream components, and carry out a cleanliness test |
| Red or brown, with a burnt smell | Local overheating or carbonisation | Stop the machine immediately, cool it down and check the heating and the oil circuit |
| Darkening together with flocculent or gummy deposits | The oil has deteriorated and formed insoluble products | Take it out of service and carry out cleanliness and water content tests before deciding |
How to confirm which kind it is
First, make a comparison: keep a sample before treatment and a sample after treatment, and compare the two cups against the same background. Second, check the odour: a burnt smell or a sour smell means a completely different nature, and the detailed judgement is given in the article on odour. Third, check the settling: after standing for several hours, is there layering? Layering means the sediment has been disturbed. Fourth, check the figures: compare the cleanliness against the NAS 1638 class 5 line, and if the particle count does not meet it, the problem is not the colour but contamination.
Any action that involves opening a cover or taking a sample has to be preceded by stopping the machine, releasing the pressure and disconnecting the power, and it should only be done once the oil temperature has fallen to a level that is not too hot to touch, since hot oil is both a scald hazard and under a slight vapour pressure.
If the oil colour darkens, do not panic: first look for an unusual odour and for sediment, then look for layering after the oil has stood. Uniform darkening is normal; it is a black or red colour that calls for a shutdown.