I was asked recently by a client why does the lab not test viscosity at 100’C when there is a high water contamination. The client was quite concerned as he had quoted his client a test suite including viscosity and was concerned he may have to refund the client. This should never be the case as the suites are always assuming the sample is suitable for the tests requested e.g. I have had clients ask for a ppm water value on samples that are 90% water and sediment contents on bags of sand neither of which are possible. Therefore the client needed help explaining to their client why the test could not be performed.
The answer is quite simple in that water boils at 100’C, so the sample would boil and bubble whilst passing through the viscometer tube meaning that the viscosity would be impossible to measure accurately as the water vapour would be trying to travel upwards whilst the oil would want to travel downwards under gravity. In addition if measuring using automated viscometer baths, as most commercial labs use, rather than someone watching by eye with a stopwatch, the water bubbling can sometimes confuse the sensors that determine the start and stop points for kinematic viscosity giving answers of 0 or 9999 cSt.
Therefore, most labs often just quote the viscosity at 40’C in these cases, which is usually sufficient for trending purposes.
For some clients this is not acceptable and it is critical they have a value for viscosity at the high temperatures or perhaps need a viscosity index value. The solution is often to remove the water to give a viscosity at 100’C as if the water were not present. This usually involves slowly heating the sample to remove water. The slow heating is done to minimise boiling of the light ends (smaller molecules in the mix) of the oil which if from a mineral oil base will always be present to some extent. The viscosity can then be performed.
I would not recommend going to this effort on anything but the most critical or costly to change lube systems and most labs will most likely charge for this request as it can be quite time consuming to perform.
One thing to remember is the viscosity given only gives an indication of how the oil viscosity will act once corrective action of removing the water through filtration or purification is performed, as at 100’c in your machinery it will likely boil (assuming roughly atmospheric pressure) meaning loss of lubrication at that point it boils and potential wear to the system.
With repeated boiling of the water the gas bubbles will collect on the same contact points each time, termed condensation nuclei, meaning that repeated bubbles will be produced at the same point. These bubbles may appear small and innocuous, but they collapse and implode causing a mini explosion on that surface causing pinhole corrosion over time. This type of corrosion you may be familiar with in water or water/glycol mix coolant systems where the most common cause of a coolant leak is down to this microscopic corrosion drilling through an entire liner.
So in summary, if there is too much water to perform a viscosity this is likely to be too much water for your system and so the next step should be to correct the water in the system rather than spend too much time worrying about how to measure the viscosity. If it is absolutely critical you can always ask the lab to do some cleaning up of the sample to be able to perform the test for you.
Find out how viscosity is tested below, what is it’s significance and how to diagnose viscosity changes. Then test what you learn in our test your knowledge quiz below.
