Why you need to measure air contamination in oil

Despite the fact that most people who work with industrial lubricants seem to be only too aware of the steps needed to measure moisture in oil samples, and the importance of doing so, too few people seem to be familiar with the methods used to ascertain the levels of air in a lubricant sample. This is despite the fact that air is a major oil contaminant, which can cause a serious degradation in oil quality and machinery in a number of circumstances.

In normal circumstances, the amount of dissolved air in mineral oils is likely to be 10% by volume of lubricant. If there are significantly higher levels of air in an industrial lubricant, it could cause an acceleration in oxidation and additive depletion, which would obviously have knock on effects to the efficiency and effectiveness of oil and the machinery in which it is located.

Entrained air

One type of problematic air contamination is that of entrained air. This is air that is unstable and comes in the form of suspended microscopic air bubbles that can cause clouding of lubricant.

If entrained air is present in oil, it can affect its ability to be compressed, as well as compromising heat transfer, and causing oxidation, varnishing and cavitation, amongst other things. This is why it is so important to check for this kind of contamination.

If entrained air is present in an oil sample, air could rise to the surface.

Free air

Free air can be found in oil where there are dead zones, standpipes and high regions that are trapping air. This kind of air-based lubricant contamination can affect everything from loss of system controls to corrosion, hydraulic compressibility and even vapor lock.


The third kind of air contamination comes in the form of foam, and typically occurs when oil reaches levels of over 30% air contamination. When this happens, foam can often be seen on the surface of lubricants in sumps and aerated oil tanks.

An excess of foam can often leak out of machines and causes machinery compressibility problems, along with loss of system controls and corrosion.

As you can see, an excess of air in lubricants can be harmful to machinery, but it would be fair to say that entrained air is the most harmful type of air contamination as it has the most potential to cause a wide range of problems, including overheating and pump cavitation. This is why it really is important to test air levels on a regular basis, even in high-quality and high-performance oils like Mobil SHC Gear 220.