Best Ways to Measure Air in Oil

Noria Corporation

"What is the best way to measure the amount of air in your oil?"

Air should be considered a contaminant in oil because it can lead to several adverse effects. For example, the oil may become compressible, which is undesirable for circulation systems and hydraulic applications. Air also speeds up oxidation, thus reducing the oil’s life.

In addition, air contamination may produce cavitation or micro-dieseling (generation of soot in the oil), as well as increase the oil level. If the air level rises significantly, it may even create oil leaks, which would be a safety concern.

Air contamination is not typically measured. When it is identified, the recommendation is to eliminate the root cause, which may be related to the oil’s condition or the result of excessive turbulence or poor equipment design.

The standard laboratory method for measuring an oil’s foam tendency is ASTM D892. This test determines the volume of foam produced after blowing air into an oil sample. It also reports the remaining air volume in the sample following a 10-minute settling period.

Keep in mind that air contamination may be present in four different states: dissolved air, entrained air, foam and air pockets.

Dissolved Air

Dissolved air can be described as micro air bubbles dispersed in the oil. These bubbles are not visible to the naked eye. This form of contamination is quite common, as most new and in-service lubricants will have dissolved air. When performing particle count tests, it will be important to degas the oil. This involves exposing the oil sample to a vacuum (below atmospheric pressure) to extract the dissolved air. The volume of air that can be extracted from a new oil is approximately 20 percent of the oil volume. This step can prevent a false positive particle count when using optical particle counters. Once it has been degassed, the oil volume change is minimal.

Entrained Air

With entrained air, the bubbles will be very small and frequently in large numbers, causing the oil to look hazy or milky. These bubbles are often so small that they cannot be seen with the naked eye. Entrained air is sometimes confused with moisture contamination. Just because an oil sample looks hazy, don’t assume it is due to moisture contamination. It might be entrained air. If the lubricant has good air-release properties, the air will be dissipated in 15 to 30 minutes. Entrained air tends to be more stable in higher viscosity oils, since the viscosity reduces the air-release speed. When there is significant entrained air, it will increase the oil level in the machine.


Foam sits on the oil’s surface and is the result of turbulence or plunging in return lines. It also can be caused by entrained air that rises to the oil’s surface. Foam is considered a significant problem when the oil level is difficult to measure or when it reaches a reservoir’s filling ports or hatches.

Air Pockets

This type of air contamination refers to air that has become trapped in lines when the oil was changed out or after flushing. When identified, it must be purged as soon as possible.

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