Air can be present in the oil in the following states: dissolved, entrained, free and foam. A certain quantity of dissolved air is normal – typically around 10% by volume for mineral oils.
Suspended microscopic air bubbles in oil are described as entrained air and result in clouding of the oil. Entrained air can affect compressibility, heat transfer characteristics, oxidation, cavitation and varnishing (especially through microdieselling).
Free air may be found in air pockets, which are most common in dead zones and high regions. It can impact hydraulic compressibility and lead to vapour lock causing starvation of oil supply and loss of control.
Foam occurs when there is more than 30% air in the lubricant. Excessive foam can lead to poor lubrication, loss of control, poor heat transfer and accelerated oxidation.
An oil’s Air Release properties can be assessed via the ASTM D3427/IP313 test methods.
“Compressed air is blown through the test oil, which has been heated to a temperature of 25, 50, or 75 degrees C. After the air flow is stopped, the time required for the air entrained in the oil to reduce in volume to 0.2% is recorded as the air release time.”