Year of publication: 2007
How much air is in my oil?
We are repeatedly asked this question, in particular when examining hydraulic and turbine oils, but also increasingly in the intended use of online sensors.
Unfortunately, however, there is no straight answer to this question, because air is contained in every oil. Even in completely transparent oil, in which air cannot be seen in the form of air bubbles, approx. 8 to 11 % can be present in the form of "dissolved air". It depends on its saturation behavior how much air a fresh oil can absorb. This is significantly influenced by oil temperature, oil type, viscosity, pressure in the system, additives and other factors.
Air that no longer remains dissolved in the oil, for example because the air absorption capacity decreases with increasing temperature, forms bubbles. These have a negative effect on the lubricating properties, among other things. If air bubbles form between two friction partners, no lubricant film can form at this point. Pressure fluctuations can lead to a so-called diesel effect or cavitation. Particularly in hydraulic applications and in the lubrication of plain bearings, this generates wear in the form of erosion particles.
The amount of air dissolved in the oil cannot be readily determined. Once visible air bubbles have formed in the oil at operating temperature, the exact determination of the air content does not help much in solving the problem. Due to dwell and storage times on the way of a sample to the laboratory, the air content can change so much that a meaningful evaluation of the value is no longer possible.