The fluids used as pressure media in fluid power systems are often polluted with undesired air bubbles. This entrained air affects the system behaviour, stability and safety. Knowledge of the amount of entrained air inside a hydraulic fluid plays a decisive role in predicting the system behaviour. In addition, this information is necessary when a system or components should be optimised to obtain better air release properties. The content of entrained air highly depends on the static pressure as air is always dissolved in hydraulic pressure fluids up to a certain equilibrium condition.
In this paper, different physical principles (optical, mechanical and electrical) are presented to determine the amount of entrained air in an oil-hydraulic system. Starting from these theoretical ideas, different methods are selected and corresponding test devices are designed and built up. These devices are experimentally investigated by including entrained air into a commonly used mineral oil in a hydraulic system. The tested devices are based on different physical principles. In the end, the methods are compared against each other in terms of accuracy of the results and effort to perform these measurements.