If the oil in a system is intended to remain practically free from air (e.g. in aircraft hydraulics), the entire circuit must be placed under a slight excess pressure (approx. 0.3-0.4 bar). This happens using a gas diaphragm or membrane in a closed container so that no external air enters into the container during "breathing" (fluctuations of the oil level); the balance is instead created using a diaphragm or membrane (Figure V 39). The pre-load of the container may also be a suitable measure to improve the suction behaviour of pumps.
Figure V 39: Pre-charged tank