The approach flow system connects the stock preparation facility with the paper machine. Here, the pulp for the paper machine is diluted to the required consistency. Additionally, the approach flow system ensures a smooth flow of stock. A particularly important element is the stock deaerator upstream of the headbox. It removes any entrained air under vacuum to ensure a constant, smoothly running process. For effective performance, the stock deaerator always has to be filled to a predefined, millimetre-exact level.