Fill level monitoring relates to a method for controlling the fill level of a collecting vessel, wherein the collecting vessel could be filled with a dielectric medium. A fill level is monitored by a monitoring circuit with a measuring capacitor, the capacity of which changes steadily in correlation with the fill level, at least beyond a fill level range of the collecting vessel. The method includes: determining a capacity of the measuring capacitor; determining the quality of the measuring capacitor; determining a permittivity of the dielectric medium on the basis of the quality of the measuring capacitor; determining a fill level of the collecting vessel on the basis of the determined capacity of the measuring capacitor and the permittivity of the dielectric medium; and fill level-dependent drainage of the dielectric medium from the collecting vessel.