A radar level gauge system for determining the filling level of a filling material in a tank is disclosed. The system comprises a transmitter for transmitting measuring signals towards the surface of the filling material; a receiver for receiving echo signals from the tank; processing circuitry for determining the filling level of the tank based on said echo signal and adapted to compare said determined filling level with at least one preset threshold value and to produce an alarm signal in dependence of said threshold value; and a storage means for storing the preset threshold value, wherein said threshold value in said storage means is replaceable and/or adjustable. The alarm signal could indicate high level and/or overfill. The possibility of adjusting/replacing the threshold values enables adjustment of the alarm levels when the system is already installed.A corresponding method of operation is also disclosed.