A method is provided for operating a device for sterilising containers, wherein the containers are transported along a predetermined transport path by a transport device and during the transportation at least one wall of the containers is irradiated with radiation, in particular charge carrier radiation, by at least one first radiation device, and wherein a sensor device is also provided which detects charge carrier radiation and/or radiation caused by the charge carriers and which is arranged such that the radiation incident on the sensor device fluctuates due to the movement of the containers along the transport path, and this sensor device detects at least one characteristic radiation variation. Wherein at least one first limit value is defined and a comparison between this limit value and the radiation variation is performed. According to the method, the limit value has a value that changes over time.
