Device for monitoring a tank to prevent fuel from being stolen from a fuel tank

The device (10) has a tank sensor (32), where fuel quantity is measured based on a resistance value of the sensor. A freely programmable microprocessor (34) periodically detects and stores changes of the resistance value of the sensor, and releases a silent alarm signal during changing of the resistance value. A multistage potentiometer (28) cooperates with the sensor. The microprocessor considers only changes of the resistance value, which lies below a lower threshold or above an upper threshold, where the upper and/or lower thresholds are selected based on time and electric units.