Sensor for monitoring the air in enclosed spaces with gas-filled operating equipment to measure different trace gases.The sensor unit detects the smallest concentrations of the gas to which it is tuned/calibrated and permanently displays the current measured value. None of the DILO sensors contain a radioactive source, so no special precautions are required with regard to radioactive hazardous substances during operation. The sensors also feature a fast response time with a stable measurement result. The settings for the warnings and alarms of each sensor can be freely configured. If the standard configuration is to be changed, this can be done via the connected monitor.