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Based on a single semiconductor (sc) gas sensor and sensors for temperature, relative humidity and gas flow we developed a system for fire detection in coal mines. These sensors are read-out by a high-dynamic-range hardware control and data acquisition platform, in which the sc-sensor is operated with an application-optimized temperature cycle to improve the stability and selectivity. A hierarchical evaluation strategy not only allows identification of smoldering fires (signified by CO and ethene in a specific ratio) but is also suitable for separating or quantifying typical interfering compounds (such as CH4, CO, NOX or H2) caused by operational situations thus helping to avoid costly false alarms. Currently, the developed field test system is running underground at different control points in the production line. After initial calibration, which is carried out in the lab, special algorithms for online re-calibration were developed.