Abstract: A gas sensing system is provided for the detection of gas concentrations in a flowable medium. The gas sensing system has a gas permeable membrane structure to separate the flowable medium from a chamber where a gas sensor is positioned for detecting the gas concentration in the chamber. The sensor has predetermined environmental operating ranges and the system has a heat drain with a heat sink to maintain the temperature of gas in the chamber within the operating ranges, together with gas nozzles leading into the chamber to purge gas in the chamber and to maintain the gas concentration from exceeding the sensor operating range.