A sensor and method for measuring respiratory gas properties are presented. A thermal conductivity sensor is used to measure the thermal conductivity of a gas with unknown composition and/or mass flow rate at different temperatures. The measured thermal conductivities at different temperatures are compared with known thermal conductivities of gases at different temperatures. In an exemplary application the sensor and method are installed in a tube to determine a mass of respiratory air flowing through the tube and a concentration of CO2 therein.