Apparatus for sensing combustible gases employing an oxygen-activated sensing element

An apparatus for sensing a constituent gas in a gaseous mixture, said apparatus comprising:

• a sensor element having an exposed surface, said sensor element being composed of metal chosen from the group consisting of noble metals and noble metal alloys;
• a first substrate to which the sensor element is affixed;
• means for heating at least the exposed surface of the sensor element to a temperature above the oxygen activation temperature and below the oxygen desorption temperature of the noble metals or noble metal alloys of the sensor element;
• a reference element composed of an electrically conducting material having a surface that is incapable of becoming oxygen activated;
• a second substrate to which the reference element is affixed;
• a circuit adapted to detect change in a fluctuating current, said circuit including a capacitor, the sensor element and the reference element comprising the plates of said capacitor;
• structure supporting the sensor element and the reference element with their surfaces in a confronting, spaced-apart configuration to form the capacitor;
• means for varying capacitance of the capacitor to cause a fluctuating current in said circuit;
• and
• means for exposing the sensor element to the gaseous mixture to cause a dynamic adsorption and desorption of said constituent gas onto said exposed surface;
• whereby the presence of said constituent gas is indicated by detecting change in the fluctuating current caused by change in work function of the sensor element resulting from adsorption and desorption of said constituent gas on said sensor element.