A system for sensing a gas stream constituent comprises: (a) a thermally conductive, electrically insulative substrate, (b) a gas-sensing element mounted on the substrate and capable of sensing the constituent, (c) a reference element mounted on the substrate having electrical properties congruent with the gas-sensing element and being insensitive to the constituent, (d) an electronic circuit interconnecting the gas-sensing element and the reference element. The circuit is capable of actuating both of the elements and measuring the voltage difference between the elements. The voltage difference is proportional to the concentration of the constituent in the gas stream.