NOVEL ROOM-TEMPERATURE CARBON-MONOXIDE SENSOR UTILIZING RATE OF CHANGE OF RESISTANCE IN THICK-FILMS OF TIN OXIDE

A room temperature thick-film semiconductor gas sensor has recently been developed in this laboratory for the measurement of carbon monoxide (CO) concentrations down to 10 ppm. Novel features of the fabrication process are the addition of a catalytic additive to enhance the sensitivity to CO, and a binding agent which results in an open porous structure to increase the effective surface area. The resistance R0 of sensors before exposure to CO varies in the range 100 kOMEGA-30 MOMEGA, and the rate of change of sensor resistance dR/dt after exposure to CO is large, attaining a maximum value after several minutes. It is shown that the average rate of change of sensor resistance dR/dtBAR is directly proportional to R0 with proportionality constant 10(-3) s-1. Circuitry is described, based on detection of the rate of change of sensor resistance, for a CO gas sensor capable of detecting small concentrations at the onset of a fire, before smoke particles are emitted.