Surface adsorption and gas consumption in restricted flow, thermally driven, gas sensors

Abstract

The operation of a new type of thermally cycled gas sensor is examined. Taguchi-type tin oxide semiconductor gas sensors are mounted within a small chamber, along with a platinum ribbon, and access to the outside atmosphere is restricted to a narrow opening. During the thermal cycling of either the sensor heater voltage or the platinum ribbon current, gas will 'breathe' in and out of the chamber. The outside atmosphere contains the reducing gas to be tested mixed in normal air. When cycling the sensor temperature, gas will enter as the tin oxide cools, and adsorb onto the surface. When the tin oxide is reheated the adsorbed gas will quickly oxidise, producing a sharp rise in conductance. Increasing the rate of bulk gas consumption in the enclosed chamber, by increasing the constant current supplied to the platinum ribbon, the authors are able to examine this process in more detail. In other experiments the sensor heater is kept at a constant voltage and the gas is drawn in by cycling the platinum ribbon current. In this case low-temperature surface adsorption does not take place and direct comparison of gas consumption rates can be made. They examine the response of the TGS Hash 812 and the TGS Hash 813 to ethanol, propane and carbon monoxide gas.