The semistor: a new concept in selective methane detection

Technology associated with both catalytic and semiconductor-based gas sensors has been combined to produce hybrid devices capable of detecting low levels of combustible gases. First-generation non-selective sensors, which respond to a wide range of reducing gases including hydrogen, carbon monoxide, lower hydrocarbons and ethanol vapour, were obtained by coating narrow diameter platinum wire coils with a low resistivity tin dioxide-based formulation. As in the case of catalytic sensors, semiconductor-based pelli stors (semistors) are typically operated in a bridge-type circuit. However, in contrast to catalytic sensors, which are used to detect percentage levels of combustible gases, semistors are responsive to sub-100 ppm concentrations. Optimisation experiments have been carried out on factors affecting performance, such as sensing layer composition, coil pitch and firing time. A further study on the effects of incorporating an additional catalyst-containing surface filter layer has led to the development of a second-generation methane-selective sensor. Undesirable humidity and ambient temperature effects, normally associated with tin dioxide-based sensors, can be eradicated by operating a semistor in conjunction with a compensating element. This utilises a filter layer, which removes all target gas response while retaining the same humidity and temperature dependence as the sensor.