Obviously, the response of the semiconductor sensor differs depending on the kind of gas present in the air. We can take advantage of this fact to increase the sensor sensitivity and selectivity to a specified gas along with the capability to discern the gases in a mixture by measuring the sensor response over a wide range of temperatures. These measurements are performed by our system both during heating as well as during the cooling of the sensor sensitive layer.
Fig. 8 shows the response of the sensor in the presence and absence of wood pyrolysis, alcohol fume, and methane during one such temperature scanning mode. One can see, for instance, that the conductivity curve corresponding to the presence of methane differs substantially from that of wood pyrolysis or alcohol fume. Thus, the analysis of the shape of the response curve may lead to an improvement of the sensor selectivity to methane, and hence to an increase of the precision of the measurements. In contrast, if the temperature of the sensor is kept constant, one may detect the presence of a dangerous gas by the analysis of the minimum of its respective curve. Operating the sensor in constant mode (always
heated) obviously disables the sensor selectivity and drastically decreases its sensitivity. Experiments on cross sensitivity to other gases can be found in the literature [13].