Compact, DC-electrical biased sulfur dioxide sensing elements for use at high temperatures

Fabrication and operation of sensing elements for the detection of sulfur dioxide (SO2) at high temperature (800–900 °C) are reported. The sensing elements consisted of three (two oxide and one Pt) electrodes on yttria-stabilized zirconia substrates. To operate the elements, a DC current (on the order of 0.1 mA) is driven between two of the electrodes and the voltage between one of these electrodes and the third electrode is used as the sensing signal. These sensing elements respond very strongly to SO2, for example when one of these sensing elements was operated at 900 °C, 2 ppmV of SO2 in a background of 7 vol% O2, balance N2 was found to produce a >10% change in the sensing signal, which could be easily detected. Sensing elements fabricated to be nominally identical were shown to yield qualitatively identical sensing behavior, and temperature, oxygen content, and flow were all found to strongly impact sensing performance. The effect of interferents, such as NOx and CO, was evaluated and found to be relatively small in comparison to the SO2 response. The sensing response, over a 1 month period, was very stable, with the ratio of the average change in sensing signal over one day to the average sensing signal magnitude being about 0.1%.