Development of instrumentation incorporating solid state gas sensors for measurement of oxygen partial pressure.

Electronic instrumentation was developed for the measurement of the oxygen partial pressure, P1 ( in a sample gas using fully-sealed zirconia pump-gauge oxygen sensors operated in an AC mode. These sensors, operated typically at 700°C, consisted of two discs of zirconia with porous platinum electrodes on each face separated by a gold seal and enclosing a small internal volume. One disc was operated as a pump enabling oxygen to be electrochemically transferred into and out of the enclosed volume; the other disc operated as a gauge, the Nernst EMF across the electrodes providing a measure of the ratio of the internal to the external oxygen partial pressure. By careful design of the circuitry it was possible to measure the oxygen partial pressure, P, without the need for a separate reference gas supply. Subsequently, a novel "tracking" mode of operation was proposed and implemented in which leakage effects generally associated with sealed pump-gauge devices were minimised: the sensor was operated in a feedback control-loop in order to adjust automatically the mean internal reference oxygen partial pressure, P0, so as to maintain the ratio (Px/P0) close to unity. The signal-to-noise ratio was markedly improved by using gauge EMFs with high amplitudes which inevitably display a distorted sinusoid due to the logarithmic term in the Nernst equation. Surprisingly, mathematical analysis predicted that the linearity of the output of the instrument using phase-sensitive detection should not be affected by the deviation from a sinusoid and this was confirmed experimentally: signal processing was practically implemented using simple analogue electronics. As anticipated there was a strong influence of sensor temperature on the output of the instrument: consequently, methods for temperature compensation were proposed and shown to be feasible with minimum hardware. The theory of Operation of leaky pump-gauge was also developed which indicated that a physical leak in the sensor should cause a phase shift and amplitude change in the sensor output. Experimental results were, in general, in agreement with the theory demonstrating the influences of the geometry and dimensions of the leak and of the operating frequency. Importantly, the theory predicted that, when operated in the AC mode, devices with major leakage may still be used for oxygen partial pressure measurement: again this was confirmed by experiment and the additional benefit of a concomitant substantial simplification of the electronic circuitry also realised. Interestingly an unexpected but small influence of oxygen concentration on the phase shift was observed: this requires additional study