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Figure 3: Image of a real commercial oxygen potentiometric sensor made of YSZ. The sensing cell consists in a Pt electrode exposed to the unknown gas, the ceramic electrolyte (YSZ), and another Pt electrode exposed to air via the reference channel, which is not in contact with the measured gas in the normal operation mode of the sensor. Also, a protective and diffusive layer is put over the measuring electrode. The layer is visually indistinguishable from the electrode.