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The Zirconia Oxygen
Sensor
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General Information
The inside and outside of the zirconia oxygen sensor are coated with
porous platinum, forming two electrodes. The sample gas flows past
the inside of the sensor, while atmospheric air circulates freely on the
outside of the sensor. This atmospheric air is used as the reference gas
for making oxygen measurements. See Figure 1-4 below.
Oxygen ions migrate through the zirconium oxide
along the concentration gradient.
From Outside
Electrode
When O concentration in sample gas falls, the cell voltage rises
2
with increased oxygen migration through the zirconium oxide.
Figure 1-4: Oxygen Migration in the Zirconia Sensor
At the operating temperature of the oxygen sensor, the atmospheric
reference oxygen is electrochemically reduced at the outer electrode,
and the resulting oxygen ions seek to equalize with the lower oxygen
concentration on the sample side of the cell by migrating through the
porous ceramic toward the inner electrode. At the inner electrode they
give up electrons to become oxygen molecules again, and are swept
away by the sample flow.
The lower the concentration of oxygen in the sample gas, the greater
the rate of ion migration through the ceramic, and the higher the cell
voltage due to electron exchange at the electrodes. The cell voltage
rises logarithmically as the amount of oxygen in the sample gas falls,
allowing the accurate measurement of very low levels of excess
oxygen.
O
O
2
2
O
2
Atmospheric O
Outside Cell
O
2
O
O
2
2
Sample O
2
Inside Cell
Zirconium Oxide Ceramic
with Lattice Imperfections
From Inside
Electrode
Volts
December 2004
2
1-5
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