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The Oxygen Measuring Cell
1-2 | Thermox CG1000 Oxygen Analyzer
The sensing element itself is a closed-end tube or disk made from ceramic
zirconium oxide stabilized with an oxide of yttrium or calcium. Porous
platinum coatings on the inside and outside serve as a catalyst and as
electrodes. At high temperatures (generally above 1200°F/650°C), oxygen
molecules coming in contact with the platinum electrodes near the sensor
become ionic. As long as the oxygen partial pressures on either side of the
cell are equal, the movement is random and no net flow of ions occurs.
If, however, gases having different oxygen partial pressures are on either
side of the cell, a potentiometric voltage is produced (See Figure 1-1). The
magnitude of this voltage is a function of the ratio of the two oxygen par-
tial pressures. If the oxygen partial pressure of one gas is known, the volt-
age produced by the cell indicates the oxygen content of the other gas. A
reference gas, usually air (20.9% O
Since the voltage of the cell is temperature dependent, the cell is main-
tained at a constant temperature. Some newer high temperature insitu
models use the heat from the process to heat the sensor, and the process
temperature is continuously measured and used in the software calcula-
tion. The oxygen content is then determined from the Nernst equation:
where R and F are constants, T is absolute temperature, and O
are the oxygen partial pressures on either side of the cell.
Figure 1-1.
Zirconium oxide cell principle of operation.
), is used for one of the gases.
2
O
RT
1
E =
In
O
4F
2
and O
1
2
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