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CGA 351 User’s Manual 5
Chapter 1.
1.1.5 The Zirconium Oxide Oxygen Sensor
The inside and outside of the zirconium oxide 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 4
below.
Figure 4: Oxygen Migration in the Zirconium Oxide 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.
1.1.6 The Heater Control Circuit
The oxygen sensor temperature in the CGA 351 is maintained by a heater, which is part of a complex temperature
control loop. This circuit constantly monitors the temperature input from an RTD, compares it to the set point
temperature, and turns the heater ON or OFF accordingly. The specific type of control circuit, a Proportional Integral
Derivative ( PID ) loop, is used because of the adjustable parameters involved:
• Proportional Band: Because the system cannot respond instantaneously to temperature changes, the actual
temperature of the oxygen sensor oscillates about the set point. In general, increasing the proportional band
reduces the magnitude of these temperature oscillations.
Volts
O
2
2
O
O
2
O
2
2
O
O
2
Sample O
Inside Cell
Oxygen ions migrate through the zirconium oxide
along the concentration gradient.
From Outside
Electrode
From Inside
Electrode
Zirconium Oxide Ceramic
with Lattice Imperfections
Atmospheric O
Outside Cell
When O concentration in sample gas falls, the cell voltage rises
with increased oxygen migration through the zirconium oxide.
2
2
2