Yokogawa Single Channel Oxygen Analyzer System ZR22/ZR402 - Manual (Page 143)

IM 11M13A01-02E 9-2
0.1 0.5 1 5 10 21.0 50 100
120
100
80
60
40
20
0
-20
-40
0.51% 0 2 ,81.92mV(Zero origin of calibration)
21.0% O 2 , 0mV
(Span origin of calibration)
Oxygen concentration (vol %)
Cell
voltage
(mV)
F9.1E.EPS
Figure 9.1 Oxygen concentration in a Measurement Gas vs Cell Voltage (21% O
2
Equivalent)
The measurement principles of a zirconia oxygen analyzer have been described above.
However, the relationship between oxygen concentration and the electromotive force of
a cell is only theoretical. Usually, in practice, a sensor shows a slight deviation from the
theoretical value. This is the reason why calibration is necessary. To meet this require-
ment, an analyzer calibration is conducted so that a calibration curve is obtained, which
corrects the deviation from the theoretical cell electromotive force.
9.1.2 Calibration Gas
A gas with a known oxygen concentration is used for calibration. Normal calibration is
performed using two different gases: a zero gas of low oxygen concentration and a span
gas of high oxygen concentration. In some cases, only one of the gases needs to be used
for calibration. However, even if only one of the gases is normally used, calibration
using both gases should be done at least once.
The zero gas normally used has an oxygen concentration of 0.95 to 1.0 percent oxygen
by volume with a balance of nitrogen gas (N
2
). The span gas widely used is clean air (at
a dew-point temperature below -20
Њ C and free of oily mist or dust, as in instrument air).
For best accuracy, as the span gas use oxygen whose concentration is near the top of the
measurement range, in a nitrogen mixture.