Yokogawa Integral Oxygen Analyzer ZR202 - Manual (Page 130)

<9. Calibration>
9-2
IM 11M12A01-02E 8th Edition : Jan.13,2012-00
Figure 9.1 Oxygen concentration in a Sample Gas vs Cell Voltage (21 vol%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 requirement, 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 vol%O
2
with a balance of
nitrogen gas (N
2
). The span gas widely used is clean air (at a dew-point temperature below -20
°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.
0.1 0.5 1 5 10 21.0 50 100
120
100
80
60
40
20
0
-20
-40
0.51 vol% O 2 ,81.92mV(Zero origin of calibration)
21.0 vol% O 2 , 0mV
(Span origin of calibration)
Oxygen concentration (vol % O 2 )
Cell
voltage
(mV)
F9.1E.EPS