Instruction manual

SM425 INSTRUCTION MANUAL 9.0 CALIBRATION
Figure 9-1 Cell Voltage vs. Oxygen Concentration
The measurement principles of a zirconia oxygen analyzer have been described above. However, this
relationship between oxygen concentration and the electromotive force of a cell is only theoretical. In
practice, a sensor shows a slight deviation from the theoretical value and calibration is necessary to
obtain accurate measurements. Calibration generates a curve, which compensates for this deviation.
9.1.2 Calibration Gases
Gases with known oxygen concentrations are used for calibration. A two-point calibration is performed
using two different gases: a zero gas of low oxygen concentration and a span gas of high oxygen
concentration. It is recommended that a two-point calibration be conducted at least once even if the one-
point calibration is normally used. A one-point calibration uses only one gas.
Zero gas normally has an oxygen concentration of 0.95 to 1.0 vol% O
2
with a balance of nitrogen gas
(N
2
). Span gas can be instrument air (clean air with a dew-point temperature below -20°C and free of oil
and particulate). For best accuracy, use a span gas with an oxygen concentration near the top of the
measurement range mixed with nitrogen.
9.1.3 Calibration Curves
Calibration curves are generated either by the two-point method shown in Figure 9-2 or the one-point
method shown in Figure 9-3.
Figure 9-2 shows a two-point calibration. Cell electromotive forces for a span gas with an oxygen
concentration p1 and a zero gas with an oxygen concentration p2 are measured and determine the
calibration curve passing through these two points. Subsequent process gas measurements are
determined from this calibration curve.
The generated calibration curve is compared to a theoretical calibration curve to calculate the zero-point
and span-point correction ratios. In Figure 9-2 the zero correction ratio is represented by B/A x 100 (%)
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