Yokogawa Single Channel Oxygen Analyzer System ZR22/ZR402 - Manual (Page 144)

IM 11M13A01-02E 9-3
9. Calibration
9.1.3 Compensation
The deviation of a measured value from the theoretical cell electromotive force is
checked by the method in Figure 9.2 or 9.3.
Figure 9.2 shows a two-point calibration using two gases: zero and span. Cell electromo-
tive forces for a span gas with an oxygen concentration p1 and a zero gas with an
oxygen concentration p2 are measured while determining the calibration curve passing
between these two points. The oxygen concentration of the measurement gas is deter-
mined from this calibration curve. In addition, the calibration curve corrected by
calibration is compared with the theoretical calibration curve for determining the zero-
point correction ratio represented by B/A
ϫ 100 (percent) on the basis of A, B and C
shown in Figure 9.2 and a span correction ratio of C/A
ϫ 100 (percent). If the zero-
point correction ratio exceeds the range of 100
Ϯ 30 percent or the span correction ratio
becomes larger than 0
Ϯ 18 percent, calibration of the sensor becomes impossible.
21.0 p1 p2 0.51
81.92
ez
e2
e1
es
0
C
A B
Zero-point correction factor = (B/A) x 100 (percent) Correctable range: 100 ± 30 percent
Span correction factor = (C/A) x 100 (percent) Correctable range: 0
±
18 percent
Cell
electromotive
force, mV
Calibration curve
before correction
Corrected calibration
curve (theoretical
calibration curve)
Zero origin
Span-gas concentration Zero-gas concentration
Span origin
Oxygen concentration (percent oxygen by volume)
F9.2E.EPS
Figure 9.2 Calculation of a Two-point Calibration Curve and Correction Factors
using Zero and Span Gases
Figure 9.3 shows a one-point calibration using only a span gas. In this case, only the cell
electromotive force for a span gas with oxygen concentration p1 is measured. The cell
electromotive force for the zero gas is carried over from a previous measurement to
obtain the calibration curve. The principle of calibration using only a span gas also
applies to the one-point calibration method using a zero gas only.