Yokogawa Single Channel Oxygen Analyzer System ZR22/ZR402 - Manual (Page 137)

IM 11M12A01-03E 9-1
9. Calibration
9. Calibration
The following describes the calibration procedures for the EXA XT ZR Zirconia High-
temperature Humidity Analyzer.
9.1 Calibration Briefs
9.1.1 Measurement Principle of Zirconia Humidity Analyzer
A solid electrolyte such as zirconia allows the conduction of oxygen ions at high
temperatures. Therefore, when a zirconia-plated element with platinum electrodes on
both sides is heated up in contact with gases having different partial-oxygen pressures on
each side, oxygen ions flow from a high partial-oxygen pressure to a low partial-oxygen
pressure, causing a voltage. When a sample gas introduced into the zirconia-plated
element with the measurement electrode, and air (21.0 vol % O
2
) is flowed through the
reference electrode, an electromotive force (mV) is produced between the two elec-
trodes, governed by Nernst’s equation as follows:
E = - RT/nF log e y/a ………………………… Equation (1)
where, R = Gas constant
T = Absolute temperature
n: 4
F = Faraday’s constant
y = O
2
vol % on the zirconia element measurement electrode
a = O
2
vol % to 21.0 vol % O
2
on the zirconia element reference electrode
The humidity analyzer uses a sample gas composed of water vapor and air.
(A) For the vol % H
2
O measurement
x: Assuming that H
2
O vol % in a mixed gas is measured:
y = (100 – x)
ϫ 0.21 …………………. Equation (2)
From the above equations (1) and (2), we obtain:
E = -K log y/a = -Klog [(100 – x)
ϫ0.21] /21
= - K log (1 –0.01 x) ……………… Equation (3)
where, K = Constant
Using the above equation (3), we can calculate the water vapor in vol % from the
electromotive force.