Yokogawa Integral Oxygen Analyzer ZR202 - Manual (Page 118)

IM 11M12A01-05E 9-1
9. Calibration
9. Calibration
The following describes the calibration procedures for the ZR202G Zirconia High-
temperature Humidity Analyzer (integrated model).
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) 3 0.21 …………………. Equation (2)
From the above equations (1) and (2), we obtain:
E = -K log y/a = -Klog [(100 – x) 30.21 /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.