Yokogawa Multi Channel Oxygen Analyzer System ZR22/AV550G - Manual (Page 151)

IM 11M12D01-01E 9-1
9. Calibration
9. Calibration
9.1 Calibration Briefs
9.1.1 Principle of Measurement
This section sets forth the principles of measurement with a zirconia oxygen analyzer
before detailing calibration.
A solid electrolyte such as zirconia allows the conductivity 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, the element shows the action of the concentration cell. In other words, the
electrode in contact with a gas with a higher partial-oxygen pressure acts as a negative
electrode. As the gas comes in contact with the zirconia element in this negative
electrode, oxygen molecules in the gas acquire electrons and become ions. Moving in
the zirconia element, they eventually arrive at the positive electrode on the opposite side.
There, the electrons are released and the ions return to the oxygen molecules. This
reaction is indicated as follows:
Negative electrode: O
2
+ 4e 2 O 2-
Positive electrode: 2 O 2- O
2
+ 4 e
The electromotive force E (mV) between the two electrodes, generated by the reaction,
is governed by Nernst’s equation as follows:
E =
RT
nF
ln
Px
Pa
................................. (1) -
where, R: Gas constant
T: Absolute temperature
n: 4
F: Faraday’s constant
Px: Oxygen concentration in a gas in contact with the positive
zirconia electrode (%)
Pa: Oxygen concentration in a gas in contact with the negative
zirconia electrode (%)
Assuming the zirconia element is heated up to 750
ЊC, then we obtain equation (1)
above.
E = log
Px
Pa
................................. (2) -50.74
With this analyzer, the sensor (zirconia element) is heated up to 750
ЊC, so Equation (2)
is valid. At that point, the relationship as in Figure 9.1 is effected between the oxygen
concentration of the measurement gas in contact with the positive electrode and the
electromotive force of the sensor (= cell), where a comparison gas of air is used on the
negative electrode side.