Yokogawa AC8 Calibration Units - Manual (Page 4)

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IM 11M06D03-01E-A
I. INTRODUCTION
There are two types of single channel calibration units offered by Yokogawa. These calibration units are used with
the ZR402G Single Channel Oxygen Analyzer, to perform calibration of either ZR22G or ZO21D (obsolete) Oxygen
Probes.
The AC1 Automatic Calibration Unit utilizes electrical solenoids that are wired to and actuated by the ZR402G. Once
activated by the ZR402G, the calibration gases flow to the ZR22 without the user opening valves or adjusting flow
rates. The MC1 Manual Calibration Unit is not electronically controlled by the ZR402G. During calibration mode, the
user must open valves and adjust the flow rates of the calibration gases. Both the MC1 and the AC1 provide a single
point of control for the calibration system.
II. OVERVIEW
2.1 CALIBRATION PRINCIPLES
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: O2 + 4e 2 O 2-
Positive electrode: 2 O 2- O2 + 4 e
The electromotive force E (mV) between the two electrodes, generated by the reaction, is governed by Nernst’s equa-
tion as follows:
E = -RT/nF ln PX/PA........................................Equation (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˚C, then we obtain equation (1) above.
E = -50.74 log (Px/Pa) ................................. Equation (2)