Instruction manual

9.0 CALIBRATION SM425 INSTRUCTION MANUAL
42500001 REV A 143
9.0 CALIBRATION
9.1 Calibration Principles
9.1.1 Measurement Principle
This section explains the principles of measurement and calibration for a zirconia oxygen analyzer.
The sensor cell is a zirconia element plated on both sides with platinum forming two electrodes. Zirconia,
a solid electrolyte, becomes a conductor of oxygen ions at high temperatures resulting in a voltage being
generated between the electrodes. This voltage is dependent on the difference between the partial
pressures of oxygen in the reference and process gases. The electrode exposed to the higher partial
pressure of oxygen becomes the negative electrode where the oxygen molecules are ionized by adding
electrons. The ions pass through the zirconia element to the positive electrode and then release the extra
electrons. This reaction is summarized as follows:
Negative electrode: O
2
+ 4e → 2 O
2-
Positive electrode: 2 O
2-
→ O
2
+ 4 e
The electromotive force E (mV) or voltage between the two electrodes, generated by the reaction, is
expressed by Nernst’s equation:
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 (%)
At 750°C (1023°K), the equation can be reduced to:
E = -50.74 log Px/Pa........................................Equation (2)
The sensor element of the SM425 is heated to 750 °C so Equation (2) can be used to calculate the cell’s
theoretical output voltage. Figure 9-1 plots the cell voltage generated by the ratio of the process gas
concentration (positive electrode) to the reference gas concentration (negative electrode) where the
reference gas is assumed to be 21.0%.