There are two different types of Zirconia measurement systems.

1      amperometric

2      potentiometric

Figure shows a schematic of the amperometric system. An amperometric sensor consists of  zirconia cell that has a high voltage being applied across the two platinum electrodes. Oxygen then flows through  the diffusion hole and comes into contact with the cathode of the heated zirconia element. As the Oxygen interacts with the cathode a current is generated. The measured current is proportional to the diffusion rate from the sample gas to the cathode. This output current is linear to the oxygen concentration.

Advantages to this type of system are this it is capable of measuring trace oxygen concentration from ppm to percentage level, and calibration is required only on the span side (air). The disadvantages are that if the sample gas contains a flammable gas, a measurement error occurs, and presence of dust causes clogging of the gas diffusion holes on the cathode side; a filter must be installed in a preceding stage.

Figure  shows the basic potentiometric method which uses a solid electrolyte such as zirconia with platinum electrodes attached that when heated the device acts as an oxygen concentration cell.

Basically a concentration gradient of Oxygen ions is established within the Zirconia lattice which producesa voltage potential between the platinum electrodes according to the NERNST equation:
E = − RT/nF ln P𝑋/PA
Where, R: Gas Constant
                N: 4
                F: Faraday’s constant
                 T: Absolute temperature
               PX: Oxygen concentration in a gas (measurement gas) in contact with the positive electrode   
               PA: Oxygen concentration in a gas (reference air) in contact with the negative electrode (%)

The Zirconia becomes conductive only at high temperatures: above 600°C so use of  heater that controls the temperature of the Zirconia to 750˚C.

At that temperature the equation becomes: E = E0 − 50.74 log P𝑋/PA , which equals E = −50.74 log (O2/21)  when instrument air is used as reference gas and when pressure is equal on both sides.

If the Oxygen concentration on both sides of the Zirconia is equal then the ions move randomly through the crystal and no voltage is generated. However, if the concentration is different on both sides, then the ions move to the low concentration side because there is equilibrium between Oxygen molecules and ions. The relationship between the voltage generated and the O2 partial pressure is logarithmic. The Oxygen molecules react in the following manner:

Negative electrode (Higher oxygen partial pressure side) O2 + 4e → 2O2-
Positive electrode (Lower oxygen partial pressure side) 2O2- → O2 + 4e

When plant air is used as the reference gas, the theoretical relationship between the oxygen concentration of the measurement gas and the electromotive force of the zirconia sensor (= cell) is as shown in Figure  In practice, sensors will exhibit slight deviations from the theoretical value but this deviation is corrected when the sensor is calibrated.

It is important to note that the sensor (zirconia element) is heated to 750°C during measurements. If the process gas contains combustible gases such as carbon monoxide, hydrogen, and methane, these gases burn at the detector and consume oxygen, causing the oxygen concentration measured to be less than the actual value. Therefore, zirconia oxygen analyzers should be used only when the effect of coexisting combustible gases can be ignored or when their effect on oxygen concentration can be corrected. Generally, exhaust gases after combustion that are emitted from combustion equipment such as boilers and industrial furnaces have been completely burned; therefor the volume of combustible gases is very small in comparison with oxygen and their influence can be ignored.

Leave a Reply