When the oxygen concentration is reference gas (air) > sample gas
Figure 3: Structure and operating principle of an oxygen gas analyzer based on a concentration cell type in a zirconium oxide method
Equation 1:Nernst equation
Solid electrolyte zirconia elements have conductivity for oxygen ions under high temperature conditions.
When a zirconia element with platinum electrodes attached to both sides is heated to over 400 degrees and each gas of different oxygen concentration contacts on the both sides, "electromotive force" is generated by a similar reaction into a cell.
This is called an oxygen concentration cell.
On the electrode side with high oxygen concentration, oxygen molecules gain electrons and become oxygen ions, which reach through the zirconia element to the other electrode side, where they release electrons and return to oxygen molecules. The greater the difference in oxygen concentration on both sides, the greater the electromotive force is generated.
The electromotive force increases as the temperature of the zirconium oxide element is heated. By measuring this electromotive force and the temperature of the zirconia element, the oxygen concentration in the sample gas can be obtained.
The sequence of these electrochemical reactions is represented by the Nernst equation (Equation 1).
When the oxygen concentration is reference gas (air) > sample gas
Figure 3: Structure and operating principle of an oxygen gas analyzer based on a concentration cell type in a zirconium oxide method
Equation 1:Nernst equation
When electric current is applied between the electrodes on both sides of a heated zirconia element, oxygen ions move between the electrodes, and oxygen is transported by pumping action from one electrode to the other. This electrochemical action is called the oxygen pumping action and amount of transported oxygen by pumping action is proportional to applied current.
When the oxygen pumping action is performed in the gas diffusion chamber restricted by the gas diffusion holes, the current keeps a constant value though the zirconia element even if the voltage applied to the zirconia element is increased. This constant current is called limiting current. This limiting current is proportional to the oxygen concentration in sample gas, so the oxygen concentration can be measured by measuring limiting current.
The structural feature of the analyzer is that a sample gas diffusion chamber, reference gas chamber, electrodes, and gas diffusion holes built into the zirconia element, and the entire zirconia element is constantly heated by a heater. (Figure 4-1)
Using this structure and the following operations, oxygen concentration can be measured by the combination method of a oxygen concentration cell and a limiting current. (Figure 4-2, Figure 4-3, Figure 4-4)
Figure 4-1: Structure of oxygen analyzer by using limiting current type in zirconium oxide method.
Figure 4-2: Oxygen pumping action
Current (IP-34) is applied between electrodes 3 and 4 to transfer some oxygen in the sample gas diffusion chamber to the reference gas chamber with oxygen pumping action, and the reference gas chamber is filled the 100% oxygen concentration.
The current required for oxygen pumping action is called the pumping current. (Figure 4-2)
Figure 4-3: Forming concentration cell
At the same time, a pumping current (IP-12) is applied between electrodes 1 and 2, and the oxygen in the sample gas diffusion chamber is exhausted to the outside by the oxygen pumping action to reduce the oxygen concentration in the sample gas diffusion chamber to 0%.
This creates a zirconia concentration cell between the reference chamber and the sample gas diffusion chamber, generating a constant electromotive force (350 mV).
The oxygen concentration of 0% in the gas diffusion chamber and 100% in the reference gas chamber generate the reference condition for the analyzer. (Figure 4-3)
Figure 4-4: Oxygen concentration measurement by limiting current
If there is a concentration difference between the external sample gas and the oxygen in the sample gas diffusion chamber, the sample gas flows into the sample gas diffusion chamber through the gas diffusion holes and the oxygen diffuses.
A pumping current (IP-12) corresponding to the concentration of diffused oxygen is applied, and all oxygen is exhuasted to the outside (0% oxygen concentration) by the oxygen pumping action, maintaining the reference condition shown in Figure 4-3 at all times.
The constant current that maintains this state is called the limiting current (limit).
Since the limiting current is proportional to the concentration of oxygen flowing in through the gas diffusion holes, the oxygen concentration is measured by detecting the limiting current. (Figure 4-4)
Gas analyzers with a zirconium oxide method can be inserted directly into measuring points, and use oxide measurment at heat treatment furnaces, industrial furnaces, engines, boilers and other facilities that require fast response time by the direct measurment. Furthermore, the zirconium oxide method gas analyzer has a simple structure with no moving parts in the detector' sensor, making it highly resistant to the influences of vibration, allowing it to be used at measurement points with vibrations.
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