In today’s industrial environment, the content of oxygen has an important effect on industrial production, so it is often measured. There are various products on the instrument market, but in terms of measurement principles, there are four main measurement principles for oxygen analyzers:
How the oxygen analyzer works
(1) Thermal magnetic oxygen analyzer
The principle is to determine the oxygen content in the flue gas by using the physical property that the magnetic susceptibility of oxygen in the flue gas component is particularly high. Oxygen is a paramagnetic gas (a gas that can be attracted by a magnetic field is called a paramagnetic gas). It is attracted in an uneven magnetic field and flows to a stronger magnetic field. A heating wire is provided there, so that the temperature of oxygen increases and the magnetic susceptibility decreases, so the magnetic field attractive force is reduced, and the magnetic field is pushed out by the unheated oxygen molecules with higher magnetic susceptibility, which causes the magnetic field to be discharged. “Thermomagnetic convection” or “magnetic wind” phenomenon. Under certain gas sample pressure, temperature and flow rate, the oxygen content in the gas sample can be measured by measuring the size of the magnetic wind.
Because the thermal element (platinum wire) serves as both bridge arm resistance of the unbalanced bridge and heating resistance wire, a temperature gradient occurs under the action of magnetic wind, that is, the temperature of the intake side bridge arm is lower than that of the outlet side bridge.
The temperature of the arm
The unbalanced bridge will output the corresponding voltage value according to the oxygen content in the gas sample.
(2) Zirconia sensor type oxygen analyzer
Zirconia (ZrO2) is a ceramic, a solid with ion-conducting properties. At room temperature, it is a monoclinic crystal. When the temperature rises to 1150 ° C, the crystal form changes into cubic crystals, and at the same time, it shrinks by about 7%. When the temperature decreases, it becomes a monoclinic crystal. If heating and cooling are repeated, ZrO2 will break. Therefore, pure ZrO2 cannot be used as a measuring element. If a certain amount of calcium oxide (CaO) or yttrium oxide (Y2O3) is added to ZrO2 as a stabilizer, and then baked at high temperature, it will become a stable zirconia material. At this time, tetravalent zirconium is replaced by divalent calcium or Trivalent yttrium replacement, while generating oxygen ion holes, so ZrO2 belongs to anionic solid electrolyte. ZrO2 conducts electricity mainly through the movement of holes. When the temperature reaches above 600 ° C, ZrO2 becomes a good oxygen ion conductor.
Oxygen analyzer principle
A platinum electrode is sintered on each side of the zirconia electrolyte. When the oxygen partial pressures on the two sides of the zirconia are different, the oxygen on the side with the higher oxygen partial pressure migrates to the side with the lower oxygen partial pressure in the form of ions, resulting in the oxygen partial pressure. The platinum electrode on the high side loses electrons and becomes positive, while the platinum electrode on the low oxygen side receives electrons and produces negative electricity, thus generating an oxygen potential difference between the two platinum electrodes. This potential is only related to the difference in oxygen content (oxygen concentration difference) in the gas on both sides when the temperature is constant. If the oxygen content on one side is known (for example, the oxygen content in the air is constant), the oxygen content on the other side (such as the oxygen content in the flue gas) can be expressed by the oxygen concentration potential. If the oxygen concentration potential is measured, the smoke can be known, The amount of oxygen in the gas.
(3) Fuel cell oxygen analyzer
Fuel cell oxygen analyzer uses a completely sealed fuel cell oxygen sensor, which is one of the most advanced oxygen measurement methods in the world. The fuel pool oxygen sensor is composed of a highly active oxygen electrode and a lead electrode, and is immersed in a KOH solution. Oxygen is reduced to hydroxide ions at the cathode and lead is oxidized at the anode.
The KOH solution is separated from the outside by a polymer film. The sample gas does not directly enter the sensor, so the solution and lead electrodes do not need to be cleaned or replaced regularly. Oxygen molecules in the sample gas diffuse into the oxygen electrode through the polymer film for electrochemical reaction. The current generated in the electrochemical reaction is determined by the number of oxygen molecules diffused into the oxygen electrode, and the diffusion rate of oxygen is proportional to that in the sample gas.
In this way, the output signal of the sensor is only related to the oxygen content in the sample gas, and has nothing to do with the total amount of gas passing through the sensor. Through the connection of an external circuit, the charge transfer or the current in the reaction is proportional to the oxygen participating in the reaction.
Using this method for oxygen measurement, it is not affected by the reducing gas in the measured gas, and many sample gas processing systems are eliminated. It is faster than the old-fashioned “Gold Net-Pb” primary battery for oxygen measurement, and does not require a long start-up blow-off process. The gas sample of the “Gold Net-Pb” primary battery directly enters the solution, resulting in a large amount of instrument maintenance and fuel The battery method sample gas does not directly enter the solution, and the sensor can work very stably and reliably for a long time. In fact, fuel cell oxygen sensors are completely maintenance-free.
Oxygen analyzer principle
(4) Electrochemical oxygen analyzer
A chemical gas analysis instrument
It measures the composition of a gas based on changes in the amount of ions or changes in current caused by a chemical reaction. In order to improve the selectivity, prevent the measurement electrode surface from being stained and maintain the electrolyte performance, a diaphragm structure is generally used.
There are two types of commonly used electrochemical analyzers: constant potential electrolytic and galvanic cell. The working principle of the constant potential electrolytic analyzer is that a specific potential is applied to the electrode, and the measured gas generates electrolysis on the electrode surface. As long as the potential applied to the electrode is measured, the unique electrolytic potential of the measured gas can be determined, so that the meter has the ability to select and identify the gas to be measured.
Galvani battery analyzer is used to electrolyze the test gas diffused into the electrolyte through the diaphragm, and measure the formed electrolytic current to determine the concentration of the test gas. By selecting different electrode materials and electrolytes to change the internal voltage on the electrode surface, selectivity to gases with different electrolytic potentials is achieved.
Application of oxygen analyzer:
- Pharmaceutical and food packaging, storage, and purge gas monitoring.
- Air separation plant, nitrogen purge welding system, cylinder gas quality.
- Glove box, workshop air monitoring, medical monitoring.
The oxygen analyzer is an oxygen sensor composed of zirconia solid electrolyte, and the signal change is an intelligent online analysis instrument composed of a new microprocessor as the core. Mainly used for air separation, automatic analysis of oxygen content in chemical processes; semiconductor, magnetic material production; float glass, cement building materials industry; automatic analysis of oxygen content in various industrial furnaces, heat treatment processes, and scientific research on electronic components, biopharmaceuticals, etc.
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