Zirconia Oxygen Measurement Principle

Since zirconia has a simpler structure and a shorter response time (0.1 s to 0.2 s) than existing oxygen meters (such as magnetic oxygen analyzers, electrochemical oxygen meters, and gas chromatographs), the measurement range is wide (from (ppm to 100%), high operating temperature (600°C～1200°C), reliable operation, easy installation, low maintenance, etc., so it is widely used in metallurgical, chemical, power, ceramics, automotive, environmental protection and other industrial sectors .

A porous platinum (Pt) electrode is sintered on both sides of the zirconia electrolyte (ZrO2 tube). At a certain temperature, oxygen molecules on the high-concentration side (air) are adsorbed on the platinum electrode when oxygen concentrations on both sides of the electrolyte are different. Combined with the electron (4e) to form the oxygen ion O2-, the electrode is positively charged, and the O2- ions migrate through the oxygen ion vacancy in the electrolyte to the Pt electrode on the low oxygen concentration side to emit electrons, which are converted into oxygen molecules to make the electrode With negative electricity. The reaction of the two electrodes are: reference side: O2+4e - 2O2 - measurement side: 2O2--4e - O2

In this way, a certain electromotive force is generated between the two electrodes. The zirconia electrolyte, the Pt electrode, and the gas with different oxygen concentrations on both sides constitute an oxygen probe, which is a so-called zirconia concentration cell. The electromotive force E between the two stages is obtained from the Nernst formula: E = (1) where EmV is the concentration cell output, n 4 - the number of electron transfers, which is the R ideal gas constant, 8.314 W·S /mol-T (K) F96500 C; PP1 - oxygen concentration of test gas 0 - reference gas oxygen concentration percentage - Faraday constant, - absolute temperature

This fraction is the basis for oxygen measurement by the oxygen probe. When the temperature at the zirconia tube is heated to 600°C to 1400°C, the gas at the high concentration side is used as a reference gas with a gas having a known oxygen concentration. If air is used, P is used as the reference gas. Combining this value with the constant term in the formula, the actual zirconia battery has a thermoelectric potential, a contact potential, a reference potential, and a polarization potential to generate a local potential CmV. The actual calculation formula is: (0=20.6% (0.2095 /P1) ± CmV) (C local potential (new head is usually ± 1mV) = visible, if you can measure the output electromotive force E of the oxygen probe and the measured gas absolute temperature T, you can calculate the oxygen partial pressure of the measured gas (concentration) P1, which is the basic detection principle of zirconia oxygen probe Third, the structure type and working principle of zirconia oxygen probe According to different detection methods, zirconia oxygen probe is divided into two categories: sampling detection oxygen probe and straight Plug oxygen probe

1. Sampling Detection Oxygen probe sampling detection method is through the guide tube, the measured gas is introduced into the zirconia detection chamber, and then the zirconia is heated to the working temperature (750 °C or more) through the heating element. Zirconia is generally tubular and the electrode is a porous platinum electrode. The advantage is that it is not affected by the temperature of the detected gas. By using different diverting pipes, the oxygen content in various temperature gases can be detected. This flexibility is used in many industrial on-line inspections. The disadvantages are that the reaction time is slow; the structure is complex, and it easily affects the detection accuracy; when there are many impurities in the gas to be detected, the sampling tube is easy to block; the porous platinum electrode is vulnerable to corrosion by sulfur and arsenic in the gas and clogging of fine dust; The heater is usually heated by electric wire, and the service life is not long. When the temperature of the detected gas is low (0 °C ~ 650 °C), or when the measured gas is relatively clean, suitable sampling type detection methods, such as oxygen measurement by a nitrogen generator, oxygen measurement in a laboratory, etc.

2、In-line detection Oxygen probe In-line detection is to directly insert zirconia into the gas under high temperature and directly detect the oxygen content in the gas. This detection method is suitable for the gas temperature to be detected at 700°C～1150°C (special structure It can also be used for high temperatures up to 1400°C. It uses the high temperature of the gas being measured to bring the zirconia to operating temperature without the need for additional heaters (see Figure 3). The technical key of the in-line oxygen probe is the high temperature sealing and electrode problems of the ceramic material. The following shows the structure of two types of in-line oxygen probes. (1) Integral zirconia tube This form is developed from the form of a zirconia tube used in the sampling test method, that is, the original zirconia tube is elongated so that the zirconia can directly extend into the high-temperature gas to be measured. This structure eliminates the need for high temperature sealing. (2) In-line zirconia oxygen probe Because of the need to directly insert zirconia into the test gas, there is a high requirement for the length of the oxygen probe. The effective length is about 500mm to 1000mm, and the special environment length can reach 1500mm. And the detection accuracy, working stability and service life have very high requirements, so the in-line oxygen probe is difficult to use the traditional zirconia oxygen probe of the overall zirconia tubular structure, and take more technically demanding zirconium oxide and oxidation Aluminum tube structure. Sealing performance is one of the most critical technologies of this zirconia oxygen probe. At present, the most advanced connection method in the world is the permanent welding of zirconia and alumina tubes. The sealing performance is excellent. Compared with the sampling detection method, the inline detection has obvious advantages: zirconium oxide directly contacts the gas. The detection accuracy is high, the reaction speed is fast, and the maintenance amount is small.