Incomplete gas sensor classification principle and application The so-called gas sensor is used to detect the presence of a specific gas within a certain area and/or to continuously measure the gas component concentration. In gas, coal, petroleum, chemical, municipal, medical, transportation, and home security, gas sensors are often used to detect the concentration of flammable, flammable, and harmful gases or their presence or absence, or the consumption of oxygen. In the production and manufacturing fields such as the power industry, gas sensors are also commonly used to quantitatively measure the concentrations of various components in the flue gas to determine the combustion conditions and the emission of harmful gases. In the field of atmospheric environmental monitoring, the use of gas sensors to determine environmental pollution is even more common. Gas sensor classification, from the detection of gas species, often divided into combustible gas sensors (usually catalytic combustion, infrared, thermal conductivity, semiconductor type), harmful gas sensors (generally using electrochemical, metal semiconductor, photoionization, flame Ionized), harmful gas sensors (usually using infrared, ultraviolet, etc.), oxygen (usually paramagnetic, zirconia) and other types of sensors; from the use of the instrument, is divided into portable and fixed; from the gas The method of the sample is divided into a diffusion type (ie, the sensor is directly installed in the measured object environment, the measured gas directly contacts the sensor detection element through natural diffusion), and the suction type (refers to the use of a suction pump and the like, and will be tested The gas is introduced into the sensor detection element for detection. According to whether the measured gas is diluted, it can be further subdivided into a completely inhaled type and a dilution type, etc.; from the analysis of gas components, it can be classified into a single type (only for specific gases) And compound type (simultaneous detection of multiple gas components); according to sensor detection principle, divided into thermal, electrochemical, magnetic, optical , semiconductor, gas chromatography and so on. Thermal gas sensors Thermal gas sensors are mainly classified into thermal conductivity and thermochemical types. The thermal conduction type uses the thermal conductivity of gas to measure the concentration of one or more gas components through the change of the resistance of the thermal element. Its application in the industry has been for decades and its instrument type. More, the gases that can be analyzed are also more extensive (such as H2, CO2, SO2, NH3, Ar, etc.). The thermochemical formula is based on the thermal effect of the chemical reaction of the gas being analyzed. Among them, the oxidation reaction (ie, combustion) of the gas is widely used. It is typically a catalytic combustion gas sensor. The key component thereof is a bridge coated with a combustion catalyst. Detection of combustible gases, such as gas generating stations, gas plants to analyze CO, H2, C2H2 and other combustible gases in the air, coal mining pits for the analysis of CH4 content in tunnels, oil production vessels to analyze the leakage of methane on site, fuel And chemical raw material storage warehouse or raw material workshop analyzes the petroleum vapor and alcohol vapor in the air. Electrochemical gas sensor The electrochemical gas sensor utilizes the electrochemical activity of the gas being measured to electrochemically oxidize or reduce it to distinguish gas components and detect gas concentrations. The more common types of electrochemical sensors are the galvanic cell type (which works like a fuel cell), the constant-potential electrolytic cell type (operates under current forcing, and is a Coulomb analytical type sensor). At present, electrochemical sensors are the most common and most mature sensors for detecting toxic and harmful gases. It is characterized by its small size, low power consumption, good linearity and repeatability, resolution of up to 0.1ppm, and long life. Insufficiency is susceptible to interference, and sensitivity is greatly affected by temperature changes. 3HH electrochemical sensor for H2S detection produced by Honeywell's British City Technology Corporation, with a measurement range of 0 to 50 ppm, a maximum allowable of 500 ppm, a resolution of 0.1 ppm, and an external dimension of approximately 42 mm x 18 mm high, Its main cross-interference sources are CO, SO2, NO, NO2, H2, and so on. The zirconia oxygen sensor is one of the late developments in electrochemical component analysis sensors. It began to appear in the 1960s. Its basic principle of operation is based on the principle of the concentration cell, by measuring the gas to be analyzed and the reference gas due to the oxygen concentration. The differential electromotive force caused by the difference measures the oxygen content in the gas to be analyzed. Because it has the advantages of simple structure, reliable work, high sensitivity, good stability, fast response, and easy installation and use, it develops rapidly. It is often used for oxygen analysis of multi-component gases such as sulfuric acid, air separation, boiler combustion, and oxygen measurement of molten metals. (Alfa, UK) Magnetics Gas Analysis Sensors In magnetic gas analysis sensors, the most common is a magnetic oxygen analysis sensor that uses oxygen's high magnetization characteristics to measure oxygen concentration. Its oxygen measurement range is the widest. A very effective oxygen measuring instrument. Commonly used thermo-magnetic convection oxygen analysis sensors (according to the composition of different methods, but also subdivided into the speed of thermal magnetic, pressure-balanced thermal magnetic) and magnetic mechanical oxygen analysis sensor. Typical applications include chemical fertilizer production, cryogenic air separation, thermal power plant combustion systems, natural gas acetylene and other industrial production of oxygen in the control and chain, emissions, tail gas, smoke and other emissions of environmental monitoring.
Optical Gas Sensors Optical gas sensing technology is one of the latest developments but one of the fastest growing technologies. The types commonly used in the industry include infrared gas analyzers, ultraviolet analyzers, photoelectric colorimetric analyzers, chemiluminescence analyzers, and light scattering analyzers. The principle of infrared type is to use the infrared absorption spectrum characteristic or thermal effect of the measured gas to achieve gas concentration measurement. The commonly used spectral range is 1 ~ 25μm. Common types are DIR dispersion infrared type and NDIR non-dispersive infrared type. The continuous SOA-307/307Dx sulfur dioxide analyzer manufactured by Shimadzu, Japan uses a single-source, two-beam, non-dispersive infrared absorption method that selects a specified frequency band by radiating broadband infrared radiation to the gas to be measured and using wavelength selective detectors. To measure the absorption of infrared radiation at a specific wavelength of SO2, the measurement range is a minimum of 0 to 100 ppm and a maximum of 0 to 1 vol%. The commonly used ultraviolet analyzers are non-dispersive ultraviolet analyzers and ultraviolet fluorescence analyzers. The former is similar to the infrared absorption principle, and is also based on the measurement of natural gas to selectively absorb ultraviolet rays. Its absorption characteristics also obey Beer's law, and the used ultraviolet wavelength range It is 200 to 400 nm. The latter, such as the ultraviolet fluorescence type SO2 analyzer, is a dry-type analyzer. Its working principle is based on that SO2 molecules receive UV energy and become excited SO2 molecules. When returning to the steady state, they produce characteristic fluorescence, and the intensity of the emitted fluorescence is comparable. The SO2 concentration is proportional. UV fluorescence can continuously measure SO2 content in the atmosphere without damaging the sample. Its sensitivity can reach 0～2×10-7 of the measuring range, and the stability can achieve ±2% of full-scale drift at 24h, repeatability of ±2% full-scale, and coexistence of background gas on the measurement. Smaller, with long life and significant maintenance workload. The photoelectric colorimetric method is based on Beer's law to achieve automatic photoelectric colorimetric measurement. Its applicable analysis objects include SO2, NO, hydrocarbons, and halogen compounds. Chemiluminescence analyzers use the principle of photothermal generation accompanied by chemical oxidation. The commonly used chemiluminescence analyzers include an ozone analyzer (using O3-C2H4 to generate photons emitted from a chemiluminescent reaction to measure ozone) and chemistry. Light-emitting NOx analyzer (Using the strong oxidation of O3, so that NO and O3 chemiluminescence reaction to achieve measurement). The light scattering analyzer is a kind of analyzer that is used to measure the turbidity or opacity of gas by the interaction between the light beam and the particles in the gas to generate the scattering (pre-scattering, side-scattering and back-scattering). It is one of the most commonly used analytical instruments in environmental emission monitoring. . Semiconductor Gas Sensors Semiconductor gas sensors are conductive or volt-amperes characterized by the adsorption or reaction of a surface when interacting with a gas, caused by a sensing element made of a metal oxide or a metal-semiconductor oxide material, which causes the carrier to move. Measurement of gas concentration by measurement of characteristics or surface potential changes. From the mechanism of action can be divided into surface control type (using gas adsorption on the semiconductor surface to produce changes in the conductivity of the sensor), surface potential type (using semiconductors to generate gas surface potential or interface potential changes after the gas sensor), volume The control type (based on the change of the volume when the semiconductor reacts with the gas and thus the working principle of the conductivity change). Percentage concentrations of combustible gases can be detected, and ppm levels of toxic and hazardous gases can also be detected. The utility model has the advantages of simple structure, high detection sensitivity, fast response speed and many other practical advantages, but its main disadvantages are that the linear range of the measurement is small, the interference by the background gas is relatively large, and it is easily affected by the ambient temperature. (Japan Figaro) Gas chromatographic analyzer The gas chromatograph analyzer is based on chromatographic separation technology and detection technology. It separates and determines the concentration of each component in the gas sample. Therefore, it is a full analytical instrument. In power plant boiler tests, there are applications. During work, a certain volume of gas sample is periodically taken from the sample introduction device and carried by a clean carrier gas (mobile phase) with a certain flow rate. The column is filled with a solid or liquid called a stationary phase. The relative absorption or dissolving ability of the components of the stationary gas sample is different, so that the components are repeatedly distributed in the two phases, so that the components are separated and flow out of the column in sequence to enter the detector for quantitative determination. According to the detection principle, it is subdivided into two types: concentration detector and quality detector. Concentration detectors measure the instantaneous change in the concentration of a component in a gas, that is, the response of the detector is proportional to the concentration of the component.
A mass detector measures the change in velocity of a component of a gas entering the detector, ie, the response of the detector is proportional to the amount of a component entering the detector per unit of time. The most commonly used detectors are TCD thermal conductivity detectors, FLD hydrogen flame ionization detectors, HCD electron capture detectors, and FPD flame photometric detectors. Among them, TCD detector and HCD detector belong to concentration type, and FLD detector and FPD detector belong to quality type. TCD detector is the earliest and the most widely used universal detector. It has the characteristics of suitable sensitivity, strong versatility, good stability and simple structure. The FLD detector has a high sensitivity to most organic compounds and is generally about 3 to 4 orders of magnitude more sensitive than TCD, capable of detecting trace amounts of ppb species, and has a fast response time. The HCD detector is a selective, high-sensitivity detector with a very high sensitivity to electronegative substances and is 2 to 3 orders of magnitude more sensitive than FID. FPD is widely used for analysis of SO2, H2S, and the like. In summary, the main advantages of the gas chromatograph are high sensitivity, suitable for trace and trace analysis, and the ability to analyze complex multiphase gasses. The disadvantage is that regular sampling cannot achieve continuous sample analysis. The system is more complex and used for laboratory analysis. It is not suitable for industrial site gas monitoring. At present, there are gas chromatographs that use the operation of computer-controlled instrumentation systems and perform data calculations, and can perform component-limited alarms, and also have the ability to automatically check instrument failures.
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