The so-called gas sensor refers to a device for detecting the presence or absence of a specific gas within a certain area and/or continuously measuring the concentration of the gas component. In coal, petroleum, chemical, municipal, medical, transportation, home security and other aspects, commonly used in the detection of combustible gas sensor, the concentration of flammable, toxic gases or existence, or the consumption of oxygen and so on. 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), toxic gas sensors (usually used electrochemical, metal semiconductor, photoionization, flame ionization type), hazardous gas sensors (often with infrared, ultraviolet, etc.), oxygen (often using paramagnetic, zirconia) and other other types of sensors; from the meter to use, portable and fixed into; gas obtained from the way the sample is divided into diffusion (i.e., the sensor is mounted directly measured object environment, found by natural diffusion of gas into direct contact with the sensor detection element), the suction means of formula (aspirator means through the use of the like, to be tested gas is introduced into the sensor detecting element for detecting whether the measured gas diluted according subdivided into completely inhaled and dilution type, etc.); from the analysis of the gas components, into a single formula (to detect only a specific gas) And compound type (simultaneous detection of multiple gas components); according to sensor detection principle, divided into thermal type, electric Chemical formula, magnetic, optical, semiconductor type gas chromatography type.
Thermal gas sensor
Thermal gas sensors mainly include thermal conductivity and thermal chemical 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, of which the oxidation reaction (ie, combustion) of the gas is widely used. It is typically a catalytic combustion gas sensor. The key component is the Wheatstone bridge coated with a combustion catalyst. For the detection of combustible gases, such as gas generating stations, gas plants for the analysis of CO, H2, C2H2 and other combustible gases in the air, coal mining pits for the analysis of the CH4 content in the tunnel, the leakage of methane on the analysis site of oil mining vessels , Fuel and chemical raw materials storage warehouse or raw material workshop to analyze the oil vapor in the air. The FGM-3100 catalytic combustion type flammable gas detector produced by RAE Systems Inc. of the United States has a sampling mode of diffusion, with a detection accuracy of ±2% of full scale and a response time of <15 seconds. The main advantage of catalytic combustion type gas sensor has a broad spectrum in response to all of the combustible gas, ambient temperature, humidity insensitive, near-linear output signal, and its simple structure and low غير مجاز مي باشدt. But its main disadvantage is a low accuracy, high operating temperatures (up to an internal temperature of 700 ~ 800 ℃), high current consumption, is susceptible to the adverse effects of sulfide poisoning, halogen compounds and the like.
electrochemical gas sensor is an electrochemical gas sensor using electrochemically active gas to be measured, which is an electrochemical oxidation or reduction, whereby resolution gas composition, gas concentration detected. 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. Honeywell's British company produced by City Technology 3HH electrochemical sensor for the detection of H2S, the measurement range 0 ~ 50ppm, 500 ppm maximum allowed value, a resolution of 0.1 ppm or, the outer diameter dimension of approximately 18mm high 42mmX The 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 sulphuric acid, air separation, boiler combustion, and oxygen measurement of molten metals.
Magnetic sensors
gas analysis in gas magnetic sensor analysis, the most common is the use of oxygen measured magnetization characteristic of the magnetic high oxygen concentration analysis oxygen sensor measuring oxygen in the widest range , is 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, exhaust gas, smoke and other emissions of environmental monitoring.
Optical Gas Sensors
Optical Gas Sensor Technology is one of the fastest-growing technologies to start late. 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 irradiating the measured gas with broadband infrared rays 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 they return to a steady state, they produce characteristic fluorescence, and the intensity of fluorescence emitted by them The SO2 concentration is proportional. UV fluorescence can continuously measure the SO2 content in the atmosphere without damaging the sample. Affect the sensitivity of the measurement range of up to 0 ~ 2 × 10-7, stability can be achieved at 24h drift of ± 2% of full scale, repeatability of ± 2% of full scale, and the coexistence of the background gas 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, edge scattering and backscattering). It is one of the most commonly used analytical instruments in environmental emission monitoring. .
semiconductor gas sensor is a semiconductor gas sensor detecting element made of a metal oxide or a metal oxide semiconductor material, or surface adsorption generated upon interaction with the reaction gas in accordance with, causing movement of carriers is characterized The gas concentration measurement is performed by changing the conductivity or volt-ampere characteristic or surface potential. From the mechanism of action, it can be divided into surface control type (sensitive element that uses gas adsorption on the surface of the semiconductor to generate conductivity change), surface potential type (gas sensing element that produces surface potential or interface potential change after the semiconductor adsorbs gas), and 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. Having a simple structure, high sensitivity, fast response, and many practical advantages, but its main drawback is small linear measurement range, greater interference by background gas, susceptible to environmental temperature and the like.
GC analyzers
gas chromatograph analyzers are based on chromatographic separation techniques and detection techniques, separation and determination of the concentration of each component in the gas sample, thus the whole analytical instruments. 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 fixed gas sample is different, so that the components are repeatedly distributed in the two phases, so that the components are separated, and the chromatographic column flows out of the column and enters the detector for quantitative determination. According to the detection principle, it is subdivided into two types: concentration detector and mass 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.
The mass detector measures the change in the velocity of a component of the gas entering the detector, ie the response of the detector is proportional to the amount of a component entering the detector per unit 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. Currently, there are gas chromatographs that use the operation of computer-controlled instrumentation systems and perform data calculations, and can perform component-limited alarming, and also have the ability to automatically check instrument failures.
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