Detailed explanation: Gas sensor classification principle and application

A gas sensor is a meter for detecting the presence or absence of a specific gas within a certain area and/or continuously measuring the concentration of a gas component. In coal mines, petroleum, chemical, municipal, medical, transportation, home and other safety protection, gas sensors are often used to detect the concentration of flammable, flammable, toxic gases or their presence or absence, or the consumption of oxygen. In the field of manufacturing such as the power industry, gas sensors are also commonly used to quantitatively measure the concentration of each component in the flue gas to determine the combustion situation and the emission of harmful gases. In the field of atmospheric environmental monitoring, it is more common to use gas sensors to determine environmental pollution.

Gas sensor classification, from the detection gas type, is often divided into flammable gas sensors (usually using catalytic combustion, infrared, thermal conductivity, semiconductor), toxic gas sensors (usually using electrochemical, metal semiconductor, photoionization, flame) Ionized), harmful gas sensors (usually using infrared, ultraviolet, etc.), oxygen (often using paramagnetic, zirconia) and other types of sensors; from the use of the instrument, divided into portable and fixed; from the gas The method of the sample is divided into a diffusion type (that is, the sensor is directly installed in the environment of the object to be measured, the measured gas is directly in contact with the sensor detecting element through natural diffusion), and the suction type (refers to the method to be tested by using a suction pump or the like) The gas is introduced into the sensor detecting element for detection. It can be subdivided into a complete inhalation type and a dilution type according to whether the measured gas is diluted or not. From the analysis gas component, it is divided into a single type (only for specific gas detection). And composite (simultaneous detection of multiple gas components); according to sensor detection principle, divided into thermal, electric Chemical formula, magnetic, optical, semiconductor type gas chromatography type.

Thermal gas sensor

Thermal gas sensors are mainly of two types: thermal conductivity and thermochemical. Thermal conductivity is the use of the thermal conductivity of a gas to measure the concentration of one or several gas components by changing the resistance of the thermistor. It has been used in industry for decades, and its instrument type More, the gas that can be analyzed is 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. The oxidation reaction (ie combustion) of the gas is widely used. It is typically a catalytic combustion gas sensor. The key component is a Wheatstone bridge coated with a combustion catalyst. It is used to detect combustible gases, such as gas generating stations, gas plants for analyzing combustible gases such as CO, H2, C2H2 in the air, coal mines for analyzing CH4 content in tunnels, and oil extraction vessels for analyzing methane content leaked at the site. , fuel and chemical raw materials storage warehouse or raw material workshop to analyze petroleum vapor, alcohol ether vapor, etc. in the air. The FGM-3100 catalytic combustion combustible gas detector produced by RAE Systems of the United States has a sampling mode of diffusion type with a detection accuracy of ±2% of full scale and a response time of "15s." The main advantage of the catalytic combustion gas sensor is that it has broad spectrum response to all combustible gases, is insensitive to environmental temperature and humidity effects, and has a nearly linear output signal, and has a simple structure and low cost. However, its main disadvantage is low precision, high operating temperature (internal temperature can reach 700-800 ° C), high current consumption, and is susceptible to adverse effects such as sulfides and halogen compounds.

Electrochemical gas sensor

The electrochemical gas sensor utilizes the electrochemical activity of the gas to be measured, electrochemically oxidizes or reduces it, thereby distinguishing the gas component and detecting the gas concentration. The more common types of electrochemical sensors are galvanic type (which works similarly to fuel cells), constant potential electrolytic cell type (operating under current forcing, and Coulomb analysis type sensors). At present, electrochemical sensors are the most common and mature sensors for detecting toxic and harmful gases. It is characterized by small size, low power consumption, good linearity and repeatability, resolution of generally 0.1ppm, and long life. Insufficient is susceptible to interference, and sensitivity is greatly affected by temperature changes. The 3HH electrochemical sensor for detecting H 2 S produced by Honeywell's British Urban Technology Company has a measurement range of 0 to 50 ppm, a maximum allowable value of 500 ppm, a resolution of 0.1 ppm, and an external dimension of approximately 42 mm. It is 18mm high and its main cross-interference sources are CO, SO2, NO, NO2, H2 and so on. The zirconia oxygen sensor is a relatively late development in electrochemical component analysis sensors. It began to appear in the 1960s. Its working principle is based on the principle of concentration battery, by measuring the oxygen concentration of the gas to be analyzed and the reference gas. The concentration electromotive force caused by the difference is used to measure the oxygen content in the gas to be analyzed. Because of its simple structure, reliable operation, high sensitivity, good stability, fast response, convenient installation and use, it has developed rapidly. It is commonly used in the analysis of oxygen content of multi-component gases such as sulfuric acid, air separation, boiler combustion, and oxygen determination of molten metals.

Magnetic gas analysis sensor

Among the magnetic gas analysis sensors, the most common magnetic oxygen analyzer that uses the high magnetization characteristics of oxygen to measure the oxygen concentration has the widest range of oxygen measurement and is a very effective oxygen measuring instrument. Commonly used are thermomagnetic convection oxygen analysis sensors (which can be subdivided into speed-type thermomagnetic and pressure-balanced thermomagnetic types) and magnetic mechanical oxygen analyzers. Typical applications include chemical fertilizer production, cryogenic air separation, thermal power plant combustion systems, natural gas acetylene and other industrial production of oxygen control and chain, environmental monitoring of emissions of exhaust gas, exhaust gas, flue gas, etc.

Optical gas sensor

Optical gas sensing technology is one of the fastest-growing technologies to start late. Common types used in the industry include infrared gas analyzers, ultraviolet analyzers, photoelectric colorimetric analyzers, chemiluminescence analyzers, and light scattering analyzers.

The infrared working principle is to realize the gas concentration measurement by utilizing the infrared absorption spectrum characteristics or the thermal effect of the gas to be measured. The commonly used spectral range is 1 to 25 μm, and the commonly used types are DIR dispersive infrared type and NDIR non-dispersive infrared type. The SOA-307/307Dx sulfur dioxide continuous analyzer produced by Shimadzu, Japan is measured by a single-source dual-column non-dispersive infrared absorption method, which is to select a specified frequency band by radiating broadband infrared rays to the gas to be measured and using a wavelength selective detector. This measures the absorption of infrared radiation of a specific wavelength of SO2, and its measurement range is a minimum of 0 to 100 ppm, and a maximum of 0 to 1 vol%.

Commonly used ultraviolet analyzers are non-polarized ultraviolet analyzers and ultraviolet fluorescent analyzers. The former is similar to the infrared absorption principle. It is also based on the measured gas to selectively absorb ultraviolet light. Its absorption characteristics also comply with Beer's law. The ultraviolet wavelength range used. It is 200 to 400 nm. The latter, such as the UV-fluorescent SO2 analyzer, is a dry-type analyzer. The working principle is based on SO2 molecules that receive ultraviolet energy into SO2 molecules, which produce characteristic fluorescence when returning to steady state, and the fluorescence intensity emitted by it. The SO2 concentration is proportional. Ultraviolet fluorescence can continuously and automatically measure the SO2 content in the atmosphere without destroying the sample. The sensitivity can reach 0~2&TImes;10 -7 in the measurement range. The stability can be ±2% of the full scale drift at 24h, the repeatability is ±2% full scale, and the influence of coexisting background gas on the measurement. Smaller, with a long life and a small maintenance workload.

The photoelectric colorimetric method is based on Beer's law to achieve automatic photoelectric colorimetric measurement, and the applicable analysis objects are SO2, NO, hydrocarbons, halogen compounds, and the like. Chemiluminescence analyzers work by using the principle of photothermal generation associated with chemical oxidation reactions. Commonly used chemiluminescence analyzers include an ozone analyzer (photons emitted by the chemiluminescence reaction using O3-C2H4 to measure ozone) and chemistry. Illuminated NOx analyzer (using the strong oxidation of O3 to make a chemical reaction between NO and O3 to achieve measurement).

The light scattering analyzer uses the light beam to interact with the particles in the gas to generate scattering (front scattering, side scattering, back scattering) for gas turbidity or opacity measurement. It is one of the most commonly used analytical instruments in environmental emission monitoring. .

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