Principle, classification and use of gas sensor!

A gas sensor is a converter that converts a certain gas volume fraction into a corresponding electrical signal. 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, and toxic gases or their presence or absence, or the consumption of oxygen.

Gas sensors are mainly used to detect a specific gas and measure whether the gas exists near the sensor, or the content in the air near the sensor. Therefore, gas sensors are often indispensable in safety systems.

From the working principle, characteristic analysis to measurement technology, from the material used to the manufacturing process, from the detection object to the application field, an independent classification standard can be formed, and a complex classification system can be derived, especially in the issue of classification standards. It has not been unified yet, and it is quite difficult to classify them strictly and systematically.

gas sensor

Classification of gas sensors

From the detection of gas types, it is usually divided into combustible gas sensors (often using catalytic combustion, infrared, thermal conductivity, semiconductor type), toxic gas sensors (generally using electrochemical, metal semiconductor, photoionization, flame ionization), Harmful gas sensors (often infrared, ultraviolet, etc.), oxygen (often paramagnetic, zirconia) and other types of sensors.

From the method of use, it is usually divided into portable gas sensors and fixed gas sensors.

From the way of obtaining gas samples, it is usually divided into diffusion gas sensors (that is, the sensor is directly installed in the environment of the measured object, and the measured gas is in direct contact with the sensor detection element through natural diffusion), and suction gas sensors (meaning that by using suction The gas to be measured is introduced into the sensor detection element for detection by means of an air pump, etc. According to whether the measured gas is diluted, it can be subdivided into complete suction type and dilution type, etc.).

From the analysis of gas composition, it is usually divided into a single gas sensor (detecting only a specific gas) and a composite gas sensor (simultaneous detection of multiple gas components).

According to the sensor detection principle, it is usually divided into thermal gas sensors, electrochemical gas sensors, magnetic gas sensors, optical gas sensors, semiconductor gas sensors, gas chromatography gas sensors, etc.

 

Gas sensors are a large category of chemical sensors. From the working principle, characteristic analysis to measurement technology, from the material used to the manufacturing process, from the detection object to the application field, independent classification standards can be formed, and a complex classification system can be derived, especially in the issue of classification standards. Without uniformity, it is quite difficult to classify them strictly and systematically. Let's first understand the characteristics of the gas sensor:

1. Stability

Stability refers to the stability of the basic response of the sensor over the entire operating time, depending on the zero drift and interval drift. Zero drift refers to the change in the sensor output response during the entire working time when there is no target gas. Interval drift refers to the change of the output response of the sensor continuously placed in the target gas, which is manifested as the decrease of the output signal of the sensor during the working time. Ideally, a sensor has a zero drift of less than 10% per year under continuous operating conditions.

 

2. Sensitivity

Sensitivity refers to the ratio of the change in the sensor output to the change in the measured input, and is mainly dependent on the technology used in the sensor structure. Most gas sensors are designed using biochemical, electrochemical, physical, and optical principles. The first consideration is to choose a sensitive technique that is sensitive enough to detect the valve limit or percentage of the lowest explosive limit of the target gas.

 

3. Selective

Selectivity is also known as cross-sensitivity. It can be determined by measuring the sensor response produced by a certain concentration of interfering gas. This response is equivalent to the sensor response produced by a certain concentration of target gas. This characteristic is very important in applications tracking multiple gases, since cross-sensitivity reduces measurement repeatability and reliability, and ideal sensors should have high sensitivity and high selectivity.

 

 

4. Corrosion resistance

Corrosion resistance refers to the ability of the sensor to be exposed to high volume fractions of the target gas. When a large amount of gas leaks, the probe should be able to withstand 10 to 20 times the expected gas volume fraction. When returning to normal operating conditions, sensor drift and zero correction values ​​should be as small as possible. The basic characteristics of gas sensors, namely sensitivity, selectivity and stability, are mainly determined by the choice of materials. Select appropriate materials and develop new materials to optimize the sensitive properties of gas sensors.

Next is about the detection principles, characteristics and uses of different gas sensors:

1. Semiconductor gas sensor

According to the detection element made of metal oxide or metal semiconductor oxide material, the surface adsorption or reaction occurs when it interacts with the gas, causing the electrical conductivity or voltammetry characteristic or surface potential change characterized by the movement of carriers to carry out the gas concentration. measured.

 

From the mechanism of action, it can be divided into surface-controlled type (a sensitive element that uses gas adsorbed on the surface of a semiconductor to produce a change in conductivity), surface potential type (a gas-sensitive element that uses a semiconductor to adsorb gas to generate a change in surface potential or interface potential), volumetric Control type (based on the working principle that the volume of the semiconductor changes when reacting with the gas, resulting in a change in conductivity), etc. It can detect combustible gas with percentage concentration, and can also detect toxic and harmful gas at ppm level.

Advantages: simple structure, low price, high detection sensitivity, fast reaction speed, etc.

Disadvantages: The measurement linear range is small, it is greatly interfered by the background gas, and it is easily affected by the ambient temperature.

Semiconductor gas sensor

2. Solid electrolyte gas sensor

A solid electrolyte is a solid substance with the same ionic conductivity properties as an aqueous electrolyte solution, and when used as a gas sensor, it is a type of battery. It does not need to make the gas dissolve in the electrolyte through the gas permeable membrane, which can avoid problems such as solution evaporation and electrode consumption. Due to the high conductivity, good sensitivity and selectivity of this sensor, it has been widely used in petrochemical, environmental protection, mining, food and other fields, and its importance is second only to metal-oxide-semiconductor gas sensors.

 

This sensor is between the semiconductor gas sensor and the electrochemical gas sensor, the selectivity and sensitivity are higher than that of the semiconductor gas sensor, and the lifespan is longer than that of the electrochemical gas sensor, so it is widely used. The disadvantage of this sensor is that the response time is too long.

 Solid electrolyte gas sensor

3. Catalytic combustion gas sensor

This sensor is actually a gas sensor based on a platinum resistance temperature sensor, that is, a high temperature resistant catalyst layer is prepared on the surface of the platinum resistance. At a certain temperature, the combustible gas catalyzes combustion on the surface, so the temperature of the platinum resistance increases, resulting in the resistance of the resistance. resistance changes.

Catalytic combustion gas sensor

Since the catalytic combustion gas sensor platinum resistance is usually surrounded by ceramic beads composed of porous ceramics, this kind of sensor is also commonly called catalytic bead gas sensor. In theory this sensor can detect all combustible gases, but there are many exceptions in practical applications. This sensor can usually be used to detect combustible gases such as methane, LPG, acetone, etc. in the air.

4. Electrochemical gas sensor

The electrochemical gas sensor is a detector that measures the current by oxidizing or reducing the gas of the measurement object at the electrode, and obtains the concentration of the object gas. Including galvanic cell type gas sensor, constant potential electrolytic cell type gas sensor, concentration cell type gas sensor and limiting current type gas sensor.

1. Primary cell type gas sensor (also known as: Gavoni cell type gas sensor, also called fuel cell type gas sensor, also called spontaneous battery type gas sensor), their principle is the same as the dry cell we use, but the battery The carbon manganese electrodes were replaced by gas electrodes. In the case of an oxygen sensor, oxygen is reduced at the cathode and electrons flow through the ammeter to the anode, where lead metal is oxidized. The magnitude of the current is directly related to the concentration of oxygen. This sensor can effectively detect oxygen, sulfur dioxide, etc.

 Electrochemical gas sensor

2. Constant potential electrolytic cell type gas sensor, this sensor is very effective for detecting reducing gas. Its principle is different from that of galvanic cell type sensor. Its electrochemical reaction occurs under the force of current, which is a real Coulomb analysis (determined by Faraday's law based on the amount of electricity consumed during electrolysis) sensor. This sensor is used in the detection of carbon monoxide, hydrogen sulfide, hydrogen, ammonia, hydrazine, and other gases, and is currently the mainstream sensor for toxic and harmful gas detection.

3. Concentration battery type gas sensor, the gas with electrochemical activity will spontaneously form concentration electromotive force on both sides of the electrochemical cell, and the magnitude of the electromotive force is related to the concentration of the gas. Examples of such sensors are oxygen sensors for automobiles, solid state Electrolyte-type carbon dioxide sensor.

4. Limiting current type gas sensor, there is a sensor for measuring oxygen concentration. The oxygen (gas) concentration sensor is prepared by using the principle that the limiting current in the electrochemical cell is related to the carrier concentration. It is used for oxygen detection in automobiles and oxygen concentration in molten steel. detection.

Main advantages: small size, low power consumption, good linearity and repeatability, resolution can generally reach 0.1ppm, long life.

The main disadvantage is that it is susceptible to interference, and the sensitivity is greatly affected by temperature changes.

5. PID - photoionization gas sensor

The PID consists of a UV light source and a gas chamber. The principle of ultraviolet light is the same as that of fluorescent tubes, but the frequency is high and the energy is large. After the gas to be tested reaches the gas chamber, the ultraviolet light emitted by the ultraviolet lamp generates a charge flow, and the gas concentration is positively correlated with the size of the charge flow. The gas concentration can be measured by measuring the charge flow. It can detect volatile organic compounds and other toxic gases from 10ppb to higher concentrations of 10000ppm. Many hazardous substances contain volatile organic compounds, and PID is highly sensitive to volatile organic compounds.

PID - photoionization gas sensor

6. Thermal gas sensor

Thermal gas sensors are mainly divided into two categories: thermal conductivity and thermochemical.

The thermal conductivity type uses the thermal conductivity of the gas to measure the concentration of one or several gas components by changing the resistance of the thermistor. Its application in the industry has a history of decades, its instrument types are more, and the gas that can be analyzed is also wider.

The thermochemical formula is based on the thermal effect of the chemical reaction of the gas to be analyzed. Among them, the oxidation reaction (ie combustion) of the gas is widely used, which is typically a catalytic combustion gas sensor. Its main working principle is that at a certain temperature, some metal oxide semiconductors The conductivity of the material changes with the composition of the ambient gas.

Its key component is a Wheatstone bridge coated with a combustion catalyst, which is mainly used to detect combustible gases, such as gas generating stations, gas plants to analyze CO, H2, C2H2 and other combustible gases in the air, and coal mining mines for analysis The content of CH4 in the tunnel, the content of methane leaked on the site of the analysis of the oil extraction vessel, the storage of fuel and chemical raw materials or the raw material workshop to analyze the petroleum vapor, alcohol ether vapor, etc. in the air.

7. infrared gas sensor

A complete infrared gas sensor consists of an infrared light source, an optical cavity, an infrared detector and a signal conditioning circuit. The sensor is made using the absorption of gases at specific frequencies of the infrared spectrum. Infrared light is emitted from the transmitting end to the receiving end. When there is gas, the infrared light will be absorbed, and the received infrared light will be reduced, thereby detecting the gas content. At present, the more advanced infrared type adopts dual wavelengths and dual receivers, which makes the detection more accurate and reliable.

Advantages: good selectivity, only detect gases of specific wavelengths, which can be customized according to the gas; using optical detection method, it is not easy to be poisoned and aged by the influence of harmful gases; fast response speed and good stability; using physical characteristics, no chemical reaction, Good explosion-proof; high signal-to-noise ratio, strong anti-interference ability; long service life; high measurement accuracy.

Disadvantages: narrow measurement range; afraid of dust and humidity, the site environment is better, and it is necessary to regularly clean and maintain the dust on the mirror surface; it cannot be detected when there is airflow on site; the price is high.

 infrared gas sensor

8.  magnetic gas analysis sensor

Among the magnetic gas analysis sensors, the most common is the magnetic oxygen analysis sensor that uses the high magnetization characteristics of oxygen to measure the oxygen concentration. It uses the principle that oxygen in the air can be attracted by a strong magnetic field. Its oxygen measurement range is the widest, and it is a very effective oxygen measurement sensor.

Commonly used are thermomagnetic convection oxygen analysis sensors (which can be subdivided into velocity measurement thermomagnetic type, pressure balance thermomagnetic type according to the composition method) and magneto-mechanical oxygen analysis sensors.

Main application: used for the detection of oxygen, with excellent selectivity, it is the core of the magnetic oxygen analyzer. Its typical applications include chemical fertilizer production, cryogenic air separation, thermal power station combustion system, natural gas to acetylene and other industrial production oxygen control and chain, environmental monitoring of waste gas, tail gas, flue gas and other emissions.

9. Gas chromatographic analyzer

Based on chromatographic separation technology and detection technology, it separates and measures the concentration of each component in the gas sample, so it is a full analysis sensor. It has been used in power plant boiler tests.

During operation, a certain volume of gas sample is periodically taken from the sampling device, and is carried by a pure carrier gas (ie mobile phase) with a certain flow rate, and flows through the chromatographic column. The chromatographic column is filled with a solid or liquid called a stationary phase. The difference in the absorption or dissolving ability of each component of the gas sample is fixed, so that each component is repeatedly distributed in the two phases, so that each component is separated, and flows out of the chromatographic column according to time and enters the detector for quantitative determination.

According to the detection principle, gas chromatographic analyzers are subdivided into concentration detectors and mass detectors.

The concentration detector measures the instantaneous change of the concentration of a certain component in the gas, that is, the response value of the detector is proportional to the concentration of the component.

The mass detector measures the speed change of a certain component in the gas entering the detector, that is, the response value of the detector is proportional to the amount of a certain component entering the detector per unit time. The most commonly used detectors are TCD thermal conductivity detector, FLD hydrogen flame ionization detector, HCD electron capture detector, FPD flame photometric detector, etc.

 Gas chromatographic analyzer

Advantages: high sensitivity, suitable for micro and trace analysis, and can analyze complex multiphase separation gases.

Disadvantages: Regular sampling cannot achieve continuous sampling analysis, the system is more complicated, and it is mostly used for laboratory analysis, which is not suitable for industrial field gas monitoring.

10. Other gas sensors

1. Ultrasonic Gas Detector

This kind of gas detector is quite special. The principle is that when the gas leaks from the high pressure end to the low pressure end through a small leak hole, turbulent flow will be formed and vibration will be generated. Typical turbulent airflow will become a factor when the differential pressure is higher than 0.2MPa, and ultrasonic waves will be generated when the differential pressure exceeds 0.2MPa. Turbulent molecules collide with each other to generate heat and vibrations. The thermal energy dissipates quickly, but the vibrations are transmitted over considerable distances. The ultrasonic detector determines whether there is air leakage by receiving ultrasonic waves.

Such detectors are commonly used on oil and gas platforms, power plant gas turbines, compressors, and other outdoor pipelines.

Magnetic oxygen analyzer

2. Magnetic oxygen analyzer

This gas analyzer is based on the physical phenomenon that the magnetic susceptibility of oxygen is much greater than that of other gases, and it is a physical gas analysis device that measures oxygen in mixed gases. This equipment is suitable for automatic detection of oxygen content in various industrial gases, and can only be used for oxygen detection, with excellent selectivity.

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