An article to understand the classification and principle of commonly used gas sensors

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. These sensors can provide security systems with information on flammable, flammable and toxic gases, as well as the consumption of oxygen in the area, the ratio of carbon dioxide.

Common gas sensors include electrochemical gas sensors, catalytic combustion gas sensors, semiconductor gas sensors, infrared gas sensors, etc. Different types of sensors have different performances, usage methods, applicable gases and applicable occasions due to different principles and structures. Today, Instrument Control Jun will list the common types of gas sensors for everyone, hoping to be helpful to everyone.

Electrochemical gas sensor

A considerable part of flammable, toxic and harmful gases, such as hydrogen sulfide, nitrogen monoxide, nitrogen dioxide, sulfur dioxide, carbon monoxide, etc., are electrochemically active and can be electrochemically oxidized or reduced. Using these reactions, it is possible to distinguish gas components and detect gas concentrations. Electrochemical sensors are based on this principle.

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Electrochemical sensors have many subclasses:

Galvanic gas sensor

This kind of sensor is also called Gavoni cell type gas sensor, or fuel cell type gas sensor, spontaneous cell type gas sensor. Their principle is the same as the dry batteries we use every day, except that the carbon-manganese electrodes of the batteries are replaced by gas electrodes. In the case of an oxygen sensor, the oxygen cathode is reduced and the electronic ammeter flows to the anode, where lead metal is oxidized. Therefore, the magnitude of the current is directly related to the oxygen concentration. This sensor can effectively detect oxygen, sulfur dioxide, chlorine and other gases.

Constant Potential Electrolytic Cell Type Gas Sensor

This sensor is very effective for detecting reducing gases. Its principle is different from that of galvanic sensors. The electrochemical reaction is forced by current, and it is a real Coulomb analysis sensor. This sensor has been successfully 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.

Note: Coulomb analysis refers to the method of determining the content of the measured substance by Faraday's law according to the electricity consumed in the electrolysis process.

Concentration cell type gas sensor

The electrochemically active gas of this sensor will spontaneously form a concentration electromotive force on both sides of the electrochemical cell. The magnitude of the electromotive force is related to the concentration of the gas. The successful examples of this sensor are the oxygen sensor for automobiles, solid electrolyte carbon dioxide detection instrument.

Limiting current type gas sensor

This is a sensor for measuring oxygen concentration. The working principle is based on the oxygen pump action of the stable zinc oxide solid electrolyte, and the limiting current is obtained by controlling the oxygen supplied to the cathode through gas diffusion. This sensor is currently mainly used for boiler combustion control, oxygen concentration detection in molten steel, and oxygen detection in automobiles.

Semiconductor gas sensor

The semiconductor gas sensor uses the oxidation and reduction reaction of gas on the semiconductor surface, resulting in the change of the resistance value of the sensitive element:

Gases with a tendency to adsorb negative ions, such as oxygen, are called oxidizing gases—electron-accepting gases;

Gases that have a tendency to adsorb positive ions, such as hydrogen, carbon oxides, and alcohols, are called reducing gases—electron-supplying gases.

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When the oxidation (reduction) type gas is adsorbed on the N(P) type semiconductor, the carrier of the semiconductor decreases (increases), and the resistivity increases (decrease); when it is adsorbed on the P(N) type semiconductor, the carrier of the semiconductor increases (decrease), the resistivity decreases (increases). (It can be seen that oxidation and reduction semiconductors are diametrically opposed) Therefore, the corresponding gases can be effectively detected from these properties.

Semiconductor gas sensors can be effectively used for: methane, ethane, propane, butane, alcohol, formaldehyde, carbon monoxide, carbon dioxide, ethylene, acetylene, vinyl chloride, styrene, acrylic and many other gas detection. In particular, this sensor is low-cost and can meet the needs of both industry and civilian use.

Disadvantages: poor stability, greatly affected by the environment, should not be used in places where accurate measurement is required.

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 change.

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Since the catalytic combustion gas sensor platinum resistance is usually surrounded by ceramic beads composed of porous ceramics, this sensor is also usually called a 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.

Based on the excellent temperature characteristics of platinum resistance, this sensor has accurate measurement and fast response. The sensor output is directly related to the environmental explosion hazard, and the field of safety detection is a class of leading sensors.

The disadvantage is that it needs to work in a sufficient oxygen environment (after all, it needs to burn); when working in a dark fire, there is a danger of ignition and explosion; most of the organic vapors of the elements have a poisoning effect on the sensor; due to the continuous consumption of the catalyst, the zero point and range will drift. Requires frequent calibration and adjustment.

Photoionizing Gas Sensor

It is usually called PID, which is the abbreviation of PhotoionizaTIon Detector (the instrument controller hereby reminds that this PID is not proportional, differential and integral). This is a very sensitive, versatile detector that can detect volatile organic compounds and other toxic gases from 10ppb to higher concentrations of 10,000ppm. Many hazardous substances contain volatile organic compounds, and PID is highly sensitive to volatile organic compounds.

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PID uses an ultraviolet light source to ionize organic molecules into positive and negative ions that can be detected by a detector. The detector captures the positive and negative charges of the ionized gas and converts them into current signals to achieve gas concentration. Measurement. When the measured gas absorbs high-energy ultraviolet light, the gas molecules are excited by the ultraviolet light to temporarily lose electrons and become positively charged ions. The gas ions are detected on the electrodes of the detector, and the gas concentration is detected according to the potential generated by the electrodes. , after detection, the ions combine with electrons to reconstitute the original gas molecules.

PID can detect aromatic hydrocarbons, ketones, aldehydes, chlorinated hydrocarbons, amines and amine compounds and unsaturated hydrocarbons.

Infrared Gas Sensor

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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.

Its advantages are: good selectivity, only detect gases of specific wavelengths, and can be customized according to the gas; adopting optical detection method, it is not easy to be poisoned and aged due to 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.

The disadvantages are: the measurement range is narrow, and only hydrocarbons of (c1, c5) can be detected; afraid of dust and humidity, the site environment is better, and the dust on the mirror surface needs to be cleaned and maintained regularly; it cannot be detected when there is airflow on site; the price is relatively high high.

Solid Electrolyte Gas Sensor

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Solid electrolyte gas sensors refer to gas sensors that use solid electrolytes as sensing materials. Commonly used solid electrolytes mainly include: stabilized zirconia, sodium ion fast conductors, proton conductors, and halides of some low-valent metals. Solid electrolyte gas sensors can be divided into balanced potential type, mixed potential type, limited current type and short-circuit current type according to the characteristics of the detection signal.

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.

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 factor when the differential pressure is higher than 0.2MPa, and ultrasonic waves will be generated above 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 often found on oil and gas platforms, power plant gas turbines, compressors and other outdoor pipelines.

Magnetic oxygen analyzer

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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 the oxygen in the mixed gas. This device is suitable for automatic detection of oxygen content in various industrial gases. This type of equipment can only be used for oxygen detection and has excellent selectivity.