Gas detector according to sensor classification

Gas detector is an instrumentation tool for detecting the concentration of gas leaks. It mainly refers to portable/hand-held gas detectors. Gas sensors are mainly used to detect the types of gases present in the environment.

Gas sensors are sensors used to detect the composition and content of gases. It is generally believed that the definition of a gas sensor is based on the classification of the detection target. That is, any sensor used to detect the composition and concentration of gas is called a gas sensor, whether it is a physical method or a chemical method. For example, sensors that detect gas flow are not considered gas sensors, but thermal conductivity gas analyzers are important gas sensors, although they sometimes use a generally consistent detection principle.

As early as the 1970s, gas sensors have become a major component of the sensor field and belong to a branch of chemical sensors.

There are about the following types of gas sensors that are currently on the market:

1. Semiconductor-type gas sensor It is manufactured by using some metal oxide semiconductor materials under the condition that the conductivity changes with the change of the composition of the ambient gas. For example, alcohol sensors use the principle that when tin dioxide is exposed to alcohol gas at high temperatures, the resistance decreases sharply.

The semiconductor gas sensor can be effectively used for the detection of many gases such as methane, ethane, propane, butane, alcohol, formaldehyde, carbon monoxide, carbon dioxide, ethylene, acetylene, vinyl chloride, styrene, acrylic acid, and the like. In particular, this sensor is inexpensive and suitable for civil gas detection needs.

The following types of semiconductor gas sensors are successful: methane (natural gas, methane), alcohol, carbon monoxide (city gas), hydrogen sulfide, ammonia (including amines, hydrazines). High-quality sensors can meet the needs of industrial inspection.

Disadvantages: poor stability, large impact on the environment; in particular, the selectivity of each sensor is not unique, the output parameters can not be determined. Therefore, it should not be used in places where the exact measurement is required.

At present, the main suppliers of such sensors are in Japan (inventors), followed by China, and recently they have newly joined South Korea. Other countries, such as the United States, have considerable work in this area, but they have not yet reached the mainstream. China has invested more manpower and time in this field than Japan, but due to many reasons such as national policy orientation and social information obstruction, the quality of semiconductor gas sensors that are popular in the market in China is far inferior to that of Japanese products. I believe that With the progress of the market and the further rise of private capital, China's semiconductor gas sensors have reached and surpassed Japan's level. 2, Catalytically combusted gas sensors are sensors that produce high-temperature catalyst layers on the surface of platinum resistors. At the temperature, the combustible gas catalyzes the combustion on its surface. The combustion is the temperature rise of the platinum resistance, and the resistance changes. The change value is a function of the flammable gas concentration.

Catalytically combustible gas sensors selectively detect flammable gases: they can be detected if they can be combusted, and sensors that do not combust have no response. Of course, there are many exceptions to the phrase “everything that can be burned”. However, in general, the above-mentioned selectivity is true.

Catalytically-fired gas sensors provide accurate metering, fast response, and long life. The output of the sensor is directly related to the explosion hazard of the environment, and is a dominant sensor in the field of safety inspection.

Disadvantages: No selectivity in the flammable gas range. Working under fire has the danger of igniting an explosion. Most of the elemental organic vapors have a poisoning effect on the sensor.

At present, the main suppliers of such sensors are in China, Japan, and the United Kingdom (inventor country)! At present, China is the largest user of this type of sensor (coal mine) and also has the best sensor production technology. Despite the fact that there are a variety of agents constantly promoting public awareness of this type of sensor, after all, catalytic combustion The mainstream manufacturers of gas sensors are at home.

3. Each gas of the thermal conductivity type gas sensor has its own specific thermal conductivity. When the thermal conductivity of two or more gases differs greatly, the thermal conductivity element can be used to distinguish one of the components. content. This type of sensor has been used in sensors for the detection of hydrogen, the detection of carbon dioxide, and the detection of high concentrations of methane.

This type of gas sensor has a narrow range of applications and many limitations.

This is an old-fashioned product with manufacturers all over the world. The quality of products in the world is very much the same.

4. Electrochemical gas sensor A considerable part of its flammable, toxic and harmful gases are electrochemically active and can be electrochemically oxidized or reduced. Using these reactions, gas components can be distinguished and gas concentrations can be detected. Electrochemical gas sensors are divided into many subcategories:

(1) The original battery type gas sensor (also known as: gas sensor type gas sensor, also known as fuel cell type gas sensor, also known as spontaneous battery type gas sensor), their principle is the same as we use the dry battery, but, The carbon-manganese electrode of the battery was replaced by a gas electrode. Taking an oxygen sensor as an example, oxygen is reduced at the cathode and electrons flow through the ammeter to the anode where the lead metal is oxidized. The size of the current is directly related to the concentration of oxygen. This sensor can effectively detect oxygen, sulfur dioxide, chlorine and so on.

(2) Constant-potential electrolyzer-type gas sensor. This type of sensor is very effective for detecting reducing gas. Its principle is different from that of the original battery type sensor. Its electrochemical reaction occurs under the force of electric current and is a A true Coulomb analysis sensor. This sensor has been successfully used in the detection of carbon monoxide, hydrogen sulfide, hydrogen, ammonia, helium, and other gases, and is currently the mainstream sensor for the detection of toxic and harmful gases.

(3) Concentrated battery type gas sensors, with electrochemically active gases, spontaneously form a concentrated electromotive force on both sides of an electrochemical cell. The magnitude of the electromotive force is related to the concentration of gas. A successful example of such a sensor is automotive use. Oxygen sensor, solid electrolyte type carbon dioxide sensor.

(4) In the limiting current type gas sensor, there is a sensor that measures the oxygen concentration. An oxygen (gas) concentration sensor is prepared by using the principle of limiting current in the electrochemical cell and the carrier concentration, for oxygen detection in automobiles, and in molten steel. Oxygen concentration detection.

At present, the main suppliers of such sensors are all over the world, mainly in Germany, Japan, and the United States. Recently, several new European suppliers have been added: the United Kingdom and Switzerland. China started very early in this field, but industrialization progressed poorly.

5. Most of the infrared gas sensors have characteristic absorption peaks in the mid-infrared region. The absorption of the characteristic absorption peak positions can be used to determine the concentration of a certain gas.

The sensors used to be large-scale analytical instruments in the past, but in recent years, with the development of the sensor industry based on MEMS technology, the size of such sensors has been reduced from 10 liters and 45 kg of giants to 2 Ml (thumb size) around. The use of an infrared detector that does not require modulation of the light source makes the instrument completely free of mechanical moving parts and is fully maintenance-free.

The infrared gas sensor can effectively distinguish the type of gas and accurately measure the gas concentration.

This sensor has been successfully used for the detection of carbon dioxide and methane.

At present this kind of "sensor" supplier is in Europe! China is currently a "half" blank in this area!

6. Magnetic Oxygen Sensor This is the core of the magnetic oxygen analyzer, but the process of “sensorization” has also been implemented.

It is based on the principle that oxygen in the air can be attracted by a strong magnetic field.

This sensor can only be used for oxygen detection with excellent selectivity. Only nitrogen oxides in the atmosphere can have a slight effect, but since the amount of these interfering gases is often very small, the selectivity of magnetic oxygen analysis technology is almost unique!

Old industrial products, manufacturers all over the world. (Of course, I'm talking about an oxygen analyzer as a meter. It can be seen as a sensor within a certain range. This kind of product produced in the form of a pure sensor is the most recent thing.)

7. Others In recent years, with the continuous emergence of new technologies, gas sensor technology has also undergone a corresponding revolution. The types of gas sensors are also adding new ones.

However, whether or not some sensors should be listed under the name of a gas sensor is controversial. For example, PID detectors, although they are also used for gas detection, are small in size, but because they are not truly maintenance-free, they are equipped with such equipment. No matter how small the volume is, it should be listed in the name of "testing equipment."

8. 0-100% LEL and 0-nPPM in the detector

(1) "LEL" refers to the lower explosion limit. The lowest concentration of combustible gases that can be exposed to an explosive fire in the air is known as the Lower Explosive Limit, abbreviated as %LEL. English: LowerExplosionLimited. The highest concentration of combustible gases that can be exposed to an explosive fire in the air is called the upper explosive limit, abbreviated as %UEL. English: UpperExplosionLimited. So what is the lower explosion limit? Concentration of flammable gas is too low or too high. It is not dangerous. It only burns or explodes when it mixes with air to form a mixture or, more precisely, it encounters a certain proportion of oxygen. Combustion is an intense oxidation reaction accompanied by light and heat. It must have three elements: a. Combustibles (gas); b. Combustion (oxygen); c. Ignition source (temperature). Combustion of combustible gas can be divided into two categories, one is diffusion combustion, ie, combustible gas that is volatilized or ejected from the equipment and leaks, and meets ignition source mixed combustion. The other type of combustion is the burning of combustible gas and air. This kind of burning reaction is intense and fast. It usually produces huge pressure and sound, also called explosion. There is no strict distinction between burning and explosion. The relevant authorities and experts have performed combustion explosion analysis on the currently discovered combustible gases and developed explosive limits for flammable gases, which are divided into the upper explosion limit (UEL in shorthand of upperexplodelimit) and the lower explosion limit (lower LLE in English lowerexplodelimit). . Below the lower explosion limit, the content of combustible gas in the gas mixture is insufficient, and combustion or explosion cannot be caused. The content of oxygen in the gas mixture above the upper limit is insufficient, and combustion or explosion cannot be caused. In addition, the combustion and explosion of combustible gas are also related to the pressure, temperature, ignition energy and other factors of the gas. The explosion limit is generally expressed as volume percent concentration. The limit of explosion is the general term of the lower explosion limit and upper explosion limit. The concentration of combustible gas in the air will only occur between the lower explosion limit and the upper explosion limit. No explosion will occur below the lower explosion limit or above the upper explosion limit. Therefore, when making explosion measurements, the alarm concentration is generally set below the 25% LEL of the lower explosion limit. Various combustible gas detectors have a measuring range of 0-100% LEL. Fixed combustible gas detectors usually have two alarm points (related to the model of the alarm host): 10% LEL is a primary alarm and 25% LEL is a secondary alarm. Portable combustible gas detectors usually have an alarm point: 25% LEL is the alarm point. For example, the lower explosion limit of methane is 5% by volume, that is to say, this 5% volume ratio is divided into one hundred equal parts so that 5% volume ratio corresponds to 100% LEL, that is, when the detector value reaches 10 When the %LEL alarm occurs, the methane content is equivalent to 0.5% by volume. When the detector value reaches the 25% LEL alarm point, the methane content at this time is equivalent to 1.25% by volume. Therefore, you do not need to worry about whether or not the alarm is dangerous at all times. At this time, you are prompted to take appropriate measures immediately, such as opening the exhaust fan or cutting off some valves. There is still a large gap, so that it will play a warning.

(2) ppm is volumetric concentration: Partspermillion.

Ppm is a measure of solution concentration (mass fraction of solute) and ppm represents 1 part per million.

For solution: 1 in 1000 liters of solute in 1 liter of aqueous solution, its concentration (mass fraction of solute) is 1 ppm.

For gas: One of the ways to express the concentration of pollutants in the ambient atmosphere (air).

Volumetric Representation: The volume of contaminants contained in one million volumes of air, ie ppm

The gas concentration measured by most gas detection instruments is the volume concentration (ppm). According to our country's regulations, especially the environmental protection department, the gas concentration is required to be expressed in units of mass concentration (eg: mg/m3). Our national standards and standards are also expressed in units of mass concentration (eg: mg/m3;).