Principle of oxygen content analyzer,sensor

Classification of gas sensors

The national standard GB7665-87 defines sensors as: The sensor is a device or a device capable of sensing a prescribed measurement and converting it into an available output signal according to a certain rule. Gas sensor is used to detect the composition and content of the gas. It is generally considered that the definition of gas sensor is based on the classification of the detection target, that is to say, any sensor used to detect the composition and concentration of gas is called gas sensor, whether it is by physical or chemical methods. For example, the gas flow sensor is not considered as a gas sensor, but the thermal conductivity gas analyzer is an important gas sensor, although they sometimes use generally consistent detection principles.
As early as the 1970s, gas sensor has become a large department in the field of sensors, belonging to a branch of chemical sensors.

There are some kinds of gas sensors that are popular in the market:

1.Semiconductor gas sensor

It is made by using some metal oxide semiconductor material, the conductivity changes with the change of the ambient gas composition at a certain temperature. For example, alcohol sensor is made by using the principle that when tin dioxide encounters alcohol gas at high temperature, the resistance will decrease sharply. The semiconductor gas sensor can be used effectively in the following fields: methane, ethane, propane, butane, alcohol, formaldehyde, carbon monoxide, carbon dioxide, ethylene, acetylene, vinyl chloride, styrene, acrylic acid and other gases. In particular, the sensor has low cost and is suitable for the needs of the civil gas detection. The following types of semiconductor gas sensors are successful: methane (natural gas, methane), alcohol, carbon monoxide (urban gas), hydrogen sulfide, ammonia (including amines, hydrazine). High quality sensors can meet the needs of industrial detection.
Disadvantage: The stability is poor, and it is influenced by the environment. In particular, the selectivity of each sensor is not unique and the output parameters cannot be determined. Therefore, it is not suitable to be used in the places where the measurement accuracy is required. At present, the main supplier of this kind of sensor is in Japan (inventor), followed by China, recently joined South Korea, other countries like the United States also have considerable work in this area, but has never been brought into the mainstream! China has invested as much manpower and time as Japan in this field, but due to the national policy guidance and the blockage of social information and other reasons for many years, the performance quality of semiconductor gas sensors popular in China is far inferior to that of Japanese products. I believe that with the market progress and the further rise of private capital, it is very time for China-made semiconductor gas sensors to reach and surpass the Japanese level!

2.Catalytic combustion type gas sensor

The sensor is to prepare a high temperature resistant catalyst layer on the surface of the platinum resistance, and at a certain temperature, the combustible gas catalyzes the combustion on the surface of the platinum resistance, the combustion is the platinum resistance temperature rise, the resistance change, the change value is a function of the combustible gas concentration.
Catalytic combustion gas sensors selectively detect combustible gases: Any one that can be burned can be detected; The sensor does not respond to anything that cannot burn. There are, of course, many exceptions to the phrase "can detect anything that can burn", but, overall, the selectivity is correct. The catalytic combustion gas sensor has the advantages of accurate measurement, fast response and long service life. The output of the sensor is directly related to the explosion risk of environment, and it is a kind of dominant sensor in the field of safety detection.
Disadvantage: In the range of combustible gases, there is no selectivity. The fire is working, and there's a danger of a fire. Most elemental organic vapor is toxic to the sensor. The main suppliers of the sensors are in China, Japan and the United Kingdom (inventor country)! China is the largest user (coal mine) of this sensor, and also has the best sensor production technology, although there are a variety of agents in the propaganda interference with the public understanding of this sensor, but after all, the catalytic combustion gas sensor mainstream manufacturers at home.

3.Heat conducting pool type gas sensor

Each gas has its own specific thermal conductivity, and when the thermal conductivity of two and more gases is quite different, the content of one of the components can be distinguished by the thermal conductivity element. The sensor has been used for hydrogen detection, carbon dioxide detection, and high concentration methane detection.
The gas sensor has narrow application range and many limiting factors. It's an old fashioned product and it's made all over the world. The quality of products worldwide is similar.

4.Electrochemical gas sensor

A considerable portion of combustible, toxic, and harmful gases have electrochemical activity and can be electrochemically oxidized or reduced. By using these reactions, the gas composition can be distinguished and the gas concentration can be detected. Electrochemical gas sensors have many subclasses:

(1)Primary battery-based gas sensors (also known as these are the batteries that we use for dry batteries, except that the carbon and manganese electrodes of the batteries are replaced by the gas electrodes. In the case of an oxygen sensor, oxygen is reduced at the cathode, electrons flow through the galvanometer to the anode, where the lead metal is oxidized. The magnitude of the current is directly related to the concentration of oxygen. The sensor can effectively detect oxygen, sulfur dioxide, chlorine, etc.

(2)Constant potential electrolytic cell gas sensor, which is very effective for detecting reducing gas, its principle is different from the original cell sensor, its electrochemical reaction occurs under the current force, is a true Coulomb analysis sensor. This sensor has been successfully used for: The detection of carbon monoxide, hydrogen sulfide, hydrogen, ammonia, hydrazine and other gases is the mainstream sensor for the detection of toxic and harmful gases.

(3)The concentration cell type gas sensor, the gas with electrochemical activity on the two sides of the electrochemical cell, will spontaneously form concentration electromotive force, the magnitude of electromotive force is related to the concentration of the gas, the success example of this sensor is the oxygen sensor for automobile, solid electrolyte type carbon dioxide sensor.

(4)The invention relates to a limit current type gas sensor, which is a sensor for measuring the oxygen concentration, and uses the principle that the limit current in the electrochemical cell is related to the carrier concentration to prepare the oxygen (gas) concentration sensor, which is used for the oxygen detection of a vehicle and the oxygen concentration detection of steel water. The main suppliers of this sensor are now all over the world, mainly in Germany, Japan, the United States, recently joined several new European suppliers: Britain, Switzerland, etc. China started early in this field, but the industrialization process has been ineffective.

5.Infrared gas sensor

Most of the gases have characteristic absorption peaks in the mid-infrared region, and the concentration of a gas can be determined by detecting the absorption of the position of the characteristic absorption peaks. This sensor used to be a large analytical instrument, but in recent years, with the development of the sensor industry based on MEMS technology, the volume of this sensor has been reduced from 10 liters, 45 kg of Big Mac, to about 2 ml (thumb size). The use of infrared detector without modulating light source makes the instrument completely free of mechanical moving parts and maintenance. The infrared gas sensor can distinguish the kinds of gas effectively and measure the gas concentration accurately.
This sensor was successfully used for: Carbon dioxide, methane detection. The current supplier of this "sensor" is in Europe! China is now a 'half blank' in this field!

6.Magnetic oxygen sensor

This is the core of the magnetic oxygen analyzer, but it has also achieved the "sensor" process.
It is prepared by the principle that oxygen in the air can be attracted by a strong magnetic field.
The sensor can only be used for the detection of oxygen and has excellent selectivity. Only NOx in the atmosphere can produce a small impact, but because the content of these interference gases is often very small, the selectivity of magnetic oxygen analysis technology is almost unique! established industrial products, with manufacturers all over the world. (I'm talking, of course, about an oxygen analyzer as a single meter, which can be seen as a sensor within a certain range.) And this product, produced in the form of pure sensors, is a recent thing. )

7.Others

In recent years, with the continuous emergence of new technology, gas sensor technology is also in the corresponding revolution. The variety of gas sensors is also adding new layers. However, whether some sensors should be listed under the name of gas sensors is quite controversial, for example: PID detector, even though it is also used for gas detection, although the size is as small as the size, but because it can not truly achieve maintenance-free, therefore, this kind of equipment, no matter how small the size, should be listed under the name of "detection instrument."
In addition, optical sensors based on optical fibers are developing rapidly, and although they do not pose an absolute "threat" to electronic sensors, their unique advantages may one day be brilliant. Finally, for their own commercial purposes, business men engaged in the sale of gas sensors sometimes exaggerate the ability of a specific sensor and confuse the classification of sensors, which are not conducive to the correct application of gas sensors by users. In fact, the application of sensors is as much a subject as the manufacture of sensors.