Classification of gas detectors and purchase methods
stacked semiconductor
It is manufactured by the principle that the conductivity of some metal oxide semiconductor materials changes with the change of the ambient gas composition at a certain temperature. For example, the alcohol sensor is prepared by using the principle that the resistance of tin dioxide will decrease sharply when it encounters alcohol gas at high temperature.
advantage
Semiconductor gas sensors 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, and acrylic acid. In particular, the sensor has low cost and is suitable for the needs of civil gas detection. The following semiconductor gas sensors are successful: methane (natural gas, biogas), alcohol, carbon monoxide (city gas), hydrogen sulfide, ammonia (including amines, hydrazine). High-quality sensors can meet the needs of industrial inspection.
shortcoming
The stability is poor and is greatly affected by the environment; in particular, the selectivity of each sensor is not unique, and the output parameters cannot be determined. Therefore, it should not be used in places where accurate measurement is required.
Combustion
This kind of sensor prepares a high temperature resistant catalyst layer on the surface of the platinum resistor. At a certain temperature, the combustible gas catalyzes combustion on its surface. The combustion is caused by the temperature rise of the platinum resistor, the resistance changes, and the change value is the concentration of the combustible gas. function.
advantage
Catalytic combustion gas sensors selectively detect combustible gases: the sensor does not respond to anything that cannot be combusted. The catalytic combustion gas sensor has accurate measurement, fast response and long service life. The output of the sensor is directly related to the explosion hazard of the environment, and it is a dominant sensor in the field of safety detection.
shortcoming
In the range of flammable gases, there is no selectivity. Working in a dark fire, there is a danger of ignition and explosion. Most elemental organic vapors are poisonous to the sensor.
Thermal conductivity pool type
Each gas has its own specific thermal conductivity. When the thermal conductivity of two or more gases is quite different, the thermal conductivity element can be used to distinguish the content of one of the components. Such sensors have been sensory used for the detection of hydrogen gas, the detection of carbon dioxide, and the detection of high-concentration methane.
This kind of gas sensor can be applied in a narrow range and has many limiting factors.
Electrochemical
A considerable part of its flammable, toxic and harmful gases 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 gas sensors fall into many subcategories:
(1) Primary cell type gas sensor (also called: 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 carbon-manganese electrodes of the battery 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, chlorine, etc.
(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 A true coulometric sensor. This sensor has been successfully used in the detection of carbon monoxide, hydrogen sulfide, hydrogen, ammonia, hydrazine, and other gases, and is the mainstream sensor for the detection of existing toxic and harmful gases.
(3) Concentration battery type gas sensor, the gas with electrochemical activity will spontaneously form a 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. Oxygen sensor, solid electrolyte carbon dioxide sensor.
(4), limiting current type gas sensor, there is a sensor for measuring oxygen concentration using the principle that the limiting current in the electrochemical cell is related to the carrier concentration to prepare an oxygen (gas) concentration sensor, which is used for oxygen detection in automobiles and molten steel. Oxygen concentration detection.
Infrared
Most gases have characteristic absorption peaks in the mid-infrared region, and the concentration of a gas can be determined by detecting the absorption at the position of the characteristic absorption peak.
This kind of sensor used to be a large-scale analytical instrument, but in recent years, with the development of the sensor industry based on MEMS technology, the volume of this kind of sensor has been reduced from 10 liters and 45 kilograms to 2 ml (thumb size) or so. The use of infrared detectors that do not require modulated light sources makes the instrument completely free of mechanical moving parts and completely 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 in the detection of carbon dioxide and methane.
