Sensor types and advantages and disadvantages of gas detection instruments
A gas detector is an instrumentation tool for gas leakage concentration detection, mainly referring to a portable/handheld gas detector. Gas sensors are mainly used to detect the types of gases existing in the environment. Gas sensors are sensors used to detect the composition and content of gases. It is generally believed that the definition of gas sensors is based on the classification of detection targets, that is to say, any sensor used to detect gas composition and concentration is called a gas sensor, regardless of whether it uses physical methods or chemical methods. 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 roughly the same detection principle.
Semiconductor
It is manufactured by using some metal oxide semiconductor materials, and at a certain temperature, the electrical conductivity changes with the change of the ambient gas composition. For example, the alcohol sensor is prepared by using the principle that when tin dioxide encounters alcohol gas at high temperature, the resistance will decrease sharply.
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, acrylic acid, etc. In particular, the sensor has low cost and is suitable for the needs of civilian gas detection. The following semiconductor gas sensors are successful: methane (natural gas, biogas), alcohol, carbon monoxide (city gas), hydrogen sulfide, ammonia (including amines, hydrazines). High-quality sensors can meet the needs of industrial detection. Industrial Scientific, US Gas Sensors
shortcoming
The stability is poor, and it is greatly affected by the environment; especially, the selectivity of each sensor is not perfect, and the output parameters cannot be determined. Therefore, it is not suitable for use in places where accurate measurement is required.
Combustion
This sensor prepares a high temperature resistant catalyst layer on the surface of the platinum resistor. At a certain temperature, the combustible gas is catalyzed and burned on its surface. The combustion is the temperature rise of the platinum resistor, and the resistance changes. 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 is not combustible. 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 type of sensor in the field of security detection.
shortcoming
In the range of flammable gases, there is no selectivity. There is a danger of ignition and explosion when working with dark fire. Most elemental organic vapors are toxic to sensors.
thermal conductivity pool
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. This sensor has been used sensorily for the detection of hydrogen, 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 formula
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. There are many subcategories of electrochemical gas sensors:
(1) Primary battery-type gas sensors (also known as Gavoni battery-type gas sensors, fuel cell-type gas sensors, and self-generating battery-type gas sensors), their principles are the same as the dry batteries we use, but, The carbon-manganese electrodes of the battery are 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 the 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 gases. Its principle is different from that of the original battery type sensor. Its electrochemical reaction occurs under the force of current. It 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 electrochemically active gas 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. The successful example of this sensor is the automobile Oxygen sensor, solid electrolyte carbon dioxide sensor.
(4), limiting current type gas sensor, there is a sensor for measuring oxygen concentration, which uses 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 of automobiles and molten steel Oxygen concentration detection.
infrared
Most gases have characteristic absorption peaks in the mid-infrared region, and the concentration of a certain 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 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 a 10 liter, 45 kg Big Mac to 2 ml (thumb size) approx. The use of an infrared detector that does not need to modulate the light source makes the instrument completely free of mechanical moving parts, and is completely maintenance-free. The infrared gas sensor can effectively distinguish the type of gas and accurately measure the gas concentration.
This sensor is successfully used for: detection of carbon dioxide and methane.
