Types of gas sensors and their applications in gas detection and monitoring equipment
With the development of the petrochemical industry, the types and application ranges of flammable, explosive and toxic gases have been increased. Once these gases leak during production, transportation, and use, it will cause poisoning, fire and even explosion accidents, seriously endangering people's lives and property safety. Due to the diffusibility of the gas itself, after a leak occurs, the gas will diffuse along the ground surface under the action of external wind force and internal concentration gradient, forming a combustion explosion or poisonous danger zone at the accident site, and expanding the hazardous area.
Once a gas leakage accident occurs, corresponding measures must be taken as soon as possible to minimize the accident loss. Timely and reliable detection of the content of certain gases in the air, timely and effective remedial measures, and correct disposal methods to reduce accidents caused by leakage are necessary conditions for avoiding major property and casualties. This puts forward higher requirements for gas detection and monitoring equipment. As an important gas detector, gas sensor has been greatly developed in recent years. The development of gas sensor makes its application more and more extensive.
Gas Sensor Overview
Foreign countries began to research and develop gas sensors in the 1930s. In the past, gas sensors were mainly used for the detection and alarm of coal gas, liquefied petroleum gas, natural gas and gas in mines. At present, the types of gases that need to be detected have expanded from the original reducing gases (H2, C4H10, CH4) to toxic gases (CO, NO2, H2S, NO, NH3, PH3).
There are many types of gas sensors. According to the different gas-sensing materials and gas-sensing characteristics used, it can be divided into semiconductor type, solid electrolyte type, electrochemical type, contact combustion type, polymer type, etc.
Semiconductor gas sensor
This sensor mainly uses semiconductor gas-sensitive materials. Since the semiconductor metal oxide gas sensor came out in 1962, it has been widely used due to its advantages of high sensitivity and fast response, and has become one of the largest and most widely used sensors in the world. According to the different ways of detecting the gas-sensing characteristic quantity, it is divided into two types: resistive type and non-resistive type.
Resistive semiconductor gas sensor works by detecting the change of gas sensor with gas content. Metal oxide ceramic gas sensitive materials are mainly used. With the research and development of new materials such as composite metal oxides and mixed metal oxides in recent years, the characteristics and application range of this gas sensor have been greatly improved. For example: WO3 gas sensor can detect NH3 concentration range from 5ppm to 50ppm, ZnO-CuO gas sensor is very sensitive to 200ppm CO.
Non-resistive semiconductor gas sensors work on the principle that the current or voltage of the gas sensor changes with the gas content. There are mainly MOS diode type and junction diode type, and field effect tube type gas sensor. Most of the detection gases are combustible gases such as hydrogen and silane.
Solid Electrolyte Gas Sensor
Solid electrolyte gas sensors use solid electrolyte gas-sensing materials as gas-sensing elements. The principle is that the gas-sensitive material generates ions when passing through the gas, thereby forming an electromotive force, and measuring the electromotive force to measure the gas concentration. Due to the high conductivity, good sensitivity and selectivity of this sensor, it has been widely used, and has entered almost all fields of petrochemical, environmental protection, mining, etc., second only to metal oxide semiconductor gas sensors. For example, YST-Au-WO3 for measuring H2S, NH+4CaCO3 for measuring NH3, etc.
Contact combustion gas sensor
It can be divided into direct contact combustion type and catalytic contact combustion type. Its working principle is: when the gas-sensitive material is energized, the flammable gas is oxidized and burned under the action of a catalyst, and the heat generated heats up the heating wire, thereby changing its resistance value, and measuring the change of resistance to measure the gas concentration. This sensor can only measure combustible gases and is insensitive to non-combustible gases. For example, sensors made by coating active catalysts Rh and Pd on Pt wire have broad-spectrum characteristics, that is, they can detect various combustible gases. The contact combustion gas sensor is very stable at ambient temperature and can detect most flammable gases with the lower limit of explosion. It is widely used in the monitoring and alarm of flammable gases in petrochemical plants, shipyards, mine tunnels, bathrooms, kitchens, etc.
Polymer gas sensor
Gas sensors using polymer gas-sensing materials have been greatly developed in recent years. When the polymer gas-sensing material encounters a specific gas, its physical properties such as resistance, dielectric constant, material surface acoustic wave propagation speed and frequency, and material weight will change. There are mainly phthalocyanine polymers, LB film, phenylcyanine acetylene, polyvinyl alcohol-phosphoric acid, polyisobutylene, aminoundecyl silane, etc. Polymer gas-sensing materials play an important role in the detection of toxic gases and food freshness due to their ease of operation, simple process, good selectivity at room temperature, low price, and easy combination with microstructure sensors and surface acoustic wave devices. According to the gas-sensing properties of the materials used, this type of sensor can be divided into: polymer resistive gas sensors that measure gas concentration by measuring the resistance of gas-sensing materials; concentration battery-type gas sensors that measure the electromotive force to determine the gas concentration by forming a concentration cell when the gas-sensing material absorbs gas; surface acoustic wave gas sensors that are made based on the principle that the propagation speed or frequency of sound waves on the surface of the material changes after the gas-sensing polymer absorbs gas; The polymer gas sensor has high sensitivity to specific gas molecules, good selectivity, simple structure, can be used at normal temperature, and can supplement the deficiencies of other gas sensors.
