The sampling method of gas directly affects the response time of the sensor. At present, the sampling method of gas is mainly through simple diffusion or inhalation of gas into the detector.
Simple diffusion takes advantage of the natural diffusion of gas everywhere. The target gas passes through the sensor in the probe and generates a signal proportional to the gas volume fraction. Because the diffusion process is gradually slowing down, the diffusion method requires the position of the probe to be very close to the measurement point. One advantage of diffusion method is that the gas sample is directly introduced into the sensor without physical and chemical transformation. Sample aspiration probes are usually used to sample near processing instruments or exhaust pipes. This technology can provide a stable and controllable airflow for the sensor, so this method is worth recommending when the airflow size and speed change frequently. The gas sample may need a distance from the measuring point to the measuring probe, and the length of the distance mainly depends on the design of the sensor, but a long sampling line will increase the measurement lag time, which is a function of the length of the sampling line and the gas flow rate from the leakage point to the sensor. For some target gases and vapors, such as SiH4 and most biological solvents, the sampling amount of gases and vapors may be reduced, because they will be adsorbed or even condensed on the wall of the sampling tube.
Gas sensor is a kind of chemical sensor. From working principle, characteristic analysis to measurement technology, from materials used to manufacturing technology, from detection objects to application fields, independent classification standards can be formed, and various classification systems can be derived, especially on the issue of classification standards, which is not uniform at present and it is quite difficult to strictly classify.
Main functions of 1
1. 1 stability
Stability refers to the stability of the basic response of the sensor during the whole working time, which depends on zero drift and interval drift. Zero drift refers to the change of sensor output response during the whole working time when there is no target gas. Interval drift refers to the change of the output response of the sensor continuously placed in the target gas, which is manifested by the decrease of the output signal of the sensor during the working time. Ideally, the annual zero drift of the sensor is less than 10% under continuous working conditions.
1.2 sensitivity
Sensitivity refers to the ratio of sensor output variation to measured input variation, which mainly depends on the technology adopted in sensor structure. The design principles of most gas sensors are biochemistry, electrochemistry, physics and optics. The first thing to consider is to choose a sensitive technology, which is sensitive enough to detect the percentage of TLV- threshold-old limit value or LEL- lower explosion limit of the target gas.
1.3 selectivity
Selectivity is also called cross sensitivity. It can be determined by measuring the sensor response produced by a certain concentration of interfering gas. This response is equivalent to the sensor response generated by a certain concentration of target gas. This characteristic is very important in the application of tracking multiple gases, because cross sensitivity will reduce the repeatability and reliability of measurement, and the ideal sensor should have high sensitivity and high selectivity.
1.4 Corrosion resistance
Corrosion resistance refers to the ability of the sensor to be exposed to a high volume fraction of target gas. When a large amount of gas leaks, the probe should be able to withstand the expected gas volume fraction 10~20 times. When returning to the normal working state, the sensor drift and zero correction value should be as small as possible.
The basic characteristics of gas sensors, such as sensitivity, selectivity and stability, mainly depend on the selection of materials. Select suitable materials and develop new materials to optimize the sensitivity of gas sensors.
2 main principles and classification
Generally, it can be classified according to gas sensing characteristics, which can be mainly divided into: semiconductor gas sensor, electrochemical gas sensor, solid electrolyte gas sensor, contact combustion gas sensor, photochemical gas sensor, polymer gas sensor and so on.
2. 1 semiconductor gas sensor
Semiconductor gas sensor is an element made of metal oxide or metal semiconductor oxide material. When it interacts with gas, it produces surface adsorption or reaction, which causes the change of conductivity or volt-ampere characteristics or surface potential characterized by carrier movement. These are all determined by the semiconductor characteristics of materials.
Since 1962 semiconductor metal oxide ceramic gas sensor came out, semiconductor gas sensor has become the most widely used and practical gas sensor. According to its gas sensing mechanism, it can be divided into resistive type and non-resistive type.
Resistive semiconductor gas sensor mainly refers to semiconductor metal oxide ceramic gas sensor, which is an impedance device made of metal oxide films (such as Sn02, ZnO, Fe203, Ti02, etc.). ), its resistance varies with gas content. The odor molecules undergo reduction reaction on the surface of the film, which causes the change of the conductivity of the sensor. In order to eliminate odor molecules, oxidation reaction must also occur. The heater in the sensor contributes to the oxidation reaction process. It has the advantages of low cost, simple manufacture, high sensitivity, fast response, long service life, insensitivity to humidity and simple circuit. Disadvantages are that it must work at high temperature, poor selectivity to odor or gas, scattered component parameters, unsatisfactory stability and high power requirements. When sulfide is mixed in the detection gas, it is easy to be poisoned. Now, in addition to the traditional three categories of SnO, SnO 2 and Fe203, a number of new materials have been developed, including single metal oxide materials, composite metal oxide materials and mixed metal oxide materials. The research and development of these new materials have greatly improved the characteristics and application scope of gas sensors. In addition, by adding precious metals such as Pt, Pd and Ir into the semiconductor, the sensitivity and response time of the element can be effectively improved. It can reduce the chemical adsorption activation energy of the measured gas, so it can improve its sensitivity and speed up the reaction. Different catalysts lead to different adsorption samples, thus having selectivity. For example, various noble metal-doped Sn02-based semiconductor gas sensing materials, Pt, Pd and Au improve the sensitivity to CH4, while Ir reduces the sensitivity to CH4; Platinum and gold increase the sensitivity to H2, while palladium decreases the sensitivity to H2. The metal oxide gas sensor manufactured by thin film technology and ultrafine particle thin film technology has the characteristics of high sensitivity (up to 10-9), good consistency, miniaturization and easy integration.
Non-resistive semiconductor gas sensors are MOS diode, junction diode and field effect transistor (MOSFET) semiconductor gas sensors. Its current or voltage varies with gas content, and it mainly detects combustible gases such as hydrogen and silicon combustion gas. Among them, the working principle of MOSFET gas sensor is that volatile organic compounds (VOC) react with catalytic metals (such as buttons), and the reaction products diffuse to the gate of MOSFET, which changes the performance of the device. Identify VOC by analyzing the change of equipment performance. By changing the type of catalytic metal and the film thickness, the sensitivity and selectivity can be optimized and the working temperature can be changed. MOSFET gas sensor has high sensitivity, but its manufacturing process is complex and its cost is high.
2.2 electrochemical gas sensor
Electrochemical gas sensors can be divided into four types: galvanic cell type, controllable potential electrolysis type, electric quantity type and ion electrode type. Galvani gas sensor detects the volume fraction of gas by detecting current. Almost all instruments for detecting hypoxia on the market are equipped with this sensor. In recent years, galvanic gas sensors have been developed for detecting acid gases and toxic gases. The controllable potential electrolysis sensor detects the volume fraction of gas by measuring the current flowing in the electrolysis process. Different from the galvanic cell sensor, it needs to apply a specific voltage from the outside, which can not only detect gases such as CO, NO, NO2, O2 and SO2, but also detect the volume fraction of oxygen in blood. The electric gas sensor detects the volume fraction of the gas through the current generated by the reaction between the gas to be measured and the electrolyte. The ion electrode gas sensor appeared earlier. The main advantage of electrochemical gas sensor is that it detects gas by measuring ion polarization current, with high sensitivity and good selectivity.
2.3 solid electrolyte gas sensor
Solid electrolyte gas sensor is a kind of chemical battery with ionic conductor as electrolyte. Since 1970s, the solid electrolyte gas sensor has developed rapidly with its high conductivity, good sensitivity and selectivity. Now it is almost used in environmental protection, energy saving, mining, automobile industry and other fields. , with large output and wide application, second only to metal oxide semiconductor gas sensors. Recently, some foreign scholars have classified solid electrolyte gas sensors into the following three categories:
1) sensors, such as oxygen sensors, in which the ions obtained from the gas to be detected are the same as those moving in the electrolyte.
2) The ions from the gas to be measured adsorbed in the material are different from the moving ions in the electrolyte, such as the gas sensor composed of solid electrolyte SrF2H and Pt electrode, which is used to measure oxygen.
3) The ions from the gas to be detected adsorbed in the material are different from the moving ions in the electrolyte and the fixed ions in the material. For example, the newly developed high-quality CO2 solid electrolyte gas sensor is composed of solid electrolyte NASICON (Na3ZR2SIP2P012) and auxiliary electrode materials Na2CO3-BaC03 or Li2C03-CaC03 and Li2C03- BaC03.
At present, most of the newly developed high-quality solid electrolyte sensors belong to the third category. Another example is a sensor made of solid electrolyte NaSiCON and auxiliary electrode NO2-li2c 03, which is used to measure NO2; Solid electrolyte YST- Gold-Tungsten Oxide Sensor for measuring H2S: A sensor made of solid electrolyte NH4-ca2o 3 for measuring NH3; A sensor for measuring NO2 made of solid electrolyte Ag0.4Na7.6 and electrode Ag-Au, etc.
2.4 Contact combustion gas sensor
Contact combustion gas sensors can be divided into direct contact combustion type and catalytic contact combustion type. Its working principle is that when gas sensitive materials (such as platinum heating wire, etc. ) electricity, combustible gas under the action of catalyst oxidation combustion, heating wire due to combustion and heat, make its resistance change. This kind of sensor is insensitive to non-combustible gases. For example, the sensor made by coating active catalysts Rh and Pd on the lead has broad-spectrum characteristics, that is, it can detect all kinds of combustible gases. This kind of sensor is sometimes called thermal conductivity sensor, which is widely used for monitoring and alarming combustible gas in petrochemical plants, shipyards, mine tunnels, bathrooms, kitchens and other places. The sensor is very stable at ambient temperature and can detect most combustible gases at the lower explosion limit.
2.5 optical gas sensor
Optical gas sensors include infrared absorption type, spectral absorption type, fluorescence type, optical fiber chemical material type and so on. , mainly infrared absorption gas analyzer. Because the infrared absorption peaks of different gases are different, gases are detected by measuring and analyzing the infrared absorption peaks. At present, the latest trend is to develop an on-line infrared analyzer with fluid switching, direct flow measurement and Fourier transform. The sensor has high anti-vibration and anti-pollution ability. Combined with computer, it can continuously test and analyze gas, and has the functions of automatic correction and automatic operation. Optical gas sensor also includes chemiluminescence, optical fiber fluorescence and optical fiber waveguide, and its main advantages are high sensitivity and good reliability.
The main body of the optical fiber gas sensor is a glass optical fiber coated with active substances at both ends. The active substance contains a fluorescent dye immobilized on an organic polymer matrix. When VOC reacts with fluorescent dye, the polarity of dye changes and its fluorescence emission spectrum shifts. When the sensor is irradiated by light pulse, the fluorescent dye will emit light with different frequencies, and VOC can be identified by detecting the light emitted by the fluorescent dye.
2.6 polymer gas sensor
In recent years, great progress has been made in the research and development of polymer gas sensing materials abroad. Polymer gas sensing materials play an important role in the detection of toxic gases and food freshness because of their easy operation, simple process, good room temperature selectivity, low price and easy combination with microstructure sensors and surface acoustic wave devices. Polymer gas sensors can be mainly divided into the following categories according to their gas sensing characteristics:
L) polymer resistance gas sensor
This sensor measures the volume fraction of gas by measuring the resistance of polymer gas-sensitive materials. At present, the main materials are cyanine polymer, LB film and polypyrrole. Its main advantages are simple manufacturing process and low cost. However, this kind of gas sensor needs to be activated by electropolymerization, which is very time-consuming and will cause performance differences between different batches of products.
2) Concentration battery gas sensor
The working principle of concentration battery gas sensor is that concentration battery is formed when gas-sensitive materials absorb gas, and the volume fraction of gas can be measured by measuring the electromotive force output. At present, there are mainly polyvinyl alcohol-phosphoric acid and other materials.
3) Surface acoustic wave gas sensor The surface acoustic wave gas sensor is made on the substrate of piezoelectric material, with the surface at one end as the input sensor and the other end as the output sensor. A polymer film capable of absorbing VOC is deposited in the region between them. The adsorbed molecules increase the mass of the sensor, thus changing the propagation speed or frequency of sound waves on the surface of the material. The gas volume fraction can be measured by measuring the speed or frequency of sound waves.