Foreword : A gas sensor is a device that converts information such as the composition and concentration of a gas into information that can be used by people, instruments, computers, and the like! Gas sensors are generally classified as a type of chemical sensor, although this classification is not necessarily scientific. The "gas sensor" includes a semiconductor gas sensor, an electrochemical gas sensor, a catalytic combustion type gas sensor, a thermal conductivity type gas sensor, an infrared gas sensor, and the like.
Gas sensor principle applicationA gas sensor is a device that converts a gas component and a concentration into an electrical signal output according to a certain law by various chemical and physical effects. With the development of society and the advancement of science and technology, the development of gas sensors has attracted more and more attention, and various gas sensors have emerged. The application of integrated gas sensors mainly has the following applications:
Toxic and flammable gas detectionToxic and flammable gas detection is the largest market for gas sensors. Mainly used in petroleum, mining, semiconductor industry and other industrial and mining enterprises as well as environmental detection and control in the home. In the petroleum, petrochemical, and mining industries, hydrogen sulfide, carbon monoxide, chlorine, methane, and combustible hydrocarbons are the primary test gases. The most important in the semiconductor industry is the detection of phosphorus, arsenic and silane. The main reason in the family is to detect the leakage of gas and liquefied gas and whether it is ventilated.
Combustion controlThe automotive industry is another important market for gas sensors. An oxygen sensor is used to detect and control the air-fuel ratio of the engine to optimize the combustion process. Controls with gas sensors are used in the combustion of large industrial boilers to increase combustion efficiency and reduce exhaust emissions, saving energy. Gas sensors can also be used to detect the amount of exhaust gas emitted from a car or chimney. These exhaust gases include carbon dioxide, sulfur dioxide and carbon monoxide.
Food and beverage processing
Sulfur dioxide sensors are extremely useful devices in food and beverage processing. Sulfur dioxide is commonly used in the storage and testing of many foods and beverages to provide the minimum concentration of sulfur dioxide required to maintain a particular taste and aroma. In addition, gas sensors are also used to detect the fermentation level of wine, beer, sorghum wine to ensure product uniformity and reduce costs.
Medical diagnostics can be used to perform patient condition diagnostic tests such as bad breath detection, blood carbon dioxide and oxygen concentration testing.
Table 1 exemplifies the main detection gases and applications of gas sensors.
The variety of gas sensors is limited, and it is impossible to describe them one by one. This article mainly introduces gas sensors that are widely used and have a certain production capacity in China. They are semiconductor gas sensors, electrochemical gas sensors, contact gas sensors and heat conduction gas sensors. For other gas sensors such as optical, please refer to other related books. Main characteristics of gas sensors
1 stability
Stability is the stability of the sensor's basic response over the entire working time, depending on zero drift and interval drift. Zero drift is the change in sensor output response over the entire working time when there is no target gas. Interval drift refers to the change in the output response of the sensor continuously placed in the target gas, which is manifested by the decrease of the sensor output signal during working hours. Ideally, a sensor drifts less than 10 [%] per year under continuous operating conditions.
2 sensitivity
Sensitivity is the ratio of the amount of change in sensor output to the amount of change in the measured input, depending on the technology used in the sensor structure. Most gas sensors are designed using biochemistry, electrochemistry, physics and optics. The first consideration is to choose a sensitive technique that is sufficiently sensitive to detect the percentage of the target gas's TLV-thresh-old limit value or the minimum explosive limit (LEL-lower explosive limit).
3 selective
Selectivity is also known as 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 produced by a target gas of a certain concentration. This feature is important in tracking multiple gas applications because cross-sensitivity reduces measurement repeatability and reliability, and ideal sensors should have high sensitivity and selectivity.
4 corrosion resistance
Corrosion resistance refers to the ability of a sensor to be exposed to a high volume fraction of target gas. When the gas leaks a lot, the probe should be able to withstand the desired gas volume fraction by 10 to 20 times. The sensor drift and zero correction values ​​should be as small as possible under normal operating conditions.
The basic characteristics of gas sensors, namely sensitivity, selectivity and stability, are primarily determined by the choice of materials. Select the appropriate materials and develop new materials to optimize the sensitivity of the gas sensor.
1 Research and development of new gas sensitive materials and production processes
Studies on gas sensor materials have shown that metal oxide semiconductor materials Zn0, SIlo2, Fe203, etc. have become more mature, especially in the detection of C ratio, C2H5OH, CO and other gases. At present, there are two main directions in this aspect: First, the chemical modification method is used to dope, modify and modify the existing gas-sensitive membrane materials, and improve and optimize the film formation process. The stability and selectivity of gas sensors; the second is to develop new gas-sensitive membrane materials, such as composite and hybrid semiconductor gas sensing materials, polymer gas sensing materials, making these new materials highly sensitive and high-selective for different gases. Sexuality and high stability. Organic polymer sensitive materials have become a research hotspot because of their rich materials, low cost, simple film forming process, easy compatibility with other technologies, and work at room temperature.
2 Development of new gas sensor
Following the traditional principle of action and some new effects, the use of crystalline materials (silicon, quartz, ceramics, etc.) is preferred, and advanced processing techniques and microstructure design are used to develop new sensors and sensor systems, such as optical waveguide gas sensors and polymer sounds. Development and use of surface wave and quartz resonant gas sensors, research on microbial gas sensors and biomimetic gas sensors. With the application of new materials, new processes and new technologies, the performance of gas sensors is more perfect, which makes the miniaturization, miniaturization and multi-function of sensors have the advantages of long-term stability, convenient use and low price.
3 gas sensor intelligence
With the continuous improvement of people's living standards and the increasing emphasis on environmental protection, the detection of various toxic and harmful gases, the monitoring of air pollution, industrial waste gas and the detection of the quality of food and living environment have all raised higher on gas sensors. Requirements. The successful application of new materials development technologies such as nanometer and thin film technology provides a good precondition for gas sensor integration and intelligence. Gas sensors will be developed on the basis of multi-disciplinary integrated technologies such as micromachine and microelectronics, computer technology, signal processing technology, sensing technology, fault diagnosis technology, and intelligent technology. The development of fully automatic digital intelligent gas sensors capable of simultaneously monitoring multiple gases will be an important research direction in this field. Gas sensor selection
1. According to the measurement object and measurement environment
The type of sensor is determined based on the measurement object and the measurement environment. To carry out a specific measurement work, we must first consider the principle of the sensor, which needs to be analyzed after analyzing various factors. Because even if it is measuring the same physical quantity, there are many kinds of sensors available for selection. Which kind of sensor is more suitable, you need to consider the following specific problems according to the characteristics to be measured and the conditions of the sensor: the size of the range; The position of the measured position on the sensor volume; the measurement method is contact or non-contact; the signal extraction method, wired or non-contact measurement; the source of the sensor, domestic or imported, the price can withstand, or self-developed. After considering the above issues, you can determine which type of sensor to use and then consider the specific performance of the sensor.
2, the choice of sensitivity
Generally, in the linear range of the sensor, it is desirable that the sensitivity of the sensor be as high as possible. Because only the sensitivity is high, the value of the output signal corresponding to the measured change is relatively large, which is beneficial to signal processing. However, it should be noted that the sensitivity of the sensor is high, and the external noise that is not related to the measurement is easily mixed in, and is amplified by the amplification system, which affects the measurement accuracy. Therefore, the sensor itself should be required to have a high signal-to-noise ratio and minimize the interference signal introduced from the outside. The sensitivity of the sensor is directional. When the measurement is a single vector and its directivity requirements are high, other sensors with small sensitivity should be selected; if the measurement is a multi-dimensional vector, the cross sensitivity of the sensor is required to be as small as possible.
3. Response characteristics (reaction time)
The frequency response characteristics of the sensor determine the frequency range to be measured. The measurement conditions must be kept undistorted within the allowable frequency range. In fact, the response of the sensor always has a certain delay, and the shorter the delay time, the better. The frequency response of the sensor is high, and the measurable signal frequency range is wide. Due to the structural characteristics, the inertia of the mechanical system is large, and the frequency of the signal that can be measured by the low frequency sensor is low. In dynamic measurement, the response characteristics should be based on the characteristics of the signal (steady state, transient, random, etc.) to avoid over-fire error.
4, linear range
The linear range of the sensor is the range in which the output is proportional to the input. In theory, within this range, the sensitivity remains constant. The wider the linear range of the sensor, the larger the range and the guaranteed measurement accuracy. When selecting a sensor, when the type of sensor is determined, it is first necessary to see if its range meets the requirements. But in reality, any sensor can't guarantee absolute linearity, and its linearity is relative. When the required measurement accuracy is relatively low, within a certain range, the sensor with less nonlinear error can be approximated as linear, which will bring great convenience to the measurement.
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