gas sensor

1 Overview Editing

A gas sensor is a device that converts information such as gas composition and concentration into information that can be used by people, instruments, computers, and the like! Gas sensors are generally classified as chemical sensors, although this classification is not necessarily scientific.

The “gas sensor” includes a semiconductor gas sensor, an electrochemical gas sensor, a catalytic combustion gas sensor, a thermal conductivity gas sensor, an infrared gas sensor, a solid electrolyte gas sensor, and the like.

2 feature editing

Gas sensors are a class of chemical sensors. From the working principle, characteristic analysis to measurement technology, from the materials used to the manufacturing process, from the test object to the application field, it can constitute an independent classification standard, resulting in a variety of complicated classification systems, especially on the issue of classification standards. There is no harmonization. It is very difficult to classify it strictly. Next, learn about the main characteristics of the gas sensor [1] :

1, stability

Stability refers to the stability of the sensor's basic response throughout the operating time, depending on zero drift and range drift. Zero drift refers to the change in sensor output response over the entire operating time when there is no target gas. Interval drift refers to the change in the output response of the sensor to the target gas continuously, which is reflected in the decrease of the sensor output signal during working hours. Ideally, a sensor drifts less than 10% per year at zero point 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, and is primarily dependent 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 sensitive enough to detect the percentage of the TLV-thresh-old limit value or the LEL-lower explosive limit of the target gas.

3, selective

Selectivity is also known as cross sensitivity. It can be determined by measuring the sensor response generated 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 the cross sensitivity will reduce the repeatability and reliability of the measurement. The ideal sensor should have high sensitivity and high selectivity.

4, corrosion resistance

Corrosion resistance refers to the ability of the sensor to be exposed to a high volume fraction of the target gas. In the event of a large gas leak, the probe should be able to withstand the desired gas volume fraction of 10 to 20 times. Under normal operating conditions, the sensor drift and zero correction values ​​should be as small as possible.

The basic characteristics of a gas sensor, namely sensitivity, selectivity and stability, are determined primarily by the choice of material. Select the appropriate materials and develop new materials to optimize the sensitivity of the gas sensor.

3 Select Edit

According to the measurement object and measurement environment

Determine the type of sensor based on the measurement object and the measurement environment. To carry out a specific measurement work, we must first consider the principle of the use of sensors, which need to analyze many factors before you can determine. Because, even if it is to measure the same physical quantity, there are many kinds of principle sensors available for selection. Which kind of principle sensor is more appropriate, we need to consider the following specific problems according to the characteristics of the measured and the use conditions of the sensor: the size of the range; The measured position of the sensor volume requirements; measurement method for contact or non-contact; signal extraction method, wired or non-contact measurement; sensor source, domestic or imported, the price can withstand, or self-developed. After considering the above issues, it is possible to determine which type of sensor to use, and then consider the specific performance indicators of the sensor.

The choice of sensitivity

Generally, in the linear range of the sensor, it is desirable that the sensitivity of the sensor is 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 advantageous for signal processing. However, it should be noted that the sensitivity of the sensor is high, and external noise that is not related to the measurement is easily mixed in, and it is also amplified by the amplification system, which affects the measurement accuracy. Therefore, it is required that the sensor itself should have a high signal-to-noise ratio and the interference signal introduced from the outside should be minimized. The sensitivity of the sensor is directional. When it is measured as a single vector and its directionality is higher, sensors with small sensitivity in other directions should be selected; if the measured is a multidimensional vector, the cross sensitivity of the sensor is required to be as small as possible.

Response characteristics (reaction time)

The frequency response characteristic of the sensor determines the frequency range to be measured. It must maintain the undistorted measurement condition within the allowable frequency range. Actually, the response of the sensor always has a certain delay. 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 influence of the structural characteristics, the inertia of the mechanical system is large, and the frequency of the measurable signal of the sensor with low frequency is low. In the dynamic measurement, the characteristics of the signal (steady-state, transient, random, etc.) response characteristics should be used to avoid overheating errors.

Linear range

The linear range of the sensor refers to the range of the output proportional to the input. In theory, in this range, the sensitivity remains constant. The wider the linear range of the sensor, the larger the range, and can guarantee a certain measurement accuracy. When selecting a sensor, when the type of the sensor is determined, it first depends on whether or not the range satisfies the requirement. But in fact, any sensor cannot guarantee absolute linearity, and its linearity is also relative. When the required measurement accuracy is relatively low, within a certain range, the sensor with smaller nonlinear error can be approximated as linear, which will bring great convenience to the measurement.

4 lack of editing

advantage

Infrared gas sensors and instruments are widely used and are suitable for monitoring nearly all kinds of flammable gases. With the advantages of high precision, good selectivity, high reliability, no poisoning, no dependence on oxygen, small environmental interference factors, long life, and other significant advantages. And gradually become the mainstream of the market in the future.

Shortcomings

Since it is in its infancy, it has high technical barriers, low market share, and low scale production, resulting in high costs, which are basically around a thousand dollars.

Category 5 Edit

Semi-conductive gas sensor

This type of sensor accounts for about 60% of gas sensors, according to its mechanism is divided into conductivity type and non-conductive type, conductance type is divided into surface type and volume control type.

(1) SnO2 semiconductor is a typical surface-type gas sensor. Its sensing principle is that SnO2 is an n-type semiconductor material. When the voltage is applied, the temperature of the semiconductor material increases, and the adsorbed oxygen receives the electrons in the semiconductor to form O2 or O2. When the gas H2, CO, and CH4 are present, the semiconductor surface resistance decreases, the conductance increases, and the conductance changes. The gas concentration turns into a drop. NiO is a p-type semiconductor, and the oxidizing gas reduces the conductance and is sensitive to O2. ZnO semiconductor sensors also belong to this type.

Semiconductor gas sensor

a. The conductance type sensor elements are divided into surface sensitive type and volume control type, and the surface sensitive type sensing materials are SnO2+Pd, ZnO ten Pt, AgO, V 205, metal gallium, Pt-SnO2. The surface-sensitive gas sensor can detect various flammable gases C0, NO2, and Freon. The gas sensor of the sensing material Pt-SnO2 can detect the gases as flammable gas CO, H2, CH4.

b. The volume control type sensing materials are Fe2O8, la1-SSrxCOO8 and TiO2, CoO-MgO-SnO2 body sensors. The detectable gases are various combustible gases CO, NO2, Freon. . Sensing material Pt-SnO2

Volume-controlled semiconductor gas sensors can detect gases such as liquefied petroleum gas, alcohol, air-fuel ratio control, and burner gas exhaust.

(2) The volume control type is the change of the lattice defect which leads to the change of conductivity, and the change of conductance is proportional to the gas concentration.

Fe2O8 and TiO2 belong to this category and are sensitive to flammable gases.

(3) The thermal linear sensor is a semiconductor sensor utilizing thermal conductivity change, also called a thermal linear semiconductor sensor. The SnO2 layer is coated on the Pt wire coil. The Pt wire has the function of detecting the temperature change in addition to the heating function. . When the voltage is applied, the semiconductor heats up and the surface absorbs oxygen, so that the concentration of free electrons decreases. When combustible gas is present, the concentration of free electrons increases due to combustion, and the thermal conductivity increases with the concentration of free electrons, and the heat dissipation rate increases accordingly. The temperature of the Pt wire decreases, the resistance decreases, and the change in the resistance of the Pt wire has a linear relationship with the gas concentration.

The sensor is small, stable, and resistant to poisons. It can detect low concentrations of gases and plays an important role in the detection of combustible gases.

(4) Non-conductive FET field effect transistor gas sensors, Pd-FET. Field-effect-transistor sensors use Pd to absorb Hz and diffuse to reach the interface between semiconductor Si and Pd, reducing the work function of Pd, which is sensitive to H2 and CO. The non-conductive FET field effect transistor gas sensor is small in size, easy to integrate and versatile, and is a promising gas sensor.

Solid electrolyte gas sensor

This kind of sensor element is an ion-pair solid electrolyte membrane conduction, called the electrochemical cell, is divided into cation conduction and anion conduction, is a selective sensor, research is more practical to use is a zirconium oxide solid electrolyte sensor, the mechanism is The potential difference between the two batteries on both sides of the diaphragm is equal to the potential of the rich battery. Stable chromia solid electrolyte sensors have been successfully applied to the determination of oxygen in molten steel and the measurement of engine air-fuel ratio components.

In order to make up for the lack of conductivity of the solid electrolyte, in recent years, a layer of gas-sensitive film was coated on the solid electrolyte, and the number of gas molecules existing in the peripheral environment was related to the number of movable particles in the medium.

Contact combustion gas sensor

The contact combustion sensor is suitable for the detection of flammable gases H2, CO, and CH4. Flammable gas surface contact catalyst

When Pt and Pd are burned and broken, the heat of combustion is related to the rich and rich gas. The application of this kind of sensor is wide, small in size, simple in structure and good in stability. The disadvantage is poor selectivity.

Electrochemical gas sensor

Electrochemical gas sensors are commonly used in two ways

(1) Constant Potential Electrolytic Sensor

The gas to be measured is ionized under a specific electric field, and the gas concentration is measured by the electrolytic current flowing through the gas. This sensor has a high sensitivity, and the selective gas detection gas may play an important role in the detection of the toxic gas.

(2) Primary cell gas sensor

In the KOH electrolyte solution, a Pt-Pb or Ag-Pb electrode constitutes a battery and has been successfully used for the detection of O2. The sensitivity is high. The disadvantage is that the water evaporates and absorbs moisture, and the electrode is easily poisoned.

Optical gas sensor

(1) Direct Absorption Gas Sensor

Infrared gas sensors are typical absorption optical gas sensors, which have their own inherent spectral absorption spectrum detection gas components according to the gas. Non-dispersive infrared absorption spectrum has high sensitivity to gases such as SO2, CO, CO2, and NO.

In addition, UV absorption, non-disperse UV absorption, correlated spectroscopy, second derivative, and self-modulated light absorption methods have high sensitivity to gases such as NO, NO2, SO2, and hydrocarbons (CH4).

(2) Light reactive gas sensor

The photoreactive gas sensor utilizes a gas reaction to generate a color change to cause changes in optical properties such as light intensity absorption, and the sensing element is ideal, but the gas light sensing variation is limited, and the degree of freedom of the sensor is small.

(3) New Sensors for Gas Optical Properties

The optical fiber temperature sensor is of this type. It coats the top of the fiber with a catalyst to react with the gas and generate heat. Temperature changes cause the temperature of the fiber to change. The use of optical fiber temperature measurement has reached a practical level, the detection of gas is also successful.

In addition, sensors that use other physical quantity changes to measure gas composition are continuously being developed. For example, SAW sensors such as SO2, NO2, H2S, NH3, and H2 have higher sensitivity.

6 harmful gas sensor selection skill editing

A major role of gas sensors for hazardous gas detection, harmful gas detection has two purposes, the first is to measure the explosion, and the second is to detect the virus. The so-called explosion test is to detect the gas content of dangerous places and exceed the standard alarm so as to avoid the occurrence of explosion accidents; the detection of poisons is to detect the contents of all poisonous gases in dangerous fields and exceed the standard alarm so as to avoid poisoning by workers.
There are three kinds of harmful gases first, non-toxic or low-toxic flammable, second, non-flammable, toxic, third, flammable and toxic. For these three different situations, generally we choose sensors that need to select different gas sensors. For example, the choice of flammable gas detection alarm instrument for measuring explosions, toxic gas detection and alarming instruments for poisoning detection, etc. Second, we need to choose the type of gas sensor, which is generally fixed and portable. Leakage detection for long-term operation of production or storage positions uses fixed gas sensors; other portable gas sensors are used for inspection, emergency detection, entry detection and inspection.
There are hundreds of types of gas sensors. There may be different selection techniques for different gas sensors. When customers choose their gas sensors, they can consult the technical personnel of the sensor manufacturers if they are not very clear. Let them choose the right gas for you. The sensor, or ask the sensor technician to investigate above in order to better select the gas sensor. [2]

7 Development Editor

First, focusing on the research and development of new gas-sensitive materials and manufacturing processes

Studies on gas sensor materials show that metal oxide semiconductor materials Zn0, SIlo2, Fe203, etc. have become mature, especially in gas detection such as C ratio, C2H5OH, and CO. There are two main directions for this work: [3]

1. It is the use of chemical modification and modification methods to do doping, modification and surface modification of existing gas-sensitive membrane materials, and to improve and optimize the film formation process to improve the stability and selectivity of gas sensors.

2, is the development of new gas-sensitive membrane materials, such as composite and hybrid semiconductor gas-sensitive materials, high-molecular gas-sensitive materials, making these new materials for different gases with high sensitivity, high selectivity, high stability. Because organic polymer sensitive materials have the advantages of rich materials, low cost, simple film making process, easy compatibility with other technologies, and working at room temperature, it has become a research hotspot.

Second, the development of a new type of gas sensor

With the traditional principle of action and some new effects, priority is given to the use of crystalline materials (silicon, quartz, ceramics, etc.), advanced processing techniques and microstructural design, development of new sensors and sensor systems, such as optical waveguide gas sensors, polymer sound Surface wave and quartz resonant gas sensor development and use, microbial gas sensor and biomimetic gas sensor research. With the application of new materials, new processes, and new technologies, the performance of gas sensors has become more perfect, making the sensors compact, miniaturized, and multi-functional with long-term stability, ease of use, and low cost.

Third, 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 atmospheric pollution, industrial exhaust gases, and the detection of food and living environment quality have raised the demand for gas sensors. The request. The successful application of new material development technologies such as nanometer and thin film technology provides a good prerequisite for gas sensor integration and intelligence. Gas sensors will be developed on the basis of full use of multidisciplinary technologies such as micro-mechanics and microelectronics, computer technology, signal processing technology, sensing technology, fault diagnosis technology, and intelligent technology. The development of fully automatic digital smart gas sensors capable of simultaneously monitoring multiple gases will be an important research direction in this field.

8 Application Editing

Applied to the construction of the Internet of Things. Gas sensors have been widely used in the detection of toxic gases, combustibles, explosives, carbon dioxide, and other gases. Environmental issues have always been one of the most concerned topics in the country and even in the world. The environment on which people depend has been severely damaged. To protect the environment requires the establishment of an environmental monitoring mechanism, and the construction of the Internet of Things becomes a necessity. Gas sensors, as necessary sensors for environmental monitoring, will contribute to the construction of an environmental Internet of Things.

The sensor is the core and the most basic link of the Internet of Things. It is a bridge of various kinds of information and artificial intelligence. It is one of the important categories of gas sensors in the technical field, spanning functional materials, electronic ceramics, optoelectronic components, MEMS technology, nanometers. Technology, organic polymers and many other basic and applied disciplines. The high-performance gas sensor can greatly improve the level of information collection, processing, and deep processing, improve the accuracy of real-time prediction of accidents, continuously eliminate potential accidents, and greatly reduce accidents, especially major accidents. It can effectively realize the electronicization of safety supervision and safety production supervision and management, change passive disaster relief into active disaster prevention, and advance safety production to scientific management. [4]