Infrared light, also called infrared light, was discovered by a British scientist. In 1800, when he studied sunlight, he made light split into a colored band through a prism. He used a thermometer to measure the heat contained in different colors in the band. During the test, he stumbled upon a strange phenomenon: a thermometer placed outside the red light of the light, higher than the other temperature in the room.
Find
After repeated trials, this so-called high-temperature zone with the most heat is always outside the red light at the edge of the light strip. So he announced that in addition to visible light, the sun emits a "hot line" that is invisible to the human eye. The "hot line" that the human eye cannot see is located outside the red light, called infrared. (However, it is important to note that the energy emitted by the sun is actually the strongest with green light at a wavelength of 580 nm.)
Infrared is an electromagnetic wave that has the same essence as radio waves and visible light. The infrared wavelength is between 0.76 and 100 μm and lies between radio waves and visible light. Any object, as long as its temperature is higher than 273 degrees below zero, emits infrared rays without exception.
principle
In nature, all objects with temperatures above absolute zero are constantly emitting infrared radiation energy to the surrounding space. The magnitude of the infrared radiation energy of an object and its distribution by wavelength - has a very close relationship with its surface temperature. Therefore, by measuring the infrared energy radiated by the object itself, it can accurately measure its surface temperature, which is the objective basis on which infrared radiation is measured.
The optical system converges the target infrared radiation energy within its field of view. The size of the field of view is determined by the optical components of the thermometer and their position. The infrared energy is focused on the photodetector and converted into a corresponding electrical signal. The signal passes through the amplifier and the signal processing circuit, and is converted to the temperature value of the measured object after being corrected according to the algorithm of the internal treatment of the instrument and the target emissivity. In addition, the environmental conditions of the target and the thermometer should be considered, such as the influence of temperature, atmosphere, pollution and interference on the performance indicators and the correction method.
Blackbody law
The black body is an idealized radiator that absorbs the radiant energy of all wavelengths. Without the reflection and transmission of energy, the surface emissivity is 1. It should be pointed out that there is no real black body in nature, but in order to clarify and obtain the law of infrared radiation distribution, it is necessary to select a suitable model in theoretical research. This is the quantized vibrator model of body cavity radiation proposed by Planck. The law of Planck blackbody radiation, that is, the blackbody spectral irradiance expressed by wavelength, is the starting point of all infrared radiation theory, so it is called the blackbody radiation law.
The effect of the emissivity of an object on the temperature measurement of radiation: the actual objects present in nature are hardly black. The amount of radiation of all actual objects depends on the wavelength of the radiation and the temperature of the object, as well as the type of material that makes up the object, the method of preparation, the thermal process, and the surface state and environmental conditions. Therefore, in order to apply the law of blackbody radiation to all practical objects, it is necessary to introduce a proportionality factor, ie, emissivity, related to the material properties and surface state. According to the law of radiation, as long as the emissivity of the material is known, the infrared radiation characteristics of any object are known.
The main factors affecting the emissivity: material type, surface roughness, physical and chemical structure and material thickness.
When using the infrared radiation thermometer to measure the temperature of the target, first measure the amount of infrared radiation in the range of the target, and then calculate the temperature of the target by the thermometer. The monochromatic thermometer is proportional to the amount of radiation in the band; the two-color thermometer is proportional to the ratio of the amount of radiation in the two bands.
Test process
Performance indicators, such as temperature range, spot size, working wavelength, measurement accuracy, response time, etc.; environmental and working conditions, such as ambient temperature, window, display and output, protection accessories, etc.; other options, such as ease of use, maintenance And calibration performance and price, etc., also have a certain impact on the choice of thermometer. With the technology and continuous development, the best design and new development of infrared thermometers provide users with a variety of functions and versatile instruments, expanding the choice.
1 Determine the temperature range
The temperature range is one of the most important performance indicators of the thermometer. If the product coverage is -50 ° C - + 3000 ° C, but this can not be done by a model of infrared thermometer. Each type of thermometer has its own specific temperature range. Therefore, the user's measured temperature range must be considered accurate and comprehensive, neither too narrow nor too wide. According to the black body radiation law, the change of the radiant energy caused by the temperature in the short wavelength band of the spectrum will exceed the change of the radiant energy caused by the emissivity error. Therefore, it is better to select the short wave when measuring the temperature. In general, the narrower the temperature measurement range, the higher the resolution of the output signal of the monitoring temperature, and the accuracy and reliability are easily solved. If the temperature range is too wide, the temperature measurement accuracy will be lowered. For example, if the target temperature is 1000 ° C, first determine whether it is online or portable, if it is portable. There are many models that meet this temperature, such as Ti315, Ti213, etc.
2 determine the target size
According to the principle, the infrared thermometer can be divided into a monochrome thermometer and a two-color thermometer (radiation colorimeter). For a monochrome thermometer, the measured target area should be filled with the field of view of the thermometer when measuring temperature. It is recommended that the measured target size exceeds 50% of the field of view size. If the target size is smaller than the field of view, the background radiant energy will enter the thermometer's visual accumulator to interfere with the temperature reading, causing an error. Conversely, if the target is larger than the field of view of the thermometer, the thermometer will not be affected by the background outside the measurement area. For a two-color thermometer, the temperature is determined by the ratio of the radiant energy in two independent wavelength bands. Therefore, when the measured target is small, there is no full scene, there is smoke, dust and blocking on the measurement path, and the radiation energy is attenuated, and the measurement result will not be affected. Even when the energy is attenuated by 95%, the required temperature measurement accuracy can be guaranteed. For targets with small targets that are in motion or vibration; sometimes moving within the field of view, or possibly partially moving out of the field of view, under these conditions, the use of a two-color thermometer is the best choice. A two-color fiber optic thermometer is the best choice if there is no direct aiming between the thermometer and the target, and the measurement channel is curved, narrow, and obstructed. This is due to its small diameter and flexibility to transmit optical radiant energy over curved, blocked and folded channels, so that it is possible to measure targets that are difficult to access, have harsh conditions, or are close to electromagnetic fields.
3 determine the optical resolution
The optical resolution is determined by the ratio of D to S, which is the ratio of the distance D between the thermometer to the target and the diameter S of the measurement spot. For example, in the infrared era, the hand-held infrared thermometer Ti213 has a distance coefficient of 80:1. If it is 80 cm away from the target, the diameter of the measurement range is 1 cm. If the thermometer must be installed away from the target due to environmental constraints, but to measure small targets, you should choose a high optical resolution thermometer. The higher the optical resolution, ie the higher the D:S ratio, the higher the cost of the thermometer.
4 determine the wavelength range
The emissivity and surface characteristics of the target material determine the spectral response or wavelength of the thermometer. For high reflectivity alloy materials, there is a low or varying emissivity. In the high temperature zone, the optimum wavelength for measuring metal materials is near-infrared, with a wavelength of 0.18-1.0 μm. Other temperature zones are available with 1.6μm, 2.2μm and 3.9μm wavelengths. Since some materials are transparent at certain wavelengths, infrared energy can penetrate these materials, and special wavelengths should be chosen for this material. For example, the internal temperature of the measuring glass should be 10μm, 2.2μm and 3.9μm (the glass to be tested should be very thick, otherwise it will pass through); the internal temperature of the measuring glass should be 5.0μm; the lower area should be 8-14μm; The polyethylene plastic film is selected to have a wavelength of 3.43 μm, and the polyacetate is selected to have a wavelength of 4.3 μm or 7.9 μm. The thickness of the electrode exceeds 0.4 mm and the wavelength of 8-14 μm is selected. For example, the wavelength of the CO2 in the flame is 4.24-4.3 μm, and the wavelength of C0 in the flame is measured by a narrow band of 4.64 μm, and the wavelength of N02 in the flame is measured by 4.47 μm.
5 determine the response time
The response time represents the response speed of the infrared thermometer to the measured temperature change, defined as the time required to reach 95% of the final reading, which is related to the time constant of the photodetector, signal processing circuitry, and display system. The new infrared thermometer has a response time of up to 1ms. This is much faster than the contact temperature measurement method. If the target moves quickly or measures a fast-heating target, use a fast-response infrared thermometer. Otherwise, sufficient signal response will not be achieved, which will reduce the measurement accuracy. However, not all applications require a fast-responding infrared thermometer. For thermal inertia in stationary or target thermal processes, the response time of the thermometer can be relaxed. Therefore, the response time of the infrared thermometer should be selected to match the situation of the target.
6 signal processing function
In view of the fact that discrete processes (such as parts production) and continuous processes are different, infrared thermometers are required to have multiple signal processing functions (such as peak hold, valley hold, average), such as when measuring bottles on a conveyor belt. To maintain with peak, the output signal of its temperature is transmitted to the controller. Otherwise the thermometer reads the lower temperature value between the bottles. If peak hold is used, set the thermometer response time to be slightly longer than the time interval between the bottles so that at least one of the bottles is always in the measurement.
7 environmental conditions considerations
The environmental conditions in which the thermometer is located have a great influence on the measurement results and should be considered and properly resolved. Otherwise, the temperature measurement accuracy or even damage will be affected. When the ambient temperature is high and there are dust, smoke and steam, you can use the manufacturer's protective cover, water cooling, air cooling system, air purifier and other accessories.
These accessories effectively address environmental impacts and protect the thermometer for accurate temperature measurement. When determining attachments, standardization services should be required as much as possible to reduce installation costs. A fiber optic two-color thermometer is the best choice when reducing smoke, dust, or other particles in a noise, electromagnetic field, vibration, or inaccessible environment, or other harsh conditions. Fiber optic colorimetric thermometers should be selected for noise, electromagnetic fields, vibrations, and inaccessible environmental conditions, or other harsh conditions.
In sealed or hazardous material applications (eg containers or vacuum boxes), the thermometer is observed through a window. The material must be strong enough to pass the operating wavelength range of the thermometer used. Also determine if the operator also needs to observe through the window, so choose the appropriate mounting location and window material to avoid mutual influence. In low temperature measurement applications, Ge or Si materials are often used as windows, which are not transparent to visible light, and the human eye cannot observe the target through the window. If the operator needs to pass through the window target, an optical material that transmits both infrared radiation and visible light should be used. For example, an optical material such as ZnSe or BaF2 that uses both infrared radiation and visible light should be used as the window material.
When there is a flammable gas in the working environment of the thermometer, an intrinsically safe infrared thermometer can be selected to perform safety measurement and monitoring in a certain concentration of flammable gas environment.
In the case of harsh environmental conditions, a separate system of temperature probes and displays can be selected for ease of installation and configuration. A signal output form that matches the current control device can be selected.
application
Infrared temperature measurement technology plays an important role in the production process, in product quality control and monitoring, equipment online fault diagnosis and safety protection, and energy conservation. In the past 20 years, the non-contact infrared thermometer has been rapidly developed in technology, its performance has been continuously improved, its functions have been continuously enhanced, its variety has been increasing, its scope of application has been continuously expanded, and its market share has increased year by year. The advantages of domestic production are high in cost performance, low in relative import prices, and convenient and quick after sale. Compared with the contact temperature measurement method, infrared temperature measurement has the advantages of fast response time, non-contact, safe use and long service life. Non-contact infrared thermometers include portable, in-line and scanning three series, and are available in a variety of options and computer software, each with various models and specifications. In all types of thermometers of different specifications, the correct choice of infrared thermometer model is very important for users.
Applications
Hot air furnace vault temperature measurement:
Hot air furnace vault is a special occasion. The measurement of the vault temperature should not only solve the high temperature and high pressure problems of the target, but also adapt to the requirements of open air environment changes throughout the year. After repeated practice and improvement, we have designed a special protection device with a sealed window. The method is that the infrared probe with the protection device is fixedly mounted on the position of the thermocouple inserted on the vault, so that the probe can be aimed at the inside of the hot air furnace through the sealing window and the thermocouple socket, and the hot air furnace vault is measured. The temperature of the probe signal is sent to the instrument box installed on the control cabinet through the cable, and the temperature value is displayed, and the standard current signal of 4-20 mA is output and sent to the recorder or computer for processing. Since the infrared probe of the system does not contact the high temperature target, it has the characteristics of stable and reliable, long service life, and can usually be operated continuously for more than five years, which can reduce the consumption of the thermocouple and greatly reduce the operating cost and maintenance workload of the equipment.
Measurement of the internal temperature of the hot air duct:
The temperature measuring device of the system is basically similar to the dome temperature measuring system, and the infrared probe equipped with the protection device is installed on the jack of the original thermocouple of the hot air pipe, and the infrared probe is transmitted through the sealing window of the protection device, and then the original electricity. The even jack is aimed at the inside of the hot air duct to measure the temperature inside the hot air duct. Since the temperature measuring probe of the system is installed outside the sealing window, the entire temperature measuring system is completely isolated from the hot air in the tube, so the system can perform the exchange of the probe or other maintenance work without affecting the hot air working state in the tube. The commonly used thermocouples must be exchanged under the windy state, and the thermocouples are consumables, and the damage is frequent. The temperature measurement system can reduce the consumption of thermocouples and reduce the operating cost of the equipment.
Measurement of molten iron temperature before the furnace:
Yatai photoelectric infrared thermometer measures the molten iron in front of the furnace: the infrared temperature probe with the blower sleeve is fixedly mounted on the bracket 5-15 meters away from the molten iron, and the iron gutter is aimed at the continuous sweep of each furnace molten iron. The aiming measurement, the temperature signal measured by the probe is connected to the recorder to print the temperature curve or sent to the computer for data processing. The biggest advantage of this system is that it can automatically and continuously measure the temperature of each furnace molten iron, which can effectively avoid the influence of human factors caused by manual insertion of temperature measurement method. By analyzing the recorded temperature curve or other computer processing information, the trend of the temperature of each furnace molten iron can be truly reflected, and the maximum temperature of each furnace can be read out intuitively, which provides a powerful basis for production management and process improvement. The system has been used by Egang, Shougang and other units, and has won praise from users.
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