How to correctly select infrared thermometer

2022-08-06
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How to correctly select infrared thermometer

selecting infrared thermometer can be divided into three aspects:

(1) performance indicators, such as temperature range, spot size, working wavelength, measurement accuracy, window, display and output, response time, protection accessories, etc

(2) environment and working conditions, such as ambient temperature, window, display and output, protective accessories, etc

(3) other options, such as ease of use, maintenance and calibration performance and price, also have a certain impact on the selection of thermometer

with the continuous development of technology, the best design and new progress of infrared thermometer provide users with various functions and multi-purpose instruments, and expand their choice. Other options, such as ease of use, maintenance and calibration performance and price. When selecting the type of thermometer, the measurement requirements shall be determined first, such as the temperature of the measured target, the size of the measured target, the measurement distance, the material of the measured target, the environment of the target, the response speed, the measurement accuracy, whether to use portable or portable, etc; Select the instrument model that can meet the above requirements from the comparison of various existing models of temperature detectors; Select the best combination of performance, function and price among the models that can meet the above requirements

1. Determine the temperature measurement range

determine the temperature measurement range: the temperature measurement range is the most important performance index of the thermometer. For example, the product coverage of Raytek is -50 ℃ - +3000 ℃, but it cannot be completed by one type of infrared thermometer. Each type of thermometer has its own specific temperature measurement range. Therefore, the user's measured temperature range must be accurate and comprehensive, neither too narrow nor too wide. According to the blackbody radiation law, the change of radiation energy caused by temperature in the short band of the spectrum will exceed the change of radiation energy caused by emissivity error. Therefore, short wave should be selected as much as possible for temperature measurement. Generally speaking, the narrower the temperature measurement range, the higher the resolution of the output signal for monitoring the temperature, and the accuracy and reliability are easy to solve. If the temperature measurement range is too wide, the temperature measurement accuracy will be reduced. For example, if the measured target temperature is 1000 ℃, for example, ordinary carbon structural steel, first determine whether it is portable or portable, and if it is portable. There are many models meeting this temperature, such as 3ilr3, 3i2m and 3i1m. If the measurement accuracy is important, it is better to select 2m or 1m model, because if 3ilr model is selected, its temperature measurement range is very wide, and the high-temperature measurement performance is poor; If the user has to take care of the low temperature target in addition to measuring the target of 1000 ℃, he has to choose 3ilr3

. 2. Determine the target size

infrared thermometer can be divided into monochrome thermometer and two-color thermometer (radiation Colorimetric Thermometer) according to the principle. For the monochrome thermometer, the measured target area shall fill the field of view of the thermometer during temperature measurement. It is recommended that the size of the measured object exceed 50% of the field of view. If the target size is smaller than the field of view, the background radiation energy will enter the visual acoustic symbol branch of the thermometer to interfere with the temperature measurement reading and cause errors. On the contrary, 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 colorimetric thermometers, the temperature is determined by the ratio of radiation energy in two independent wavelength bands. Therefore, when the measured target is very small, does not fill the field of view, and there is smoke, dust and obstruction on the measurement path, which will have a significant impact on the measurement results. For small moving or vibrating targets, the colorimetric thermometer is the best choice. This is due to the small diameter and flexibility of the light, which can transmit light radiation energy in the curved, blocked and folded channels

for Raytek two-color thermometer, its temperature is determined by the ratio of radiation energy in two independent wavelength bands. Therefore, when the measured target is very small and does not fill the site, and there is smoke, dust and barrier on the measurement path that attenuates the radiation energy, the measurement results will not be affected. Even when the energy is attenuated by 95%, the required temperature measurement accuracy can still be guaranteed. For the target which is small and in motion or vibration; Sometimes the target moves in the field of view, or may partially move out of the field of view. Under this condition, using a two-color thermometer is the best choice. If it is impossible to aim directly between the thermometer and the target, and the measurement channel is bent, narrow, blocked, etc., the two-color optical fiber thermometer is the best choice to save all the data that will help to successfully establish the same processing conditions in the future. Because of its small diameter and flexibility, it can transmit light radiation energy in curved, blocked and folded channels. Therefore, it can measure targets that are difficult to approach, in bad conditions or close to electromagnetic fields

. 3. Determine the distance coefficient (optical resolution)

the distance coefficient is determined by the ratio of d:s, that is, the ratio of the distance d between the probe of the thermometer and the target to the diameter of the measured target. If the thermometer must be installed far away from the target due to environmental conditions, and it is necessary to measure small targets, the thermometer with high optical resolution should be selected. The higher the optical resolution, i.e. increasing the d:s ratio, the higher the cost of the thermometer. Raytek infrared thermometer d:s ranges from 2:1 (low distance coefficient) to higher than 300:1 (high distance coefficient). If the thermometer is far away from the target and the target is small, the thermometer with high distance coefficient should be selected. For a thermometer with a fixed focal length, the spot at the focal point of the optical system is the minimum position, and the spot near and far from the focal point will increase. There are two distance coefficients. Therefore, in order to accurately measure the temperature at the distance close to and away from the focus, the size of the measured target should be larger than the spot size at the focus. The zoom thermometer has a minimum focus position, which can be adjusted according to the distance to the target. If d:s is increased, the received energy will be reduced. If the receiving aperture is not increased, the distance coefficient d:s is difficult to be increased, which will increase the instrument cost

4. Determine the wavelength range

the emissivity and surface characteristics of the target material determine that the corresponding wavelength of the spectrum of the thermometer has low or varying emissivity for high reflectivity alloy materials. In the high temperature area, the best wavelength for measuring metal materials is near infrared, which can be 0.8 ~ 1.0 μ m。 1.6 for other temperature zones μ m,2.2 μ M and 3.9 μ m。 Because some materials are transparent at a certain wavelength, infrared energy will penetrate these materials. The profits of petroleum processing, chemical industry, building materials, nonferrous metals and other industries increased by 25.12%, 25.64%, 17.81%, 125.22% and 18.9% respectively. Special wavelength should be selected for this material. If the internal temperature of the glass is measured, select 1.0 μ m,2.2 μ M and 3.9 μ M (the measured glass should be very thick, otherwise it will pass through) wavelength; 5.0 is selected for measuring glass surface temperature μ m; 8 ~ 14 are selected for low temperature measurement area μ M is appropriate. For example, 3.43 is selected for measuring polyethylene plastic film μ m. Polyester selection 4.3 μ M or 7.9 μ m. Selection of thickness exceeding 0.4mm μ m。 For example, narrow band 4.64 is used to measure CO in flame μ m. 4.47 for measuring NO2 in flame μ m。

5. Determine the response time

the response time represents the reaction speed of the infrared thermometer to the measured temperature change. It is defined as the time required to reach 95% of the energy of the final reading, which is related to the time constant of the photoelectric detector, signal processing circuit and display system. The response time of Raytek's new infrared thermometer can reach 1ms. This is much faster than the contact method. If the moving speed of the target is very fast or the target heated rapidly is measured, the fast response infrared thermometer shall be selected, otherwise the measurement accuracy will be reduced due to insufficient signal response. However, not all applications require fast response infrared thermometers. When there is thermal inertia in the static or target thermal process, the response time of the thermometer can be relaxed. Therefore, the choice of response time of infrared thermometer should adapt to the situation of the measured target. The response time is determined mainly according to the moving speed of the target and the temperature change speed of the target. The response time of the thermometer can be relaxed when the speed of the static target or the target participating in the thermal inertia, or the existing control equipment that can be modified appropriately when necessary, is limited

. 6. Signal processing function

in view of the difference between discrete processes (such as part production) and continuous processes, the infrared thermometer is required to have multiple signal processing functions (such as peak value holding, valley value holding and average value) for selection. For example, when measuring the bottle on the conveyor belt, the peak value should be used to hold, and its temperature output signal is transmitted to the controller. Otherwise, the thermometer reads the lower temperature value between the bottles. If peak hold is used, set the response time of the thermometer to be slightly longer than the time interval between bottles, so that at least one bottle is always under measurement

7. Environmental conditions consideration

the environmental conditions of the thermometer have a great impact on the measurement results, which should be considered and properly solved, otherwise it will affect the temperature measurement accuracy or even cause damage. When the ambient temperature is high and there is dust, smoke and steam, the protective sleeve, water cooling, air cooling system, air blower and other accessories provided by the manufacturer can be selected. These accessories can effectively solve the environmental impact and protect the thermometer to achieve accurate temperature measurement. When determining accessories, standardized services shall be required as far as possible to reduce installation costs. When smoke, dust or other particles reduce the measurement energy signal under noise, electromagnetic field, vibration or difficult to access environmental conditions, or other adverse conditions, the optical fiber two-color thermometer is the best choice. Colorimetric thermometer is the best choice. In case of noise, electromagnetic field, vibration, inaccessible environmental conditions, or other adverse conditions, it is advisable to select a light Colorimetric Thermometer

in the application of sealed or dangerous materials (such as containers or vacuum boxes), the thermometer shall observe through the window. The material must have sufficient strength and be able to pass through the working wavelength range of the used thermometer. It is also necessary to determine whether the operator also needs to observe through the window, so it is necessary to select the appropriate installation position and window materials to avoid mutual influence. In the application of low temperature measurement, ge or Si material is usually used as the window, which is impervious to visible light, and the human eye cannot observe the target through the window. If the operator needs to pass through the window target, optical materials that can both transmit infrared radiation and visible light shall be used. For example, optical materials that can both transmit infrared radiation and visible light, such as ZnSe or BaF2, shall be used as window materials

when there is flammable gas in the working environment of the thermometer, the intrinsic safety infrared thermometer can be selected to conduct safety measurement and monitoring in the flammable gas environment with a certain concentration

in case of severe and complex environmental conditions, a system with separate temperature measuring head and display can be selected to facilitate installation and configuration. The signal output form matching with the current control equipment can be selected

8. Calibration of infrared radiation thermometer

the infrared thermometer must be calibrated to correctly display the temperature of the measured target. If the used thermometer is out of tolerance during use, it shall be returned to the manufacturer or the maintenance center for recalibration

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