Theoretical Principle and Application of Infrared Thermometer
There are many ways to measure temperature. Thermometers can be divided into two types: contact temperature measuring instruments and non-contact temperature measuring instruments. The contact type includes the familiar liquid thermometer, thermocouple thermometer and thermal resistance thermometer, etc. As we all know, temperature is one of the most important parameters in heating, gas supply, ventilation and air conditioning systems. Especially in the process of thermal engineering measurement, the accuracy of temperature is often the key to the success or failure of the experiment. Therefore, a high-accuracy temperature measuring instrument is essential in engineering. Therefore, this article introduces some principles and applications of infrared thermometers in temperature measurement tools.
Theoretical principle of infrared temperature measurement:
In nature, when the temperature of an object is higher than the absolute zero, due to the existence of internal thermal motion, it will continuously radiate electromagnetic waves to the surroundings, including infrared rays with a waveband of 0.75µm~100µm. Its characteristic is that at a given temperature and wavelength, the radiant energy emitted by an object has a maximum value. This kind of material is called a black body, and its reflection coefficient is set to 1. The reflection coefficient of other materials is less than 1, called It is a gray body, because the spectral radiation power P(λT) of the black body and the maximum temperature T satisfy Planck's determination. It shows that at the maximum temperature T, the radiation power of the black body per unit area at the wavelength λ is P(λT).
As the temperature rises, the radiant energy of the object becomes stronger. This is the starting point of infrared radiation theory and the design basis of single-band infrared thermometer.
As the temperature increases, the radiation peak moves to the short-wave direction (to the left), and it satisfies the Wien displacement theorem, the wavelength at the peak is inversely proportional to the maximum temperature T, and the dotted line is the line connecting the peak. This formula tells us why high temperature thermometers mostly work at short waves, and low temperature thermometers mostly work at long waves.
The rate of change of radiant energy with temperature is larger at the short-wave than at the long-wave, that is, the thermometer working at the short-wave has a relatively high signal-to-noise ratio (high sensitivity) and strong anti-interference. The thermometer should try to choose to work at the peak wavelength. Especially in the case of low temperature and small targets, this is particularly important.
Two: Infrared thermometer is composed of optical system, photoelectric detector, signal amplifier, signal processing, display output and other parts. The radiation from the measured object and the feedback source is modulated by the modulator and then input to the infrared detector. The difference between the two signals is amplified by the anti-amplifier and controls the temperature of the feedback source, so that the spectral radiance of the feedback source is the same as that of the object. The display indicates the brightness temperature of the measured object
Performance indicators and selection of three infrared thermometers:
The performance indicators of infrared thermometers include: temperature measurement range, display resolution, accuracy, working environment temperature range, repeatability, relative humidity, response time, power supply, response spectrum, size, maximum value display, weight, emissivity, etc. Pay attention to the following when selecting:
1. Determine the temperature measurement range: The temperature measurement range is the most important performance index of the thermometer. Each type of thermometer has its own specific temperature range. Therefore, the user's measured temperature range must be considered accurately and comprehensively, neither too narrow nor too wide. According to the law of black body radiation, in the short wavelength band of the spectrum, the change of radiant energy caused by temperature will exceed the change of radiant energy caused by emissivity error.
2 Determine the target size: Infrared thermometers can be divided into single-color thermometers and two-color thermometers (radiation colorimetric thermometers) according to the principle. For a monochromatic thermometer, when measuring temperature, the area of the target to be measured should fill the field of view of the thermometer. It is recommended that the measured target size 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 and acoustic symbols of the thermometer and interfere with the temperature measurement readings, causing errors. Conversely, if the target is larger than the pyrometer's field of view, the pyrometer will not be affected by background outside the measurement area. For a two-color pyrometer, the temperature is determined by the ratio of radiant energy in two independent wavelength bands. Therefore, when the target to be measured is small, does not fill the field of view, and there are smoke, dust, and obstructions on the measurement path, which attenuate the radiation energy, it will not have a significant impact on the measurement results. For small and moving or vibrating targets, the two-color thermometer is the best choice. This is due to the small diameter of the light rays and their flexibility to transport light radiant energy over curved, blocked and folded channels.
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 to the target and the diameter of the measured target. If the thermometer must be installed far away from the target due to environmental conditions, and a small target must be measured, a 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 pyrometer. If the thermometer is far away from the target and the target is small, a thermometer with a high distance coefficient should be selected. For a pyrometer with a fixed focal length, the focal point of the optical system is the smallest position of the spot, and the spot near and far from the focal point will increase. There are two distance factors.
4. Determine the wavelength range: The emissivity and surface characteristics of the target material determine the corresponding wavelength of the pyrometer spectrum. For high reflectivity alloy materials, there is low or variable emissivity. In the high temperature area, the best wavelength for measuring metal materials is near-infrared, and 0.8-1.0 μm can be selected. Other temperature zones can choose 1.6μm, 2.2μm and 3.9μm. Since some materials are transparent at a certain wavelength, infrared energy will penetrate these materials, and a special wavelength should be selected for this material.
5 Determine the response time: The response time indicates the reaction speed of the infrared thermometer to the measured temperature change, which is defined as the time required to reach 95% of the energy of the final reading, and it is related to the time constant of the photodetector, signal processing circuit and display system related. If the moving speed of the target is very fast or when measuring a fast-heating target, a fast-response infrared thermometer should be selected, otherwise the sufficient signal response will not be achieved, and the measurement accuracy will be reduced. However, not all applications require a fast-response infrared thermometer. For static or target thermal processes with thermal inertia, the response time of the pyrometer can be relaxed.
6. Signal processing function: In view of the difference between the discrete process (such as parts production) and the continuous process, the infrared thermometer is required to have multiple signal processing functions (such as peak hold, valley hold, average value) to choose from, such as temperature measurement on the conveyor belt When the bottle is used, it is necessary to use the peak value to hold, and the output signal of its temperature is sent to the controller. Otherwise the thermometer reads a lower temperature value between the bottles. If using peak hold, set the thermometer response time to be slightly longer than the time interval between bottles so that at least one bottle is always under measurement.
7 Consideration of environmental conditions: The environmental conditions of the thermometer have a great influence on the measurement results, which should be considered and properly resolved, otherwise it will affect the temperature measurement accuracy and even cause damage. When the ambient temperature is high and there is dust, smoke and steam, the protective cover, water cooling, air cooling system, air purger and other accessories provided by the manufacturer can be selected. These accessories can effectively address environmental influences and protect the thermometer for accurate temperature measurement. When specifying accessories, standardize service should be requested as much as possible to reduce installation costs.
8. Calibration of the infrared radiation thermometer: the infrared thermometer must be calibrated so that it can correctly display the temperature of the measured target. If the temperature measurement of the thermometer used is out of tolerance during use, it needs to be returned to the manufacturer or the repair center for re-calibration.
Features of Four Infrared Thermometer
1. Non-contact measurement: It does not need to touch the inside or surface of the measured temperature field, so it will not interfere with the state of the measured temperature field, and the thermometer itself will not be damaged by the temperature field.
2. Wide measurement range: Because it is a non-contact temperature measurement, the thermometer is not in a higher or lower temperature field, but works at a normal temperature or under the conditions allowed by the thermometer. Under normal circumstances, it can measure minus tens of degrees to more than three thousand degrees.
3. Fast temperature measurement speed: that is, fast response time. As long as the infrared radiation of the target is received, the temperature can be fixed in a short time.
4. High accuracy: Infrared temperature measurement will not destroy the temperature distribution of the object itself like contact temperature measurement, so the measurement accuracy is high.
5. High sensitivity: As long as there is a small change in the temperature of the object, the radiation energy will change greatly, which is easy to detect. It can measure the temperature of tiny temperature field and
6. Temperature distribution measurement, and temperature measurement of moving or rotating objects. Safe and long service life.
Disadvantages of five infrared thermometers:
1. Vulnerable to environmental factors (ambient temperature, dust in the air, etc.)
2. It has a great influence on the temperature reading of the bright or polished metal surface
3. Only limited to measuring the external temperature of the object, it is inconvenient to measure the temperature inside the object and when there are obstacles
Precautions for the use of six infrared thermometers:
(1) The emissivity of the object under test must be accurately determined;
(2) Avoid the influence of high temperature objects in the surrounding environment;
(3) For transparent materials, the ambient temperature should be lower than the temperature of the measured object;
(4) The thermometer should be aligned vertically to the surface of the object to be measured, and under no circumstances should the angle exceed 30°C
(5) It cannot be used for temperature measurement on bright or polished metal surfaces, and cannot be used for temperature measurement through glass;
(6) Correctly choose the follow-up coefficient, the target diameter must fill the field of view;
(7) If the infrared thermometer is suddenly exposed to an ambient temperature difference of 20°C or higher, the measurement data will be inaccurate, and the measured temperature value will be taken after the temperature is balanced. .
Seven improvement plans:
Since the ordinary infrared thermometer is only limited to measuring the external temperature of the object, it is inconvenient to measure the temperature inside the object and when there are obstacles, so a section of optical fiber can be added to the detection head, and a lens with a small viewing angle can be installed at the front end, so that The radiant energy of the measured object passes through the lens to the inside of the optical fiber. After multiple reflections in the optical fiber, it is transmitted to the detector. Because the optical fiber can be bent freely, the radiation can be turned freely, which solves the problem of measuring the internal temperature of the object, and can measure the temperature of places such as corners blocked by obstacles.
