[ Abstract ] This article mainly introduces the working principle and characteristics of radiation thermometers, as well as practical application experience. It focuses on analyzing the influence of different usage environments on the measurement accuracy and error of radiation thermometers, and describes how to select a suitable radiation thermometer in a steel rolling mill. [ Keywords ] Blackbody, radiation thermometer, brightness temperature Introduction Radiation thermometers are widely used in high-temperature measurement in the metallurgical industry, mainly applied in smelting, casting, rolling, heat treatment, and refractory material production processes. In physics, when any object is above absolute zero, the movement of its internal charged particles radiates energy outward in the form of electromagnetic waves of a certain wavelength, but this energy changes with temperature according to a certain trend. Radiation thermometers are made using the principle that the radiant energy of an object changes with its temperature. During measurement, simply align the thermometer's optical receiving system with the object being measured (without contact) to measure the temperature of the moving object without disrupting its temperature field. Because the temperature-sensing element of a thermometer has the characteristic of receiving radiant energy without needing to reach the temperature of the object being measured, theoretically, there are no physical limitations to the measurement, and high temperatures are feasible in the measurement process. A radiation thermometer consists of a radiation sensor and a display instrument. It is used to measure high temperatures of 400-2000 ℃. Recently, with the development of infrared technology, the lower limit of temperature measurement has reached the room temperature range, greatly expanding the application range of radiation thermometry. 1. Working Principle of Radiation Thermometers 1.1 Planck's Law (Law of Monochromatic Radiation Intensity) In physics, objects above absolute zero possess energy that can be exchanged with surrounding objects in the form of radiation. An object radiates energy to its surroundings and absorbs energy from other surrounding objects. The radiated or absorbed energy is distributed according to wavelength, ranging from 0 to ∞. Ordinary objects radiate or absorb more energy at certain wavelengths or bands, and less or none at others. For ease of study, scientists have hypothesized an ideal object—a blackbody—that radiates and absorbs energy from the outside world in all wavelength bands. The energy W(λ,T),D emitted by a blackbody conforms to Planck's formula, as shown in equation (1): ………………………………(1) In equation (1), C1 and C2 are constants, λ is the wavelength, and T is the absolute temperature of the blackbody. Blackbody radiation has the following characteristics: ① The total radiative exitance W(λT) increases rapidly with increasing temperature; the higher the temperature, the greater the spectral radiative exitance. ② When the temperature is constant, the spectral radiative exitance changes according to a certain law with different wavelengths. The curve has a maximum value λm. When the wavelength is less than λm, the radiative exitance increases with increasing wavelength; when the wavelength is greater than λm, the change law is reversed. ③ When the temperature increases in the high-temperature band, the peak wavelength of the spectral radiative exitance will shift towards shorter wavelengths. With the name blackbody, other objects are called non-blackbody. Since the actual object being measured is not a blackbody, radiation thermometers made according to the radiation formula of a blackbody have a correction problem when measuring the temperature of an actual object, usually called emissivity ε(λ,T) correction. This is an inherent shortcoming of this type of thermometer. In fact, the emissivity ε(λ,T) of an object is not only related to the wavelength λ and the temperature T, but also to the surface condition of the object being measured, such as roughness. 1.2 Stefan-Boltzmann law is also called: total radiation intensity law, fourth power law. This law states that for an absolute blackbody at temperature T, the total wavelength of radiation emitted by a unit area element in the hemispherical direction is proportional to the fourth power of the temperature T, as shown in equation (2) ………………………(2) σ——Steffen-Boltzmann constant, which is 5.66961×10-3W/(m3K4) Equation (2) above is the theoretical basis for the temperature measurement of radiation thermometers. The total radiation intensity theorem is the result of integrating the monochromatic radiation intensity theorem over the entire wavelength. 2. Classification and Characteristics of Radiation Thermometers 2.1 Classification of Radiation Thermometers ① According to the number of wavelengths used, they can be classified into single-band, dual-band, and multi-band thermometers. ② According to the width of the wavelength, they can be classified into wide-band and narrow-band thermometers. ③ According to the position of the wavelength in the spectrum, they can be classified into short-wave and long-wave thermometers. ④ According to the temperature range they can measure, they can be classified into high-temperature and low-temperature thermometers. ⑤ According to the geometric sub-classification of the measured target, they can be classified into point thermometers and surface thermometers. Each classification, besides being related to the thermometer's structure and the selected components, is also related to the application environment; improper installation and use at the site will inevitably affect the accuracy of temperature measurement parameters in the production process. For users, selecting the appropriate thermometer according to different classification methods is the primary prerequisite for achieving the expected temperature measurement objectives in the production process. 2.2 Single-band thermometers have the advantage of easily determining the operating wavelength of the thermometer based on the spectral characteristics of the object being measured to obtain the desired results. For example, glass is transparent in the visible light spectrum, with an emissivity close to 1, very close to that of a blackbody. In this case, using the 4.8-5.6 μm wavelength band as the operating wavelength of the thermometer results in very high temperature measurement accuracy. However, when measuring generally opaque objects such as steel plates, it is better to choose a thermometer with a shorter operating wavelength. The emissivity ε(λ,T) of generally opaque objects is not 1 and varies significantly with wavelength. If the operator does not know the specific value of the emissivity and relies on experience, only partial correction can be made, but the fluctuations in emissivity that affect the measurement results, especially the part that changes with temperature, are difficult to correct. Generally speaking, emissivity and its changes have little impact on the temperature measurement accuracy of short-wavelength thermometers, but a greater impact on long-wavelength thermometers. 2.3 Dual-band thermometer This type of thermometer measures temperature by utilizing the functional relationship between the ratio of signals on two bands and the measured temperature, as shown in equation (3): ………………………………(3) Where V[sub]1[/sub] and V[sub]2[/sub] are the signals generated by the first and second bands, respectively. From the equation, we can see the characteristic of this thermometer: when the signals on both bands increase or decrease by the same amount, the ratio V[sub]1[/sub]/V[sub]2[/sub] remains unchanged, and thus the temperature value measured by the thermometer remains unchanged. This type of thermometer is used in situations where the target object is obscured by completely opaque low-temperature solid particles. Because the completely opaque low-temperature particles cause V[sub]1[/sub] and V[sub]2[/sub] to attenuate in the same proportion, the thermometer reading remains unchanged. This is something that a single-band thermometer cannot do. However, when the particles blocking the target are wavelength-selective, meaning they do not attenuate proportionally to V_sub and V_sub, the resulting temperature measurement error will be much larger than that of a single-wavelength thermometer. 3. Temperature Correction for Radiation Thermometers When the object being measured is not a blackbody, correction is necessary to obtain the true temperature of the non-blackbody. For radiation thermometers, the instrument is calibrated based on the radiation of an absolute blackbody, so the value measured by the instrument is called the radiation temperature. The definition of radiation temperature is: when the total radiant energy of a blackbody is equal to the total radiant energy of a non-blackbody, the temperature of the blackbody at this point is the radiation temperature of the non-blackbody. The temperature measured by the radiation pyrometer is called the radiation temperature TF. According to the total radiation intensity theorem, the total radiation energy is equal, as shown in equation (4): …………………………………(4) Where T——-the true temperature of the non-blackbody; T[sub]F[/sub]——-the radiation temperature of the non-blackbody; εT[sub][/sub]——-the total radiation emissivity coefficient of the non-blackbody (related to temperature T) Since ε[sub]T[/sub]＜1 for a non-blackbody, then TF＜T. Therefore, the temperature measured by the radiation pyrometer is lower than the true temperature of the object. In different usage environments, the setting of the emissivity coefficient directly affects the accuracy of the measured temperature and is a reference value. With different on-site environments and changes in the distribution of the medium, the measured value of the radiation thermometer fluctuates greatly. Generally speaking, the coefficient range is 0.7-0.85 when measuring steel plates, while it is generally around 0.3-0.6 when measuring aluminum products. 4. Causes and Countermeasures of Measurement Errors in Metallurgical Production Radiation thermometers inevitably experience interference from mechanical vibration, temperature changes, and electromagnetic fields during signal conversion and processing. Various interferences already exist before the radiation from the target object enters the radiation thermometer. These interferences typically include interference in the optical path, external light interference, and emissivity interference. Cause 1: In steel plate production, to prevent sticking during rolling and to descal the steel plate, a water film often remains on the surface of the target object. Absorbent gases such as water vapor, whose concentration changes frequently, also frequently exist near the target surface. These gaseous media selectively absorb the radiation energy from the steel plate in the optical path, weakening the radiation energy incident on the thermometer and causing measurement errors. Solution: When water vapor and a water film are present in the optical path, measuring in the short-wavelength band can reduce measurement errors. Simultaneously, using compressed air to disperse the water film and cleaning the optical path with instrument air are also effective methods. Reason 2: A lot of dust is suspended in the air at the production site. This dust absorbs radiation energy non-selectively and often causes diffuse reflection, weakening the radiation energy incident on the thermometer and causing measurement errors. Solution: Add air circulation and cleaning devices, and maintain a clean working environment to make the measurement environment more suitable. Reason 3: Interference from external light (referring to light from other light sources that is reflected off the surface being measured and mixed with the measuring light). When measuring the surface temperature of steel plates, daylight interference causes reflection, resulting in measurement errors. Solution: When installing the thermometer, avoid the reflection positions and angles of sunlight and ambient lighting. For some fixed and unavoidable light sources, shielding devices should be installed. Reason 4: The emissivity of an object is not only related to temperature and wavelength, but also to factors such as surface roughness, corrosion, and oxidation even for the same substance. Solution: Improve production processes, strive to increase the roughness of the measured target, reduce the degree of corrosion, and reduce the oxidation of the measured target. 5. Measurement of Surface Temperature of Hot-Rolled Steel Plate After the steel billet is pushed out of the heating furnace, it passes through the inlet and outlet of the roughing mill, the inlet and outlet of the finishing mill, and the inlet and outlet of the straightening mill. Temperature measurement points are required at each of these points according to process requirements, serving as key operational points in the rolling process to measure the surface temperature of the steel plate. The hot rolling process has the following metrological requirements for temperature measurement: ① Due to the high rolling speed, a radiation thermometer with a fast response time is required; ② Throughout the rolling process, the initial temperature measurement range of the steel billet after it exits the heating furnace and reaches the straightening mill is generally 500℃-1300℃ (with a large temperature difference between the high and low ends); ③ Due to repeated rolling and cooling, the blackness of the steel plate surface and the surrounding water vapor concentration vary. The measurement errors at the high and low ends should be similar; ④ Water films often appear in certain areas of the strip surface, and the size and adhesion points of these films are unpredictable. Furthermore, flaky rust, scale, and dust often adhere to the steel plate surface. The temperature measuring instrument should have appropriate corrections. Therefore, the environmental characteristics and metrological requirements of the target object should be considered when selecting a radiation thermometer. After comparing working principles and practical application, the silicon photovoltaic pyrometer, with its fast response speed, short working wavelength (around 1μm), ability to avoid the main absorption peak of water vapor, and high water film transmittance below 1μm, is a suitable choice for measuring the surface temperature of targets in the rolling process. 6. Summary A radiation thermometer is a non-contact measuring instrument. Its working principle is based on the total radiation intensity theorem, which states that the total radiation intensity of an object is proportional to the fourth power of its temperature. To meet metrological requirements in the high-temperature environment of the rolling process, adding a water-cooling jacket to the radiation sensor is a good approach. Currently, the main challenge for this type of thermometer is that a more effective instrument type and technical measure to fundamentally eliminate the influence of emissivity and environmental media on temperature measurement accuracy has not yet been found, requiring further exploration. [Author Biography] Yue Linping (1972–), female (Han nationality), from Jinan, Shandong Province, works at the Medium and Heavy Plate Plant of Jinan Iron & Steel Group Co., Ltd., holds a master's degree, and is engaged in the application and maintenance of field computers and instruments. Contact Information: Medium and Heavy Plate Plant, Jinan Iron & Steel Group, Shandong Province, Postcode: 250101, Mobile: 13864172994