1. Introduction Based on the working characteristics of heat treatment furnaces and the requirements of heat treatment processes for temperature parameter measurement and control, radiation pyrometers are used to measure the temperature of high-temperature salt bath furnaces. This article discusses how to minimize measurement errors, eliminate interference from the surrounding environment, ensure unobstructed access on site, and ensure proper installation, use, and maintenance. 2. Temperature Measurement Different objects in nature emit different amounts of thermal radiation energy at the same temperature. According to Kirchhoff's law: E = εT·σ·T⁴ Where E is the thermal radiation energy; εT is the absorption coefficient; and T is the surface temperature. An object's radiation capacity is directly proportional to its absorption capacity. An object that can completely absorb the thermal radiation energy irradiated onto it is called a blackbody, and its absorption coefficient is 1. The absorption coefficients of other objects are all less than 1. Generally, a blackbody is chosen as the standard body, and calibration is based on the temperature of the blackbody. [b]3 Structural Principle[/b] The radiation pyrometer is designed and manufactured based on the functional relationship between the radiant energy of an object and its temperature over the entire wavelength range. It uses a radiation temperature sensor as the primary instrument and an electronic potentiometer as the secondary instrument. It is a lens-focusing temperature sensor with an aluminum alloy shell. The objective lens is located at the front, and a thermopile compensation aperture is installed inside the shell. There is an adjustment plate on the field aperture close to the thermopile. The function of the adjustment plate is to adjust the radiant energy irradiated onto the thermopile, so that the product has a uniform scale value. An eyepiece is installed on the detachable rear cover to observe the image of the object being measured. The radiation thermometer focuses the radiation energy of the object being measured onto the thermistor through a lens. The thermistor converts the radiation energy into electrical parameters. The thermoelectric potential is measured by a secondary instrument based on the known relationship between the thermoelectric potential and the object's temperature, and the temperature value is displayed. This temperature value must be corrected using the object's total radiation blackbody coefficient or by directly inserting a platinum-rhodium 10-platinum thermocouple into a high-temperature salt bath furnace with a DC potentiometer to measure the temperature. The temperature is then compared with the temperature displayed by the instrument to calibrate the accuracy of the pyrometer's temperature measurement. [b]4 Measurement Error[/b] The main factors affecting the accurate temperature measurement of the radiation thermometer are as follows: (1) Influence of the radiation emissivity coefficient The object being measured is not an absolute blackbody, but a gray body, i.e. (ε≠1), which will inevitably cause measurement error. The true temperature T and the radiation temperature TF of the object have the following relationship: If the emissivity coefficient of the blackbody furnace is measured, the error caused by the standard source in the transmission can be calculated. (2) Influence of installation distance The ratio of the distance between the radiating object and the thermometer to the diameter of the radiating body should be 20, otherwise it will cause error. (3) Influence of intermediate medium: Since the intermediate medium absorbs radiation energy, the radiation energy received by the radiation temperature sensor is weakened, causing measurement error. Therefore, compressed air equipment can be installed in the field to blow away intermediate medium such as soot. (4) Influence of ambient temperature: If the ambient temperature is high, the heating of the objective lens will cause the temperature of the thermopile reference end to rise, and the output will decrease. The reference end temperature compensator cannot fully compensate, which will bring measurement error. The experiment shows that when the ambient temperature is 80℃ and the indicated temperature is 1000℃, the incomplete compensation can produce a measurement error of ±10℃. Therefore, the ambient temperature should not be higher than 50℃ in the field. If it is higher, its shell should be placed in a cooling device to reduce the working environment temperature and improve the measurement accuracy. (5) Influence of thermal inertia: Since the thermopile in the radiation temperature sensor has a certain thermal inertia, it is generally required to be stable for 4 to 12 seconds. Therefore, the observation time should exceed 10 seconds before reading the temperature. [b]5 Interference Factors[/b] Since the radiation pyrometer does not come into contact with the object being measured, environmental factors at the installation site can significantly affect the measurement. External light interference refers to light from an external light source that is incident on the surface being measured and reflected into the measuring light. Examples include sunlight during outdoor measurements, indoor lighting, and nearby heating furnaces and flames. For some fixed and unavoidable external light sources, shielding devices can be installed. If operating in a very high ambient temperature, the shielding device can also become a new heat source, requiring cooling with water or air to reduce its radiation. Changing the measurement direction can also help avoid external light exposure. The spatial distance between the surface being measured and the radiation pyrometer during measurement is called the optical path. In the air at the production site, there are water vapor, carbon dioxide, floating scum, smoke, oil mist, dust, and other substances. Water vapor and carbon dioxide gases selectively absorb radiation energy, absorbing certain wavelengths while easily transmitting others. Floating scum, smoke, oil mist, and dust absorb radiation energy non-selectively, but scatter it, weakening the radiation energy incident on the thermometer and causing measurement errors. Clean compressed air can be used to clear the optical path and disperse smoke. Regular deoxidation and scum removal are necessary for the salt bath furnace to maintain the normal brightness of the salt surface. [b]6 Installation, Use, and Maintenance[/b] The heating medium of the salt bath furnace is a molten liquid, mainly relying on convection heat transfer. The furnace temperature is highly uniform and can represent the workpiece temperature. The installation of the radiation sensor greatly affects the accuracy of temperature measurement. First, choose an easily accessible and convenient location for maintenance. The lens of the radiation sensor should be 1m from the salt bath surface, and the lens should be installed perpendicular to the liquid surface. The shell temperature must not exceed 100℃. If it exceeds 100℃, a water-cooling device should be used to cool it down, and the water pressure should not exceed 4 kg·cm². The objective lens of the radiation pyrometer must be kept clean. A ventilation device should be used to blow compressed air into the measuring channel. The compressed air must be filtered before being sent into the ventilation duct to prevent water vapor, dust, and contamination of the lens from causing errors. Regular cleaning and wiping are necessary. The connecting cable should be led out from the radiation pyrometer, and the wires should be placed in a flexible metal conduit to ensure good electrical shielding and reliable mechanical protection. The image of the measured object seen through the eyepiece of the radiation pyrometer must completely cover the thermopile to ensure that the thermopile fully receives the energy radiated by the measured object. If the image of the measured object is incorrect, the distance between the radiation pyrometer and the measured object can be adjusted to magnify the image. The secondary instrument of the radiation pyrometer must have the same calibration as the radiation pyrometer and be calibrated regularly. After installation, the lens dust should be cleaned regularly, and attention should be paid to the surrounding environment for mechanical vibration and probe deviation. If any issues are found, they should be addressed immediately, and the probe should be aligned regularly to ensure accurate reflection of the furnace temperature.　[b]References:[/b] ［1］Li Junyi. Furnace Temperature Instruments and Thermal Control［M］. Beijing: Machinery Industry Press, 1985, 57-63 ［2］Gao Kuiming. Thermal Measurement Instruments［M］. Beijing: Metallurgical Industry Press, 1985, 210-261