Infrared pyrometers have been applied to various occasions in metallurgical production lines. However, in the measurement of the slab temperature in the second cold zone of the continuous casting machine, there are always questions about accuracy of measurement and the reliability of system. In order to measure the surface temperature of the secondary cooling slab accurately, and improve the quality of slab. In response to various problems reported by users, the feasibility of below infrared temperature measurement solutions is proposed.
2. Various factors that affect casting slab temperature measurement
(1) Surface state
The measuring object is a stainless steel plate, and the emissivity is related to the surface roughness and oxidation degree of the object. For smooth and clean surfaces, the emissivity of the object is 0.05-0.5. If oxidation degree of the surface increases, the emissivity becomes larger. When the oxide film on the object surface is above 10um, the emissivity tends to be stable.
The use of one-color pyrometer will be affected by emissivity change, especially when measuring stainless steel, the emissivity change has a great influence on the temperature measurement result. When using a two-color pyrometer, the influence of emissivity can be avoided. User only need to calibrate instrument once when measuring similar metal materials.
(2) Influence of thermal radiation
The temperature of the cast slab is very high and the volume is large. Ordinary instruments can only withstand temperature below 85°C, and it is generally recommended to use in abmbient temperature less than 50°C. Even if military-grade products cannot meet this requirements. The fiber-optic pyrometer, which sensor head and the detector are separated, must be used. The sensor head can withstand a high temperature of 250°C without cooling. The detector is installed in places where the ambient temperature is relatively low. When the ambient temperature is relatively high, the detector can also be cooled by water, which can greatly increase the service life of the instrument.
(3) Iron oxide scale
Using a special software algorithm can avoid the influence of the oxide scale on temperature measurement results. To judge the emissivity characteristics of the oxide scale, the sampling time of the system must be fast enough (above 10ms), the signal needs to be synchronously collected without delay, and the collected signal must be selected. The comprehensive application of various software algorithms can avoid influence of oxide scale on the temperature measurement accuracy.
(4) Water vapor and dust in optical channel
Using a two-color pyrometer can basically avoid the influence of water vapor and dust. Even if the signal of the measured object attenuates 95%, the temperature measurement accuracy will not be affected. Choose a dual-wavelength pyrometer in 0.8-1um band can avoid the influence of water vapor. The water vapor is basically transparent in 0.8-1um band.Related information is used to explain the influence of water vapor on temperature measurement under different measuring distances and measuring temperatures.
The influence of water vapor on temperature measurement at different measuring distance and temperature is explained by reference to relevant data.
Change manner of emissivity of water vapor with the wavelength of 0.8-1.0μm varies is similar to absorption change manner of water vapor with the wavelength of 1.6μm, which is almost “transparent” absorption band of water vapor. Therefore, the error of the pyrometer with operating wavelength of 0.8-1.0μm is almost negligible.
The influence of water vapor and certain dust on two-color pyrometer is basically negligible, but to completely avoid influence of these factors, an extension rod is designed and compressed air is added to keep optical channel clean. Just need to keep a distance of 20cm between pyrometer and the thickest measuring slab. In this way, the equipment is basically maintenance-free.
(5) The influence of water film on slab surface
Temperature measurement accuracy of infrared pyrometer is related to the mist of water film on slab surface. The thicker the water film medium, the more attenuation of the radiation energy of surface. Refer to following documents:
Because of the impact of cooling water, there are certain water accumulation areas and certain water film in the casting slab in the second cold zone of continuous casting. Even if a two-color pyrometer is used, these areas should be avoided as much as possible. In the actual measurement, a two-color pyrometer below 1.1um is selected for measurement and the temperature is corrected.
3. Advantage of two color pyrometer over one color pyrometer
In two-color temperature measurement, temperature will not change with the object’s surface state (such different surface roughness or chemical state ), which will not affect the accuracy of temperature measurement, but will have an impact on one-color pyrometer measurement .
The optical part of pyrometer, such as glass, will accumulate some dust after using for a period. The presence of water, gas, oil, etc. in the air will reduce the emissivity coefficient, so the temperature measured by one color pyrometer will decrease. The two-color pyrometer measures the ratio of infrared radiation intensities of two band wavelength in a specific range. With dust, water vapor and other disturbances, the signals of two wavelength fall simultaneously and the ratio remains constant after division. But this does not mean two-color pyrometer does not require maintenance. User still need to wipe the glass when it is too dirty.
One-color pyrometer cannot measure objects smaller than the field of view. When the target cannot fill the field of view, the measurement temperature will be lower than actual temperature. The two-color pyrometer can measure objects smaller than the field of view.
4. System block diagram
The controller is equipped with water cooling. In order to improve the use safety, it is recommended to use DC24V power supply, and each power supply is not less than 10W. Optical fiber and cable are protected by high temperature asbestos, to be done on site. Each detector must be equipped with purging and water cooling to ensure the reliable operation of the system for a long time.
Recommended product selection: LMM Two-Color Infrared Pyrometer