High-quality steel products require consistent process controls at every step. In order to accurately measure temperature, you must have a way to compensate for emissivity variation. The emissivity of steel varies depending on its surface character, which can vary at different temperatures as the steel is heated and cooled. So having reliable tools that can accurately take emissivity into account is vital for process efficiency.
Temperature Measurement Technology
Using the right measurement tools is essential for getting accurate temperature readings. Since the product in a steel mill can’t be touched, mill operators use non-contact devices, such as pyrometers. There are several different types of pyrometers, including single-wavelength (short and long-wavelength), ratio (two-color and dual-wavelength), and multi-wavelength – and each has its own unique applications in the steel mill.
For high and consistent emissivity conditions, single-wavelength pyrometers are often ideal. As the name implies, these pyrometers measure infrared energy at one wavelength to calculate the temperature of the product. This type of pyrometer is used when the emissivity is assumed to be a constant, known value.
There are two types of single-wavelength pyrometers: short-wavelength and long-wavelength. Short-wavelength pyrometers are less sensitive to emissivity variations than long-wavelength pyrometers, making short-wavelength the better choice. Some processes that have minor variance in emissivity get better temperature readings from short-wavelength pyrometers. Using this technology also often saves energy, which can lower costs. Some of the applications most suited for short-wavelength technology include:
- Refractories: At the reheat furnace and ladle-preheat stations, refractory temperatures are often measured for process control. Because refractory material has a high and consistent emissivity, single-wavelength (short-wavelength) technology is appropriate.
- Cold Mill: Cold rolling involves several processes. The last process is cold rolling of the steel at a lower temperature. Single-wavelength technologies are often the only type of pyrometer that can measure at low temperatures. While cold-rolled steel has a low emissivity, use the shortest wavelength possible to minimize any errors.
- General Low Temperature: General conditions such as outside temperature and storage temperatures also have to be taken into account when manufacturing a raw product. For accurate gauge of these temperatures, short-wavelength technologies are often the best measurement tools.
Unlike single-wavelength pyrometers, ratio pyrometers measure infrared energy at two different wavelengths. The ratio of IR energy at these wavelengths is used to calculate a temperature. Ratio often the best to use to compensate for emissivity variation during the steel making process.
As long as both wavelengths are affected equally by the emissivity variation, the ratio between them will remain the same, allowing you to get an accurate temperature measurement. Some of the best applications for ratio pyrometers include:
- Caster, HRM, Coiler: Steel experiences a number of physical changes at these parts of the mill and emissivity can vary greatly throughout the process. Ratio technology is much more appropriate here to eliminate any errors due to emissivity variation.
- Molten Steel: When steel is in a molten state, the emissivity can vary greatly as the surface texture is constantly changing. Dual-wavelength pyrometers are used to measure flowing molten steel when it is being poured to in order to best deal with the constantly varying emissivity.
- Annealing Line – Roller Wedge: At the annealing line, pyrometers are crucial for temperature control. Ratio pyrometers are a better choice than single-wavelength pyrometers at the roller wedge because they provide a real-time measure of emissivity. This reported emissivity value can confirm that the pyrometer is aligned to the target to ensure that you have a true, valid temperature reading.
Multi-wavelength pyrometers are customized to their specific application. They measure infrared energy at multiple wavelengths and use an application-specific algorithm to compensate for complex emissivity variation of a target material to calculate the temperature.
This technology is useful for low and variable emissivity applications where single-wavelength pyrometers are inappropriate and emissivity variation does not affect the wavelengths equally, meaning that ratio pyrometers are also not appropriate. There are a handful of materials, called non-greybody materials, that have these unique emissivity characteristics (cold-rolled steel, high-alloy steel, zinc-coated steel, stainless steel), and require multi-wavelength pyrometers for accurate temperature measurement. Some uses for multi-wavelength pyrometers include:
- Annealing – Direct View: Cold rolled steel strip has a relatively low emissivity. Therefore, when measuring this steel at high temperatures during the annealing process, it is necessary to use an application-specific multi-wavelength pyrometer.
- Galvanized/Zinc-Coated Steel: Multi-wavelength pyrometers are the best technology to use when applying metal coatings at high temperatures. Zinc-coated steels are another example of a non-greybody material that requires a multi-wavelength pyrometer to compensate for unique emissivity characteristics.
Wavelength Matters: Choosing the Right Technology for Your Application
These are just a few of the applications that need close temperature control throughout the steel mill. We understand that a steel mill is a demanding environment, and there are more factors and interferences to consider outside of emissivity variation. This can make it difficult to choose the right steel infrared temperature sensors, and it’s why we emphasize thoughtful wavelength selection of your ideal application-specific pyrometers. Our white paper, Understanding Infrared Pyrometers for Demanding Steel Mill Applications, will help you to understand the problems with traditional instruments – and how application-specific pyrometers can help. Download your free white paper to choose your ideal pyrometer, and don’t hesitate to reach out to our team with any questions.