The steel making process is one of the harshest industrial environments, one that is far from an ideal laboratory setting. Making an accurate temperature measurement contending with all of these different operating conditions and interferences can certainly be challenging. However, with the right pyrometer technology (and wavelength selection), many of these errors and interferences can be minimized and even eliminated. We have identified the 3 most common interfereces : Emissivity, Steam, and Scale and how to best deal with them.
1. Emissivity
Emissivity is defined as an object’s ability to emit infrared energy, and is measured on a scale of 0-1. Emissivity is dependent on the surface texture of an object and in the most practical terms is the opposite of reflectivity. Zinc coated steel on an annealing line has a low emissivity (around 0.3) while hot rolled steel can have a high emissivity (around 0.75 and above). Needless to say, the emissivity of steel cannot be characterized by a single emissivity value. Moreover, the emissivity of steel will vary for any given application or process – for example, the emissivity of hot rolled steel can range from 0.65 to 0.75.
Single-wavelength pyrometers assume that the emissivity of a material is known and constant, and require an emissivity input. If the true emissivity of the material is the same as the emissivity input of the pyrometer, then you will have an accurate temperature measurement. If the true emissivity value is different than the emissivity input, then you will have an error. Errors are greater at higher temperatures and errors are smaller with shorter wavelength pyrometers. In fact short wavelength pyrometers can be 4-20 times less sensitive to emissivity variation compared with general purpose long-wavelength pyrometers (such as handheld units).
While short-wavelength single-wavelength sensors may reduce emissivity error, ratio pyrometers can eliminate any errors due to emissivity variation. Ratio pyrometers, as the name implies, use the ratio of energy collected at two wavelengths that is then correlated to a temperature value. If the change in emissivity affects both wavelengths equally (as is the case for hot rolled steel and most carbon steels), then you are left with the same ratio measurement and a stable temperature – effectively eliminating any error from emissivity variation.
Certain types of steel (cold rolled, high alloy, electrical, zinc coated, shot-blasted pipe) have complex emissivity characteristics that can be difficult or near impossible for single-wavelength and ratio pyrometers to measure with precision. For the types of steel listed above, the percent change in emissivity is not equal at different wavelengths and a ratio pyrometer will not work. However, a multi-wavelength pyrometer with application specific algorithms can compensate for the complex and varying emissivity of those types of steel applications.
2. Steam
Water is used throughout the steel making process from quenching and cooling hot steel, to keeping rollers and other mechanical equipment from overheating. Naturally, when water comes in contact with a hot surface it evaporates and creates steam. Steam can be found all around a steel mill and is probably one of the most common optical interferences that steel infrared temperature sensors have to deal with. The trouble with steam is that it can cause a pyrometer to read lower than the true temperature of the target. One common way to avoid steam interference is to aim the pyrometer at a spot or in a location where there is no steam. Often times steam cannot be avoided because it is simply a part of the process and you need to view through the steam to get a temperature reading.
The other way to avoid steam interference is to use a pyrometer that is filtered at a wavelength where steam is transparent. How is this possible? Infrared radiation is part of the electromagnetic spectrum, same as microwaves, x-rays, visible, etc. If I shine a flashlight at my chest, you see the spot on my chest; the visible light does not pass through. However, if I aim an X-ray at my chest, you can see right through me. The only difference is the wavelength. So at different wavelengths, objects can either be transparent or opaque. By filtering a pyrometer at wavelengths where steam is transparent (white), and not where it is opaque (grey), you can view through the steam without any interference and get an accurate temperature measurement of your target.
3. Scale
Scale is often thought to be an interference because it is a different emissivity. However, the real reason it is an interference is because it is a different temperature emitting infrared energy at a different ratio. Single-wavelength pyrometers read an average temperature of whatever is in the field of view – meaning that if half of the field of view is hot steel and half is scale, then you would get an average between the scale and the actual hot steel temperature. Since scale can be hundreds of degrees cooler than the actual steel temperature, you can get extremely varied and inconsistently low temperature measurement readings.
Ratio pyrometers do a better job at compensating for scale interference, but it still can be an issue. Scale is to a ratio pyrometer as a bump on the floor is to a table: the greater the separation between the legs, the smaller the wobble. There are two types of ratio pyrometers, two-color pyrometers and dual-wavelength pyrometers. The two-color pyrometer has overlapping wavelengths so there is no separation between the wavelengths. The dual-wavelength uses two distinct a separate wavelengths. The greater the separation between the wavelengths means a more stable your temperature reading – just like the legs on the table. As a result dual-wavelength pyrometers are 20 times less sensitive to scale compared with two-color pyrometers. A typical 40-60 degree error due to scale with a two-color pyrometer would only be a 2-3 degree error with a dual-wavelength pyrometer. While influence from scale cannot be completely eliminated, error from scale can be greatly reduced by selecting the appropriate wavelength technology.
To recap, here are some ways these big bad interferences can be tamed:
- Emissivity: Shorter wavelengths are better than long wavelengths. Ratio pyrometers eliminate errors from emissivity variation for high emissivity steels. Multi-wavelength pyrometers are used for complex and low-emissivity steels.
- Steam: Wavelength selection is critical! Select a pyrometer filtered at a wavelength where steam is transparent.
- Scale: Ratio pyrometers are better than single-wavelength pyrometers. Due to greater wavelength separation, dual-wavelength pyrometers are 20 times less sensitive to scale than two-color pyrometers.
