Editor’s Note: This blog post was originally published in 2013 and has been updated for accuracy and relevance.
Aluminum has traditionally been thought of as a difficult-to-measure-material with an infrared pyrometer because it is so reflective and has unique emissivity characteristics. To compensate for these unique emissivity characteristics of aluminum, Williamson developed a multi-wavelength sensor with application-specific algorithms that compensate for emissivity variation to produce an accurate and repeatable temperature measurement. For over 25 years, our multi-wavelength sensors have been helping aluminum extruders achieve accurate temperature measurements during critical steps in the extrusion process. Because our multi-wavelength technology is so unique, we often run into a lot of the same questions from customers as to how it works. Here is a list of our top 7 most Frequently Asked Questions.
Aluminum Extrusion – FAQ
1) Why do I need a multi-wavelength pyrometer?
Aluminum is a non-greybody material, which means that it has low and complex emissivity characteristics. Traditional single and dual-wavelength (ratio) pyrometers are unable to provide accurate and reliable temperature measurements for non-greybody materials.
Single-wavelength pyrometers require an emissivity input. Let’s say your emissivity input is set to 0.05 and the emissivity of the material changes from 0.05 to 0.06. While that is only a small nominal change in emissivity (0.01), it is a 20% change in energy and would cause a big error in temperature reading. Because the emissivity during aluminum extrusion is never constant, a single-wavelength pyrometer is not accurate or repeatable.
For materials with varying emissivity, a dual-wavelength or ratio pyrometer is often used. However, a ratio pyrometer assumes that emissivity variation affects both wavelengths equally (greybody material). Since aluminum is a non-greybody material, it has different emissivity values at different wavelengths and dual/ratio pyrometers cannot accurately compensate for emissivity variation. Enter the multi-wavelength pyrometer with application-specific algorithms designed to characterize infrared energy, emissivity, and measured wavelengths to accurately calculate the temperature and emissivity of complex non-greybody materials.
2) Does the multi-wavelength pyrometer require an emissivity setting? Do I have to change the emissivity depending on alloy?
The short and sweet answer is no. The sensor does not require any emissivity adjustments or tuning of the algorithm to provide accurate measurements. Once you select the proper algorithm for the application, the sensor is ready for operation. When we developed the aluminum extrusion algorithms, we took data from a wide range of press sizes, alloys, shapes, and operating conditions to ensure that we could have one algorithm to cover just about any type of extrusion profile imaginable. Each sensor includes up to 8 algorithms with options for profile measurements at the exit of the press or in the quench zone as well as a variety of different billet measurements.
3) Does the pyrometer need a full field of view?
For the press exit, you do not need a full field of view to make an accurate measurement. The press exit pyrometer can actually tolerate as little as a 10% full field of view and report an accurate temperature. For other applications such as billet temperature, a full field of view is required. Alignment becomes critical for billet pyrometers, and we want to make sure that no mechanical or optical obstructions are in the way.
It is recommended that the sensor is mounted 3-5ft (1-2m) from the billet to make sure it is properly aligned with a full field of view. Special small spot optics are available if greater mounting distances are required.
4) How do we know the pyrometer temperature reading is accurate?
In addition to providing a real-time measure of temperature, the Williamson multi-wavelength pyrometer also provides a real-time measure of emissivity. For a given application we know, more or less, the expected range of emissivity values. For the Press Exit measurement , we can expect an emissivity value of between 0.03 – 0.10 (typically in the 0.05-0.06 range).
When we see the calculated emissivity value in this range, we can be very confident that we are getting an accurate temperature measurement. If we notice that the reported temperature is off and the emissivity value is way outside this expected range, we know that something is not right. We can then check to see that the correct algorithm is selected, the sensor is properly aimed, and the pyrometer is not seeing any reflected energy.
5) How does a Multi -Wavelength Pyrometer reduce error?
The benefit of using a multi -wavelength pyrometer is it will eliminate interference from common industrial obstructions. If you have the appropriate wavelength selected, obstructions such as steam, water, flames, combustible gases, plasma, and laser energy will not cause a reading error.
6) Why does the pyrometer reading disagree with my thermocouple?
Thermocouples and probes are common devices used for taking a contact temperature measurement of a surface. However, there are a couple of issues when taking a contact measurement of a surface. Thermocouple readings can be influenced by a number of different factors which may or may not represent the desired target surface temperature. Common influences on thermocouple temperatures include ambient air temperature, thermal inertia (starting temp of the probe), thermal mass of the probe, surface oxidation, loss of heat through conduction, and also the applied pressure of the thermocouple on the surface.
After some use, thermocouples can also get dull or oxidize and will not be as accurate as they once were, leading to lower reported temperature values. Moreover, some thermocouples will read 40-50°F below the actual temperature right out of the box. Some have even read as much as 100°F too low. The best way to check the accuracy of the thermocouple or probe is to check it against a NIST certified hotplate. Once you determine the accuracy of the thermocouple probe against the hot plate, you can then make a fair comparison between the thermocouple probe and the pyrometer.
7) What are the operating temperature ranges?
The short answer is, it depends. The reason is that every application for a pyrometer is going to vary. We offer 6 different infrared technologies that will allow you to get the most accurate reading. By offering different options we can guarantee that the wavelength options, multiple optical configurations, temperature spans, and accessories will be optimally configured for your application.
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For additional resources about increasing quality and productivity, download our Aluminum Extrusion Best Practices Guide:
