Message Board Thread - "Determining Emissivity of Hot Metals "

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Determining Emissivity of Hot Metals Gary Orlove 6/21/2000
We would like to know how we could accurately determine emissivity of metals. We want to determine at the hot rolled plates at the full technology and different materials. We know how to makes the emissivity determine, but we pleased if you can inform because we will make measurements for the exact value. Thank you. I would like to receive monthly newsletter.
 
RE:Determining Emissivity of Hot Metals Gary Orlove 6/21/2000
Here is some information concerning your question on emissivity and metals. Metals, in general, have very low emissivities. For clean metals, the emissivity is somewhat higher in the shortwave.

Here is one method of measuring the emissivity of metals: First determine the true temperature at a specific spot by using a thermocouple, or any other means. Next measure the background radiation, also known as the T - ambient. The actual terminology used depends on the model of the camera. This is the source of thermal radiation, which can reflect off of the surface of the metal and reach the camera. For low emissivity targets, this value is extremely important. Point the infrared camera at the spot where the thermocouple is located and adjust the emissivity of the camera until the temperature agrees with that of the thermocouple. This will give you the emissivity of the metal. If the background T-ambient changes, you will have to measure this again to get accurate readings. If the surface characteristics of the metal changes (phase change, oxidation, etc.), the emissivity will change as well. It is best to try this method several times. If the results seem consistent, they are probably reliable. If you change the angle of observation, you can expect to see emissivity changes. There is a chapter "Radiation Thermometry in the Steel Industry," found in "Theory and Practice of Radiation Thermometry," edited by D. P. DeWitt and Gene D. Nutter. I believe you would find this a useful reference. In addition, here are a few places on the Internet where you can get information:

http://naca.larc.nasa.gov/reports/1958/naca-tn-4206/
http://www.zae-bayern.de/ectp/abstracts/sabuga1.html
http://www.metallurgy.nist.gov/techactv1997/MetalDataChar.html
http://electro-optical.com/bb_rad/emissivity/matlemisivty.htm
http://www.icess.ucsb.edu/modis/EMIS/html/em.html

I hope this information is useful

Bernie Lyon
 
Re:Determining Emissivity of Hot Metals tennocrama 6/15/2005
I've been trying this method (described by Bernie Lyon) of determining the emissivity of some metals; however I am having some anomalous results with some materials. I am heating the materials on a hotplate. I measure the temperature of the metal with the thermocouple and get 180 C. When I look with the IR camera and adjust the emissivity, the closest I get is about 220 C, when I set the emissivity to 1. I get the same mismatch when I focus directly on the hotplate and look at the temperature there versus with a thermocouple.

Any thoughts or suggestions would be appreciated,
Amy
 
Re:Determining Emissivity of Hot Metals Gary Orlove 6/17/2005
Your results suggest that your camera is out of calibration, and/or your thermocouple is not actually measuring the material temperature accurately. Here is what I would do to sort this out.

Get your self a candle and hold the material with the thermocouple on it in the flame to get it coated with candle soot (e~0.95) (both the thermocouple and the material). You now have coated both items with a high emissivity material. Repeat the heating with the hot plate, let it come to steady state and view with your IR camera. If the thermocouple is attached properly, you should see the same apparent temperature on both the material and the thermocouple. If the thermocouple itself appears colder, then you know that the thermocouple is giving you a false reading that is too low.

If the thermocouple looks the same as the material, and your camera reads differently, it probably would be wise to have it calibrated.

Now since we know the emissivity of candle soot, you can measure the emissivity of your material as well. Measure the temperature of the material with the soot on it using an emissivity of 0.95, then move off to an area without the soot and change the emissivity setting until it reads the same.

Gary Orlove
Infrared Training Center
 
Re:Determining Emissivity of Hot Metals raffar 6/24/2005
Our SpectroPyrometers measure both temperature and emissivity directly. We've found that in manufacturing applications emissivity changes so much that using published values is problematic. We try to explain emissivity's behavior at http://www.pyrometry.com/emissivity.php
 
Re:Determining Emissivity of Hot Metals Michael 7/25/2005
I have a Regen Overhead line that's about 1100*F in areas. There are areas that have been previously repaired with Inconel, some with stainless, some with chrome.

I use developer from a dye penetrant test kit. Paint burns off, the developer is like baby powder so it sticks. I spray 50% of a uniform area. Get a temp off of the white area, and then adjust E value until it corresponds with "known". I've been using E=.97 for the white area, this was a guess.
Not very scientific, but it is close enough adn as long as I keep it all the same, I will find anomolies or significant increases.
 


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