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High Temperature Emissivity
04-29-2013, 12:01 PM
Post: #1
High Temperature Emissivity
I am comparing the heat transfer of different torches and fuels. To test them I am heating a steel plate. I am interested in the time to temperature from ambient up to 1500°F. I have already split it into two tests because of the temperature ranges on the T640 camera. The issue I am having is once the steel reaches about 1100°F it starts to glow orange/red and I believe a change in the emissivity is causing a spike in the temperature reading on the camera. The curve of time v temperature will be very smooth up to this point and then it jumps. This leads me to believe that my high temperature test (anything above 1100°F) is not accurate. I am not sure how to measure the emissivity at these temperatures (electrical tape and liquid paper do not smell good at 1100°F). I am also open to heating something other than steel for this test (if it would give more uniform and accurate results) but I am not sure if a different material will act differently at these temperatures. Thank you
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04-30-2013, 12:49 PM
Post: #2
Heart RE: High Temperature Emissivity
My assumption is that the steel is oxidizing at the high temperatures and therefore the emissivity is raised, giving you the bump in the heating curve.

Try using the same piece of steel for all tests, so the oxidation and resulting emissivity is uniform between tests.

Or, try using a high temperature ceramic instead of steel that won't oxidize.
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04-30-2013, 04:47 PM (This post was last modified: 04-30-2013 04:50 PM by Bobberry.)
Post: #3
RE: High Temperature Emissivity
On a metal, the emissivity should be higher at the high temperature, so I would be inclined to think that the lower temperature measurements are more likely to be inaccurate than the high temp ones. I would also expect the emissivity to be quite high once it has become incandescent. Also, at very high temperatures, in a room temperature environment, I would expect the affect of reflections to be minimal. I would start by producing the curve using an emissivity of 1, this will plot the change in apparent temperature, which could help in understanding what is happening.

I am very interested in seeing the curve that you have produced.
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04-30-2013, 06:54 PM
Post: #4
RE: High Temperature Emissivity
You may not be able to use tapes, but try a high-temperature paints. With proper surface treatment, there are paints that will withstand 1000 °C. One to try is VHT SP102. Be aware that as some paints go up in temperature, their emissivity changes in a exponetial decay-like fashion.
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05-01-2013, 10:39 AM (This post was last modified: 05-01-2013 10:40 AM by ajspires.)
Post: #5
RE: High Temperature Emissivity
Thank you for the replies. A ceramic material was my first thought since it wouldn't oxidize. The problem with that is the face of the tile that the torch touches quickly reaches 2000°F while the backside is much cooler. I will try the high temperature paint to prevent oxidation on the steel and if that doesn't work well I'll try a thinner ceramic and measure temperature on the backside of the piece.

I have backdrops around the setup to reduce reflections. When measuring from the same side as the burner, the burner itself leaves a very noticeable reflection. I said I thought the high temperature results were less accurate because I have done the emissivity check with electrical tape at low temperature and the results were consistent with emissivity tables. So if the emissivity is changing at higher temperatures, I would expect those results to be less accurate. Attached is the graph of max temp on 1/8" thick cold rolled steel plate from a propane burner and emissivity of 0.79 (the flat at 1220°F is the top of the temperature range used). The next picture is the same test but emissivity changed to 1.

Attached File(s) Thumbnail(s)
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05-03-2013, 04:13 PM
Post: #6
RE: High Temperature Emissivity
A few comments:
Gary suggests pre-treating the steel by heating it so that it oxidizes and then using it for the testing. That should remove some of the emissivity variation.
Frank suggests using high T paint. I have done similar and it is a good method, depending on your goal.
You have suggested using alternate materials. I think you will still need to establish their emissivity as a function of temperature.

You state that you want to establish the rate of heating. This will be affected by many factors. I assume you are controlling geometry and flame contact, so that they are not in play. However, the choice of material, the surrounding temperatures, AND the broad band emissivity of the object will affect the rate of temperature rise. The material's properties, including density, thermal conductivity, heat capacity, and emissivity will all enter into the rate of heating. The emissivity will affect things in terms of both the rate of absorption of energy from the flame and the rate of energy loss to the surroundings. This effect will need to be considered if you change the emissivity of the material by, for example, painting it.

Jack M. Kleinfeld, P.E.
Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

Bronx, NY 10463

Skype: JKEngineer

JKEngineer -@- aol.com or JKEngineer -@- KleinfeldTechnical.com
come see what we can do for you: http://www.KleinfeldTechnical.com
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05-24-2013, 08:34 AM (This post was last modified: 05-26-2013 05:36 PM by acardoso.)
Post: #7
RE: High Temperature Emissivity
Assuming we can keep the temperature of the surface steady and, we can vary Trefl without modifying the geometry of the space, we do a series of thermograms modifying the heat sources, contributing for Trefl on the surface where we want the temperature measured.
With the equation
[Image: attachment.php?aid=116]

We build an equation system
[Image: attachment.php?aid=117]

Solving the system in order to T4 and ε we get the solutions

[Image: attachment.php?aid=118]

T and TreflX are expressed in Kelvin, t and tsurface in degrees Celcius.
σ= 5.670373(21) × 10−8 W m−2 K−4
Wx=σ∙(tsurface+273.15)4, tsurface= apparent temperature on the surface measured with camera set for 1 at emissivity.

Excel example

.xlsx  Temperature and emissivity.xlsx (Size: 11.23 KB / Downloads: 19)

Maple script

.zip  emissivity.zip (Size: 23.13 KB / Downloads: 13)

Formulas for LabView, Matlab, Maple

Formulas for Excel

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