We are looking at a different type insulation for an application here. Wouldn't it be possible to test insulating properties of one vs the other using IR. I'm being told different by a vendor. I took a sample of the "new stuff" and put it on a carbon steel bolt on one of our spin heads, temp @ 415 degs F, where the bolt is exposed. I did the same with a piece of our existing insulation same thickness, shape, and size. Basically all parameters are identical. In "my minds eye" the better insulator will show a higher delta T between the bolt and material than the poorer insulator. Correct? I did a reflectance test (attached) on the "new stuff" as an emissivity test (sort of), by sitting a hot cup of coffee beside it and a ss steel rod coupler. You can see the reflected heat from the coffee on the rod coupler but not the insulation material. Does it sound feasible to say that the insul mat. has a high emissivity or am I way out in left field. Anyhow any feed back would be deeply appreciated (for a sanity check). I know IR has been and is used for checking insulation, refractory, etc, etc. So why not material comparison.
Kind regards,

insulation_emissivity-reflectance_test.PDF (67 Kb, 28 downloads)

Roy,
First off How the heck you doing there in the Creek. Man I miss it a bunch.

Thinking about the experiment you conducted I applied the E+R+T=1, which is emissivity + Reflectivity + Transmissivty Must equal 1.

In that case I would say that it appears to have a lower reflectivity (higher emissivity) then the steel bolt, however there are better ways to find the emissivity of an object if that is what you were trying to find. Get a piece of 3m Electrical Tape. It has an emissivity of about .93-.95, put it on the object that you are trying to find the emissivity of, let it equalize temperature and then set the emissivity on your camera to .95. Shoot the tape and the object, and note the temperature. You will be real close to the actual temp of the tape. Now shoot the object, and adjust the emissivity until the temperature is the same as the tape was. The new emissivity at the temperature of the tape, is the emissivity of that object.

Now if you want to plug it into the formula you can get a real accurate number for the reflectivity.
i.e E= .75 then E).75+(R).25+(T)0 =1 So your reflectivity should be close to .25

If I am under standing correctly you are trying to evaluate the effectiveness of two different insulation types. It sounds like you used a known temperature source, wrapped it in the insulation and let it temperature stabilize then shot the outside of the insulation. Once you have determined the emissivity by the earlier process you can get a relatively accurate temperature from the surface of the insulation and after all isn't that what you want to know. Which type permits more temperature transfer

Does this help Ron?

Roy -
IR can certainly be used to compare material properties, such as insulating values.

It is necessary to accurately account for the differences in the surface properties of the materials so that you are getting an accurate temperature measurement. I did not fully understand your attachment, so I am not sure if you are doing it correctly or not.

A note of caution, however: For the same thermal conductivity, a material with lower emissivity being used for insulation on an object above ambient T (vs cold insulation)will actually have a higher skin temperature (and lower delta T) while, in fact, performing better as an insulator. The reasoning behind this is: the high emissivity surface will radiate more energy away from the surface and generate a lower surface temperature. By comparison, the low emissivity surface will radiate less away and actually have a higher surface T. Calculations to estimate these effects can be done, and in some cases, need to be done, since the convective transfer is also affected.

As a side note, we are often told to use a high emmisivity coating or patch, like electrical tape, to measure the surface T of a reflective material. The same factors apply, so that the patch will have a different T than the adjacent area, both due to radiation and convective differences. This may be, under some conditions, significant. In others, it can probably be ignored. The thickness of the patch will also affect conduction, and depending on the situation, may be significant.

If you need help working through all this on specific cases, I am available.

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

Bronx, NY 10463

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