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Note: The E-trail has needed updating for some time. We are getting to it now, so please be patient if you see pages with different formats. They'll all look great soon and there will be more stops along the E-Trail for you to explore!
The Emissivity Trail: Stop 4

When emissivity correction is not enough to obtain accurate noncontact temperature measurements!

That sounds like my kind of measurement

Everyone knows that IR Thermometers don't read right, it's that e-missivity thing!

(Shhh! Don't tell anyone, but knowing when and why you have trouble making an emissivity correction also takes even more of the mystery out of it.)

Infrared Radiation Thermometer (IRT) measurements can often be quite difficult (read: frustrating, nervewracking, enough to make one lose sleep or begin talking to yourself.

Sometimes it can be nearly impossible to get a temperature measurement within acceptable limits even with a well-researched or measured emissivity value. In most of the difficult situations an emissivity correction by itself is not always enough.

Sometimes an emissivity correction makes things worse!

Here's a few examples of industrial IRT applications and their principle problem areas.

  • Materials semi-transparent in the waveband of the Radiation Thermometer being used
    Not always obvious from a visual inspection. Materials like silicon and germainium are opaque in the visual wavelength region of the spectrum, but at longer wavelengths in the infrared they become semitransparent.
    Conversely, plastics like polyethylene are semitransparent over a wide range of visual and infrared wavelengths. Getting good measurements of thin films of uncolored plastics can be quite challenging at times.
    Ordinary window glass is partially transparent out to about wavelengths of 2.5 microns in the near infrared.

    Approachs to problems with semi-transparent materials:
    1. Be certain you know the infrared spectral transmissivity of the material you plan to measure and try to select, if possible, a measuring waveband in a region where the material is opaque.
    2. If that's not possible and the object's thickness is constant, it is possible to calculate and/or measure an effective emissivity and correct for it with a "good*" Radiation Thermometer. One needs to take great care to insure that the region beyond the object contributes a constant amount of thermal radiation to the IRT. Zero is a very good amount that can usually be arranged if one can place a sufficiently cool source of thermal radiation at the spot on the other side of the object that the IRT sees. (Don't forget that the cooled device should not be a significant IR reflector-painting it with a High Emissivity paint can often help -High Emissivity=Low Refelctivity)
    3. If the semi-transparent object is also semi-infintely thick along the axis of measurement, the emissivity can be calculated, (if the linear spectral absorbtion coefficient is known), but the location of the effective spot of measurement will be below the surface. The distance depends upon the linear spectral absorption coefficient. It has been shown by a noted British worker in the field of Radiation Thermometry, Roy Barber, that in cases where the interior temperature gradient is linear or parabolic, that the depth of temperature measurement occurs at a distance equal to the reciprocal of the same coefficient.

  • Objects inside furnaces or ovens that are much hotter than the object itself
    Like cookie dough blobs on a moving belt that moves into an oven to bake the cookies. You know they start out a lot colder than the oven and exit at a temperature close to the oven temperature (cooked,of course, that is, neither burned nor still uncooked in the center). What is their temperature during the cooking phase? Hard to say unless one can imbed a contact sensor into the middle and let it stretch through the oven without distorting anything. But to do it consistently, where the amount of dough, its makeup (number of raisins, sugar etc ) and moisture content can vary. That's another question.
    Imagine now that the oven is a high temperature refractory-lined furnace and instead of cookies we want to optimize the heating and production rate of steel billets. They enter cold (or possibly heated a little by a previous process) and exit glowing ora-ngered. How do you measure their temperature?
    With great difficulty. But with care and the proper instrumentation it can be and has been done. The papers describing these technical achievements are in the technical literature and have been there for more than about 15 years. (It is really beyond the scope of this small web page to try to educate beyond the basics. That should be a good enough for now).

  • There are a wide range of difficult applications of Radiation Thermometers, ones where emissivity plays a central role, but many other factors must be included, too. Often it turns out that the actual emissivity value is a small part of the problem. Those who do not understand or who do not attempt to systematically seek an answer, will often blame the instrumentation or the (%$*#) emissivity.
    If you have an opportunity, invite them to wander along the E-missivity Trail. Perhaps the straight forward approach that is given here, seperating the simple from the difficult (applications types, that is) will help them sort out a workable solution. Other than that, you may need some consulting help; see our consulting page.
BOTTOM LINE:
IR Thermometer temperature measurements can also not be a "piece of cake" (even with the best instrument-we'll talk about what makes a "good" instrument later) when you are trying to perform measurements of objects in hotter surroundings, or semi-transparent objects, even those objects whose spectral emissivity you know. Again, if this is not making sense, try another mantra: E-trail mantra #4.

Repeat 200 times between waketime and breakfast for one week:

E-missivity. Even when you know it, know how to use it !

Got it?

Good, that's a lot. Perhaps there's hope after all. If you don't "have it", don't hesitate to send an email with questions. However, anyone expecting to receive an email answer must indicate by the nature of the question that they have read and tried to understand not only these pages but at least one of the references contained on the "REFs" page, too. Fair enough?


Some thought-invoking questions for this major point on the trail. It stops here for now, but will be picking up very soon with some further useful information including "What makes a good IR Radiation Thermometer.


1) If you use a 2-Color, or Ratio Radiation Thermometer, and you aim it at a semi-transparent object in hotter surroundings, what temperature do you expect to read?

2) How can you be sure you are right?


E-Trail Stop 5 (under Construction) or, the trail next followed.

More questions.

Got answers? Let us know with easy feedback.


Trail Tracks:

E-Trail Stop 4 or, the trail next followed.

E-Trail Stop 2 or, the trail's last "unholy" stop.

E-Trail Stop 1 The first stop-on The E-Trail

E-Trail Start, Where you learn about thinking spectrally!

References to Read or, the best trail to follow.

P.S. Interested in a listing on our vendor directories. It's self service at TempSensor.net, click here.


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