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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!
The Emissivity Trail: Stop 2

No Emissivity Correction Needed (Sometimes?)
That's almost unholy, like against the religion of science. Everyone knows that you have to correct an IR Thermometer reading for emissivity, or else it's no good, don't they? (even though I haven't the foggiest idea why or where the value of the correction came from)!


Emissivity: a mystery to some but not to all!

A fair question deserves another in return.

Do they correct the Radiation or IR Thermometer (pyrometer) for emissivity when it's being calibrated on one of those weird furnaces they call a "Blackbody"?

(It is actually a blackbody simulator because a blackbody is a theoretical concept, like infinity, it can't actually exist, therefore it's not real. So there is no such thing as a real blackbody, just things that approximate blackbodies and all you Astrophysicists out there will rightly ask: "have you thought of black holes, aren't they real blackbodies?" To which the practical Industrial experimentalists will answer, "Please get one for our lab, we'd love to test it to be sure.")

What do you think would happen if the emissivity were not set at 1.00 when an IR Thermometer is aimed into the blackbody calibration furnace? Suppose it were set at 0.5 instead of 1.00. The reading would be higher than the furnace temperature value. Correct? (Say, "Yes"!)
The amount of difference in reading depends upon several things, but mostly on the spectral wavelength response of the Thermometer being used.

That's kind of neat. It means you might be able to tell something about a radiation thermometer's spectral response by aiming it at a blackbody furnace and changing the emissivity setting. That's true, but a little more complicated than just that and it's another chapter in another book later; we're looking into emissivity stuff at this stop on the E-trail, so we'll stay focused.

No emissivity correction is needed when you are in an application where the measurement is like a blackbody calibration measurement.

For example, when you are looking into a small portion of a furnace that is nearly at the same temperature as the thing you are trying to measure. There are lots of applications out there like that, for instance:

1. Melting tanks and forehearths in the silica and glass industries, and,
2. Glass annealing lehrs, and
3. Calcining and drying rotary kilns near the discharge end, and,
4. Soak sections of Austenizing furnaces and other flame-free soak furnaces in the metals industries, and (one of our all-time favorites),
5. Closed-end target tubes used in place of, and just like thermocouples, in process lines, ovens and furnaces.

This last use of IR thermometers is the application that proved the value of the old (circa 1940-ye gads!)"total" radiation thermometers, the L&N Rayotube™ and the Honeywell (nee Brown Instruments) Radiamatic™. They became able, less costly and longer-lived replacements for expensive platinum thermocouples in large industrial ovens, furnaces, kilns and ore roasters.

Many compadres have rediscovered or "re-invented" this measurement method over the years and, in fact some today are calling them all sorts of names related to thermocouples or other nonsensical names (when they are not even close). What's in a name?

One of the most notable variations on this theme (and most precise devices produced in the last decade) was from the Accufiber company (now vanished) started in Portland Oregon, USA by former NIST and Pratt & Whitney researcher Dr. Rays Dils. He adopted and patented this technique in the Accufiber(tm) sapphire rod temperature sensor by vacuum depositing a high temperature opaque coating on the end of a 1 mm single crystal sapphire light pipe that was, in turn, attached to a small diameter fiberoptic light guide.

The fiber optic carried the thermal radiation output of the miniature blackbody cavity back to a radiation detector. By keeping the thermal mass of the cavity very small, a contact thermometer, based on a non-contact temperature measurement techique with fixed emissivity, was produced that could measure very high temperatures very precisely, with very fast time response by virtue of the materials used.

No emissivity correction should be used either when one attempts to measure, with a single sensor, the temperature of an object that is immersed in a hotter environment . Such applications include nearly all the entry zones of ovens and furnaces where the object of measurement comes into the furnace much cooler than the furnace walls.

The thermal radiation from the surroundings will be reflected from the object of measurement, except under the most unusual conditions, into the IR Thermometer. That results in the sensor reading a falsely high temperature (the magnitude of the error depends on several factors, not the least of which is the reflectivity of the object and the difference in temperature between the object and its surroundings) even with the emissivity control set at 1.00!

Since the action resulting when the emissivity adjustment is changed from 1.00 to any other value is an increase in the apparent temperature reading, such an increase will only make the reading error larger than it was to start with.

Such applications include nearly all the entry zones of ovens and furnaces where the object of measurement comes into the furnace much cooler than the furnace walls.

Again, if this is not making sense, try another mantra,E-trail mantra #2.

Repeat 200 times between lunch and dinner for one week:

E-missivity. One For Blackbody. One For Soak. One For Preheat.

Got it?

GREAT!
Another saddle sore from riding the E-Trail (if you really followed through).

Here's another thought-inviting question for this spot on the E-trail.

If you use a 2-Color (Ratio) Radiation Thermometer, and you aim it into a Blackbody calibration furnace (presumably set to a temperature in the sensitive range of the device) what happens when your e-slope or non-greyness adjustment is not 1.00?

If you've got answers, we've got more questions.

But, seriously, we'd really like to hear what you think the result would be, or if you really went into the lab and tried it. Try it both ways, higher and lower?

If you have some other inputs or questions, drop us a feedback. We'd really like to understand how all the people using Two Color devices are coping (just click on our highlighted link below and you'll get a feedback form).

(NOTE: One of these pages along the E-missivity trail will get to explaining what emissivity is, besides just a number you need to correct a temperature reading. The concept is important, but so, too, is some of the background.

Hang in there! Check out the rest of the trail while you wait.

The Emissivity Trail-Stop 3 or, the next trail followed.

References to Read or, the best trail to follow.

Go back to the Start of the E-trail.

The Emissivity Trail-Stop 1 or, the last stop on the trail.

Feedback on your experiences with emissivity and/or ratio thermometers

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


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