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. | 