| 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! |
