Where do you get your emissivity values?
Do
you look them up in a table?
Does the table specify spectral emissivity
values?
Great ! You've got a head start. Better still, if you had the
emissivity measured for you by a laboratory that knew their beans, you have a
better emissivity value to use to correct your Infrared or IR Radiation Thermometer,
about 2/3 of the time. If the term"Spectral Emissivity" is
foreign to you, you could have temperature measurement problems you don't even
know about. Why is spectral emissivity important? Because it is the emissivity
that one needs to correct the reading of an IR Thermometer, or a quantitative
Thermal Imager, in cases where the object being measured is substantially hotter
than anything else around and is not transparent If you use the total
emissivity found in many tables you are not likely to be using the correct values
for your instrument, unless you have a total radiation thermometer! There
are a whole bunch of them on the market at really low prices just like in the
1930's through 1950's except using modern detectors. But the low cost, simple
gadgets don't seem to be used in places where measuring accuracy is more important
than price, unless someone made a mistake. Our focus in all these pages is
not on barely adequate temperature sensors, we're thinking about ones that
are used to obtain measurements accurate and repeatable to 1% of temperature
or better. Otherwise, why bother? It's easy to get poor measurement quality,
It takes a bit more effort to do things right! That's the key to repeatable and
accurate measurements. Most modern, high accuracy IR Radiation Thermometers
have a carefully selected, relatively narrow spectral response wavelength passband.
That means that they depend upon the emission properties of the object of measurement
in that same spectral passband. Most metals, especially really clean surfaces,
and all oxides and dielectric materials have emissivity that varies with wavelength.
If you use the total emissivity instead of the spectral emissivity for that fancy
sensor that cost you a zillion pesos, you are likely to get some serious measuring
errors with that value. Even if you use a "modern" Total Radiation
Thermometer, you may need to know the correct "total" emissivity setting
to get the best possible accuracy because even modern "totals" are't
truely total i.e. very few, if any respond with a Temperature raised to the 4th
power response function despite what you may hear. The proof is in the testing,
but that's a whole seperate book on testing Infrared Radiation Thermometers and
not necesary for understanding the few points made here! Realize, too that
there are cases when you do not need to correct the sensor for emissivity: e.g.
1. when the object of measurement is immersed in an environment at about the same
temperature, and,
2 when the object is immersed in an environment that is
significantly hotter than the object.
In both of these cases one makes the
errors larger by using an emissivity correction. There's more to the story than
this, but that's coming at later stops along the E-Trail. So, what good is
the total emissivity?
It is very valuable in the engineering discipline of
radiation heat transfer. Most of the emissivity tables published are for that
discipline. Even some vendors of non-contact temperature sensors reference such
tables (incorrectly). We have found this fact to be a great screening tool to
seperate the solid technical suppliers of such devices from those who are a bit
squishy. If this doesn't make sense to you, try some faith, faith in
simple rote, like: Repeat the E-trail mantra #1 200 times before lunch for
the next week: Emissivity, no mystery. Total for heat transfer, Spectral
for temperature. Got It? Good! Does the total emissivity
help at you at all? Well...., Not really. (Check the logic here, it's pretty
hard to beat!) It helps, but only for some materials which have relatively
flat spectral emissivity curves. However, you have to know the spectral emissivity
properties in order to judge how flat the spectral emissivity curve is (otherwise
you are guessing or doing "handwaving" science or engineering e.g "it
doesn't vary much, not to worry" {followed by a wave of a hand or display
of thumb and forefinger very close together-not quite quantitative}). If you
need the spectral emissivity curves in order to be able to evaluate the total
emissivity values, why do you need the total emissivity values at all? (Duh!
Why didn't they tell us that in the first place? Gooood question, lots of
possible answers that we don't repeat in proper company or even around the campfire
on the E-Trail when we're funnin'.) Bottom line: get the spectral emissivity
values, they are the only things you will need. There are several vendors that
will measure the spectral emissivity of representative samples of your materials,
especially industrial solid materials- some do it at very low cost, sometimes
free if you are a regular customer of other products and services. Sometimes there
is a problem getting truely representative samples, but that gets into a lot more
detail than we want to just now. Hang in, we'll get there eventually! If you
come away from the E-Trail with only one saddle sore, let it be this one:
Spectral e-missivity is it! One final, thought-inviting question on this part
of the trail. If you use a Two Color Pyrometer or Ratio Radiation Thermometer,
how do you select an e-slope or non-greyness correction or an emissivity ratio
setting for it? Did someone offer you a pair of Total Emissivity values for
the device or just tell you that you can forget about emissivity with a ratio
pyro? On many materials the e-slope is a critical setting because ratio devices
are more sensitive to errors in emissivity ratio correction than comparable single
waveband IR thermometers are to errors in emissivity correction. We'll get to
the math a little later to prove it, but be careful when someone tells you that
the ratio units are emissivity-free. It's all a matter of how sensitive they are
to errors in setting the emissivity ratio and the actual value for your material.
Ask for the numbers or numerical estimates, if in doubt. If the salesman can't
back claims with facts and numbers then perhaps you need to find a more knowledgeable
salesperson, one that can. 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.
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