Emissivity: a mystery to some? But not to all!
You can't live with it. You can't live without it: emissivity. Emissivity
is linked to Infrared Radiation Thermometry (or, if you prefer, pyrometry)
tighter than a doorknob is to a door. It's a mystery to many people, however,
even to some who sell non-contact temperature sensors and thermal imagers (Surprise!!!).Part
of the mystery of emissivity is its spelling, it gets mangled more often than
consistant; emmissivity, emistivity, emystery and emisomething are just a few.
Seriously, it is the often misunderstood parameter that is always associated with
IR temperature measurement and radiation heat transfer ("consistent"
is the correct spelling, BTW and emissivity has always had only one 'm'). Heat
transfer people have no problems with their emissivities. Are they better educated
than some of the users of IR thermometers? You bet they are! They
all go to engineering colleges for years and take serious math and science courses
to become proficient. They know their beans (which includes emissivity, usually
an emissivity called total emissivity)! The solution of the mystery is then
readily apparent: Get educated or get educated help! If you are going to
fool around with emissivity either understand it or get someone who does! But
since there are no high school and very, very few college classes in Radiation
Thermometry one is stuck between a rock and a hard place! (Maybe a heat
transfer course... wrong. They study mostly a different kind of emissivity than
is used in radiation thermometry-most of the time. That's the Total Emissivity-the
WRONG kind for temperature.) Maybe a physics degree... wrong again,
although a good study of atomic physics and 19th century radiation theory might
put things in some better perspective.. but that's not a complete solution. (Even
physicists don't know everything, despite the folklore). One realistic option
is to read the literature and look at some modern books on Infrared Radiation
Thermometry. It helps to have a good technical background in math and science
to understand the literature. If you have a solid technical education, that
is a wise course. All the necessary information is all out there, but it can challenge
the mind to find it all and strain the soul to separate the truth from the fiction.
That's especially true when much of the confusion about the subject can be transmitted
to the average user by a pyrometer salesman who doesn't know beans about the theory
that underlies the technology and spouts pure science fiction by rote! (Not all
are that way, just too many!). That makes it especially hard to appreciate
the relationships that exist between some of the other parameters involved e.g
wavelength, waveband, reflectivity, transmissivity, absorptivity, absorption coefficient
etc. But you don't have to be an expert to understand something (even as
"mysterious" as emissivity) and make practical use of it. Lots of people
have been doing it. Many have been very lucky, their intended use was one of the
well-known, "been there-solved that" type of applications. Others
curse the "rotten" instruments, sometimes because they really are (and
they don't know it), but, more often because they are used incorrectly. Emissivity
is the hands-down favorite whipping boy for failure to obtain good results or
explain ones that seem impossible. ("It's that crazy E-missivity thing, we
can't deal with it because it's changing all the time jist like that there salesman
said it could!") An intelligent approach sure beats using the "crock
o' beans" approach of saying:," Well no one really knows what the emissivity
is, besides it's always changing. These things are never accurate anyhow; all
we care about is repeatability". What a crock o' "beans"
that is! (Worse, you don't fool anyone, except a possibly inept pyrometer or imager
salesman who agrees that you've got a mystery on your hands!) Better to
be honest and set the emissivity correction at 1.00 and say: "I
don't know beans about emissivity and until we get some expert advice we'll use
the apparent temperature with no emissivity correction (the radiance temperature)".
As a side note, this happened in a big way a few years ago, when a major Japanese
steel company was advising a small USA steel company about the emissivity setting
for their near-IR spot radiation thermometers on a hot strip mill. They Japanese
engineers said exactly that: they didn't know the emissivity and had all their
process temperatures in radiance temperature (emisivity set at 1.00). We in the
USA pyrometer company had to provide theoretical corrections for the USA steel
mill readings to actually compare temperatures with the Japanese ones. Surprise,
surprise everything agreed! Both the theory and the practice were correct. We
had the right emissivity settings, and then they did , too. There's a paper on
the website under steel applications which shows
the theory and how the decision on emissivity settings can be made steel mills
and by extension in many other applications of IR non-contact temperature sensors.
There's another paper which has been in the literature for about 20 years which
will be available soon; it gives numerical details. (Note,
too, that an inaccurate instrument is usually also the one with poor repeatability
so this approach might help point out an instrument problem. Of course, you won't
have the emissivity to kick around anymore). You just gained an extra 100%
in credibility by being honest and acting "beanless" instead of brainless.
Plus you may actually find that the problem lies elswhere. Here's two links
that might help in getting educated about emissivity. Take them at the risk of
losing your emissivity excuse and show them to the salesman who told you the subject
was really simple (it is, of course, if you don't understand it; understanding
takes some time and effort. It's not simple, but it's not super science either,
so don't fear the unknown. Change it into the known through focused learning).
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