MrPyrometer (MrPyro): On Emissivity Trail

On Emissivity Trail

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 measurement.)

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 and the author of this missive are one!).

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 place to begin. 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 or IR Thermometer or Thermal Imaging huckster (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 also very lucky, too. Their intended use was one of the well-known, "been there-solved that" type of applications, and about 2/3 of the known uses or applications fall into that category

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 or would!")

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 thermometer 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.

The 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 U.S.A. radiation pyrometer company had to provide emissivity=1.00 corrections for the USA steel mill readings to actually compare temperatures with the Japanese ones based on our knowledge and experiences with the emissivity of steel from many mills around the world and the lab measurements made on steel samples.

Surprise, surprise! Everything agreed! Both the theory and the practice were correct. We had the right emissivity settings, and then they did, too. (FYI: We used a value of 0.83 for oxidized steel at 1.0 micron wavelength)

(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 elsewhere.

Check out the following resources online:

MrPyrometer (MrPyro)

On Emissivity Trail

No comments:

Increasing the accuracy of your temperature measurements.