Welcome to a discussion about emissivity and how it affects temperature measurement!
A material’s emissivity is the ratio of energy radiated by the material to the energy radiated by a “black body”, which is the term used for a material that has perfect emissivity (ε = 1). That sounds complicated, but thankfully there are published charts that give us common materials’ emissivities to make life easier on us. The lower the emissivity, the less accurate a temperature reading will be, and vice versa. Anything with a smooth, reflective surface is going to have low emissivity because it reflects thermal energy as opposed to absorbing and emitting thermal energy. Thermal energy radiates infrared light, which most infrared light is undetectable by the human eye except that from certain materials when heated to extremely high temperatures, i.e. a red hot fire poker, glass during the blowing process, molten rock, etc. Other than that, you'll need a thermal imager to see the rest of the infrared light spectrum.
These imagers all have a special infrared sensor that can "see" infrared light and transpose it into shapes to see areas of different temperatures. Fluke's patented IR-Fusion
technology smoothly blends the infrared light picked up by the sensor with the clear image from the special lens in the camera. Also, you can go in and manually change the emissivity value in Fluke's infrared cameras to compensate and get a more accurate measurement. As I mentioned before, you can find plenty of emissivity tables online, but I've attached the one Fluke provides, as well as posted it as a jpeg below.
Below is Fluke's description and helpful tips on emissivity from their Ti20 manual:
Emissivity
Emissivity is the measure of an object's ability to emit infrared energy. The hotter an object, the more infrared energy it will emit. Emissivity can have a value from 0 (shiny mirror, perfect reflector) to 1.0(blackbody, perfect emitter). Most organic, painted, or oxidized surfaces have emissivity values close to 0.95. If you are performing qualitative inspections with your imager, leave the emissivity set at 1.0. If you need to measure actual temperature values, set the emissivity value according to the emissivity of the material the object you are measuring is made of. Again, if you need accuracy, you will need to find the emissivity value of the material before taking the measurement.
There are two methods that help you to find the emissivity value of materials:
•Tape method
The tape method requires the usage of the Scotch brand PVC tape (emissivity value 0.97) or equivalent. Cover the surface you want to measure with tape. Wait some seconds for the temperature to stabilize. Set the emissivity of the Imager at 0.97 and measure the temperature. Make a note of the temperature value. Then, remove the tape and measure the new temperature value. Adjust the emissivity accordingly until the temperature value found previously is displayed on the Imager. This emissivity value is the one of the material being measured. This method is good for objects that are at low temperatures (under 100 °C (212 F)),
not electrically energized, and not in motion.
•Contact thermometer method
The contact thermometer method uses a contact probe and good quality temperature meter. Initially, use the contact probe temperature meter to measure the temperature of the object you want to know the emissivity value, allowing time for the contact probe to stabilize (this may take up to a minute). Note the temperature value and adjust the emissivity on the Imager accordingly until the temperature value found previously with contact probe temperature meter is displayed on the imager display. This emissivity value is the one of the material being measured. This method is good for objects that are at moderately high temperatures (under 250°C (482°F)), not electrically energized and not in motion.
So there you have it, emissivity simplified. As always, if you have any questions please don't hesitate to post them below or contact me directly at jrapp@newark.com, I am available 9am - 5:30pm EDT. In my next discussion I will talk about the features of Fluke's SmartView software. Thank you for reading!
| Emissivity Table.pdf |
