Determination Of Emissivity; Typical Emissivity Values - Fluke Endurance Series User Manual

15.3 Determination of Emissivity

Emissivity (applicable for sensors in 1-color mode) is a measure of an object's ability to absorb and emit infrared
energy. It can have a value between 0 and 1.0. For example, a mirror has an emissivity of < 0.1, while the so-
called blackbody reaches an emissivity value of 1.0. If a higher than actual emissivity value is set, the output will
read low, provided the target temperature is above its ambient temperature. For example, if you have set 0.95 and
the actual emissivity is 0.9, the temperature reading will be lower than the true temperature.
An object's emissivity can be determined by one of the following methods:
•
Determine the actual temperature of the material using an RTD (PT100), a thermocouple, or any other
suitable contact temperature method. Next, measure the object's temperature and adjust emissivity
setting until the correct temperature value is reached. This is the correct emissivity for the measured
material.
•
For relatively low temperatures (up to 260°C / 500°F) place a plastic sticker on the object to be measured.
This sticker should be large enough to cover the target spot. Next, measure the sticker's temperature
using an emissivity setting of 0.95. Finally, measure the temperature of an adjacent area on the object
and adjust the emissivity setting until the same temperature is reached. This is the correct emissivity for
the measured material.
•
If possible, apply flat black paint to a portion of the surface of the object. The emissivity of the paint is
0.95. Next, measure the temperature of the painted area using an emissivity setting of 0.95. Finally,
measure the temperature of an adjacent area on the object and adjust the emissivity until the same
temperature is reached. This is the correct emissivity for the measured material.

15.4 Typical Emissivity Values

The following table provides a brief reference guide for determining emissivity and can be used when one of the
above methods is not practical. Emissivity values shown in the table are only approximate, since several
parameters may affect the emissivity of a material. These include the following:
•
Temperature
•
Angle of measurement
•
Geometry (plane, concave, convex)
•
Thickness
•
Surface quality (polished, rough, oxidized, sandblasted)
•
Spectral range of measurement
•
Transmission (e.g. thin films plastics)
To optimize surface temperature measurements, consider the following guidelines:
•
Determine the object's emissivity using the instrument, which is also to be used for temperature
measurements.
•
Avoid reflections by shielding the object from surrounding temperature sources.
•
For higher temperature objects, use instruments with the shortest wavelength possible.
•
For translucent materials such as plastic foils or glass, ensure that the background is uniform and lower
in temperature than the object.
Appendix 15
Determination of Emissivity
141