Page 183 - An Introduction To Predictive Maintenance

Thermography 173

~
A
~
~
~ R
~
~
~
T
~
A + R + T = 1
E = A
E + R + T = 1
Figure 8–1 Energy emissions. All bodies emit energy within
the infrared band. This provides the basis for infrared
imaging or thermography. A = Absorbed energy. R =
Reflected energy. T = Transmitted energy. E = Emitted
energy.

~
~
~
~
~
~
~
~

E = A = 1 R = 0 T = 0

Figure 8–2 Blackbody emissions. A perfect or blackbody
absorbs all infrared energy. A = Absorbed energy. R =
Reflected energy. T = Transmitted energy. E = Emitted
energy.

considerations of the actual emissivity of an object improve the accuracy of tempera-
ture measurements used for predictive maintenance. To help users determine emis-
sivity, tables have been developed to serve as guidelines for most common materials;
however, these guidelines are not absolute emissivity values for all machines or plant
equipment.

Variations in surface condition, paint, or other protective coatings and many other
variables can affect the actual emissivity factor for plant equipment. In addition to
reflected and transmitted energy, the user of thermographic techniques must also con-
sider the atmosphere between the object and the measurement instrument. Water vapor
and other gases absorb infrared radiation. Airborne dust, some lighting, and other vari-