Page 187 - Intro Predictive Maintenance

178 An Introduction to Predictive Maintenance

Radiation

Radiation is the transfer of heat by wavelengths of electromagnetic energy. The most
common cause of radiation is solar energy. Only radiated energy is detected by an
infrared imager. If the aforementioned motor were sitting outside in the slab storage
yard with slabs stacked around it, the electromagnetic energy from the sun and from
the slabs would increase the temperature.

The purpose of the previous example was to make the thermographer aware that other
causes of the thermal energy could be found or not found. In this case, was the motor
hot because of a bad bearing or because of solar radiation? Was the motor missed and
failed later because of the fan blowing on it and causing convection cooling? Con-
duction is the only mode that transfers thermal energy from location to location within
a solid; however, at the surface of a solid or liquid, and in a gas, it is normal for all
three modes to operate simultaneously.

Emissivity

Emissivity is the percentage of energy emitted by an object. Infrared energy hits an
object; the energy is then transmitted, reflected, or absorbed. A common term used in
infrared thermography is blackbody. A blackbody is a perfect thermal emitter. Its emis-
sivity is 100 percent. It has no reflection or transmittance. The objects you will be
scanning will each have a different emissivity value. A percentage of the total energy
will be caused by reflection and transmittance; however, because most of your infrared
inspection will be quantitative thermography, the emissivity value will not be as
important now.

8.5 INFRARED EQUIPMENT
Listed as follows are the criteria used to evaluate infrared equipment. It is important
to determine which model best fits your needs before a purchase is made. Some of
these points will be important to you and others will not. You will know more about
your needs after you have finished reading this book.

• Portability. How much portability does your application require? Does
weight and size of the instrument affect your data collection? What kind of
equipment will you be scanning?
• Ease of Use. How much training is required to use the imager? Can it be
used easily in your environment?
• Qualitative or Quantitative. Does it measure temperatures? If yes, what tem-
perature range will be measured? Will you need more than one range?
• Ambient or Quantitative Measurements. What are the maximum upper and
minimum lower ambient temperatures in which you will be scanning?
• Short or Long Wavelengths. Long-wavelength systems offer less solar re-
flection and operate in the 8- to 14-micron bandwidth. Short-wavelength
systems offer smaller temperature errors when an incorrect emissivity value
is entered. The operating bandwidth for a short-wave unit is 2 to 5.6 microns.