Wednesday, February 6, 2019

Why Is Emissivity Important When Using IR Inspection Windows?

A local technical college contacted your company’s Human Resources office to inquire if a group of students could visit the site and learn about electrical thermography inspections.  You were asked to host this visit and speak to them about critical applications of thermography.  You decided to teach them about emissivity and why it’s important when performing electrical thermography inspections.  Your lesson planning begins.  

A factory is full of electrical equipment that requires periodic maintenance inspections.  Your company implemented a condition-based maintenance model over a year ago using infrared windows and IR cameras that allows energized electrical maintenance inspections to be performed safely and efficiently.  The ultimate goal for the reliability team is to perform frequent inspections of critical assets to find early warning signs that an asset is deteriorating and fix it before it completely fails.  You explain to the students that a scheduled downtime to fix the asset versus an unplanned outage results in less impact to the company and their end customers.   

Infrared radiation is the part of the electromagnetic spectrum between 0.75 and 1000 microns in wavelength. Learning about emissivity is a key to conducting accurate infrared inspections.  Emissivity is defined as the relative power of a surface to emit heat by radiation.  As an object heats up, the intensity of emitted radiation increases exponentially – this is known as Stephan-Boltzmann’s Law.  Infrared cameras “see” and “calculate” the emitted radiation from a target object.  There are three sources of this emitted radiation:  reflected from other sources; transmitted through the object from a source behind it; or emitted by the object itself.   

Emissivity of a material is measured between a value of 0 and 1.0.  A perfect emitter, called a blackbody because it emits 100% of the energy it absorbs, is assigned an emissivity value of 1.0.  In the work environment, emissivity of a surface is the ratio of the radiant energy emitted by an object divided by the energy that a blackbody would emit at the same temperature, the same wavelength and under the same viewing conditions. 

Know Your Emissivity 

When performing thermography on an object, the inspector must adjust the emissivity value on the infrared camera to properly compensate for the various emissivity values of all the target components encountered during the inspection.  Slight errors in emissivity compensation can lead to significant errors in temperature readings which could result in inaccurate inspection data being recorded.  Emissivities of common materials are found below:   

As the temperature increases, radiant energy increases proportionately.  Incorrect camera settings for emissivity will result in errant temperature readings.  Using an emissivity value that is higher than the emissivity of the target will result in electrical faults appearing cooler than they actually are.  Likewise, because the relationship is exponential, the error will worsen as the component increases in temperature.  The resulting temperature calculations could be drastically understated which could lead the inspectors to misdiagnose the severity of a fault on an asset or not detect the fault at all.   

 Conclusion:  

Thermographers must understand the surface of the primary target that they are inspecting with an infrared camera.  To compensate for emissivity of other objects around the target, the emissivity value must be adjusted in the IR camera.   Slight errors in emissivity compensation can lead to significant errors in temperature readings which could result in inaccurate inspection data being recorded.  Accurate data is required for a condition-based monitoring inspection model so faults on electrical assets can be identified and fixed before the asset goes into failure mode.