Why is Emissivity so important?

Posted by Martin Robinson on 12/1/15 9:56 AM
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A typical plant is full of equipment that requires periodic infrared (IR) inspection. The challenge, as any thermographer knows, is getting an accurate indication of equipment health. Properly compensating for the various emissivity values of all the components one encounters on the factory floor is possibly the most critical factor in performing accurate and meaningful inspections. Even slight errors in emissivity compensation can lead to significant errors in temperature and Delta T (difference in temperature) calculations. Electrical cabinets are a good example, as they may contain materials with emissivity values ranging from 0.07 to 0.95.

For some components, it can be difficult to determine the correct emissivity value. In the case of a highly polished component like a bus bar, the actual emissivity may be so low as to make temperature measurement impractical. It is strongly recommended that thermographers understand the surface of the primary targets. Once identified, those surfaces should be treated with a high-emissivity covering so that all targets have a standardized emissivity. Thermographers can apply electrical tape, high-temperature paint (such as grill paint), or high-emissivity labels (like the IR-ID labels from IRISS). When all targets have a standard emissivity, refection issues are minimized and measurement errors from reflected ambient energy are greatly reduced. High-emissivity targets of varying shapes can also provide a useful point-of-reference both for the thermographer and the technician making repairs.

Summary
1.    Emissivity is one of the most important variables a thermographer must understand.
2.    Whenever possible, know the emissivity of your target and compensate for it using the emissivity setting on the camera.
3.    Incorrect emissivity settings can have a significant effect on the accuracy of qualitative and quantitative data (thermograms and temperature calculations).
4.    Using an emissivity value that is higher than the actual emissivity of the target will result in electrical faults appearing cooler than they actually are.
5.    Emissivity errors are not linear, but are exponential in nature (Stephan-Boltzmann’s Law). The exponential nature of the error also means that ?T values (differences in temperature) can be greatly affected by the errors as well.
6.    When installing IR windows it is important to standardize the emissivity of the targets while the switch-gear is open (and de-energized).
7.    Common treatments for target surfaces are: grill paint, electrical tape and IRISS IR-ID labels.

To make your job easier during your next electrical inspection, take a look at IRISS IR-ID Labels. Regardless of experience and skill level, thermographers will improve efficiency with IR-ID Labels since any fault diagnosis is only as reliable as the data collected.

 

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Topics: Electrical Maintenance, Infrared Windows, Thermography, Electrical Inspection, Energized Electrical Maintenance, Emissivity, IR-ID Labels

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