The world of thermography is expanding faster than ever before, with new applications, markets and cameras. More and more maintenance practitioners are putting this remarkable technology to use as one of the tools in their pouch. As this trend grows, infrared (IR) camera suppliers have responded with great new products that are lighter, smaller, easier to use and less expensive than ever imagined.
One thing that has not changed, though, is the need to have a solid basic knowledge, grounded in experience, in order to successfully interpret the images produced by these remarkable cameras. Unfortunately, many believe they can skip over this essential step and jump right to being an “expert.”
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Photo 3. The copper busbar heated to the point where it was severely damaged. |
The intent of this article isn’t to shame readers but to help them identify areas where they may want to gain more knowledge and/or experience in order to do a better job.
So, what is your IR IQ? The basic knowledge required for good practice by reliability thermographers includes a discussion of a number of relevant issues and questions. Have fun taking this short quiz, and then examine the answers to see how you did.
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Photos 1-2. Infrared cameras can pinpoint electrical hot spots not visible to the naked eye. |
The questions
1)Thermal imaging cameras and infrared film produce similar results when it comes to detecting heat.
a. True b. False
2) It is possible to accurately detect problems inside an electrical panel or other such equipment without removing the cover.
a. True b. False
3) The emissivity correction value of copper is:
a. .68 b. .05
c. .10 d. Any of the above
4) Which of the following is essential information to help you assess the repair priority of a component found with your infrared camera?
a. Age of the component
b. Criticality of the component
c. Temperature of the component
d. The wave band being detected
5) What standards are available to guide a thermographer who is inspecting electrical and mechanical equipment?
a. NFPA 70A
b. ASTM 1934
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Photo 4. The containers contain water at the same hot temperature, but the surface temperatures vary widely. |
c. ISO 18425
d. There are no standards relating to thermography
6) You are considering the purchase of an infrared camera that will be used primarily for indoor machine inspections at close distances (up to 10 feet from the component). Which of the following specifications is important in your choice?
a. Field of view
b. Spatial resolution
c. Minimum focus
d. All of the above
7) You see a “hot spot” on a heavily oxidized overhead disconnect switch on the plant’s incoming power pole. When you measure the temperature with your camera, it reads minus-5 degrees F. The air temperature is 50 degrees F, the sun is out and the sky is blue. What is the probable explanation?
a. You had set emissivity to .9 instead of the recommended .68
b. You are probably seeing a reflection of the sun instead of a real hot spot
c. You are not working within the limits of your camera’s measurement resolution
d. The wind has cooled the hot connection
8)Thermography is not a useful technology for diagnosing problems in rotating equipment.
a. True b. False
THE ANSWERS
1) b. False
Infrared film, which can be used in conventional visual cameras, is sensitive to much shorter wavelengths of electromagnetic radiation than a longwave thermal imaging system. Imagine a room with no light and everything having a temperature of 30 degrees C (86°F). If we began to heat an electric hotplate in the room, we could see it with an infrared camera when it reached 30.01 C (86.02°F). To see the same object with infrared film, it would need to be heated to 230 C (446°F). Of course, by the time it reaches about 550 C (1022°F), we can see it with our eyes as it glows a dull red.
Despite the differences in technologies, it isn’t uncommon for people to want to try the less expensive route of using infrared film to locate their maintenance problems. Try it if you must, but infrared film will never replace an infrared camera.
2) b. False
In this age of Internet pseudo-science, many people have come to believe you can “see through walls” with infrared cameras. However, it is not possible; in fact, it is also impossible to see through the clear plastic “touch-safe” panels found inside many control cabinets.
To reliably detect problems, enclosures must be opened and panels removed. A hot spot inside may eventually produce enough heat to produce a thermal signature on the unopened enclosure, but in many cases, it still can’t be detected due to ambient conditions. Even if it is detected, the internal
damage is, by then, typically well advanced. The thermal image (Photo 2) shows approximately a 15°C (27 F) rise over normal on the outside of the enclosure. As can be seen in the visual image (Photo 3) of the interior, the copper busbar had heated to the point where it was severely damaged.
Infrared transparent windows and view ports may offer an alternative means of seeing inside enclosures without opening the covers. The plastic “touch safe” interior panels are still a problem, regardless, as they are opaque to infrared radiation.
3) d. Any of the above
The visual equivalent of this question would be to ask “What is the color blue?” There are many choices. The exact surface of copper can vary just as widely as the color blue and, in each case, a different emissivity correction value would be needed. Even when we define the surface more precisely, such as “highly polished copper,” there can be significant variations in emissivity.
Does it matter? Yes. Especially for a surface with a relatively low emissivity, using an incorrect value can make a large difference in the temperature value obtained. Too many thermographers believe, incorrectly, that if they simply input the right correction value from a table (some of which may be in the camera software itself), all will be well. These tables may be useful as guides, but they can’t replace critical thinking.
4) b. Criticality of the component
It’s not uncommon to find a wide range of temperature indications of abnormal situations. As an example, all of the containers in the thermal image (Photo 4) contain water at the same hot temperature, but the surface temperatures vary widely for each. The same is true of the machinery we look at during an inspection.
Temperature and thermal patterns are useful information, but without an understanding of the criticality of the asset, no rational determination can be made of how to prioritize a finding. In fact, conditions can easily be such that an indication of serious concern may be so subtle as to be undetectable. Take great care, therefore, to ensure that conditions are sufficient to locate any anomalies that might exist.
Once detected, many other indicators, including temperature, age and criticality, among others, will probably be used to prioritize an appropriate course of action.
5) b. ASTM 1934
“E1934-99a(2005)e1 Standard Guide for Examining Electrical and Mechanical Equipment with Infrared Thermography” from ASTM International was written specifically to address the needs of maintenance thermographers. In fact, it is currently being revised into two separate standard guides, one for electrical inspections and one for mechanical. The current document, which provides a strong foundation for condition monitoring-related IR inspections, is available at www.astm.org. In the near future, also expect to see standards for infrared mechanical inspections from the International Standards Organization (ISO).
6) d. All of the above
Potential purchasers of infrared cameras often don’t fully understood the camera “spec sheet.” Sales people can easily make you believe a specification (one their system has) is critical while glossing over other details. It’s vital to understand what each specification actually means and how it affects your use of the camera in your plant.
Spatial resolution is getting plenty of press lately as newer, lower-cost cameras come on the market with lower resolution than has ever been offered. In fact, many of these perform quite well for many (but not all) condition monitoring (CM) applications. If you can get closer to the equipment being inspected, resolution may not be an issue.
Resolution also affects field of view (FOV). This is the area you see though the lens at a given distance. In many instances, access is limited, and knowing what the minimum focus distance is may be critical to success. The bottom line is to understand specifications and your real needs before you look at what camera to buy.
7) c. You are not working within the limits of your camera’s measurement resolution
It’s most likely that you are averaging the temperature of the clear sky, which is extremely cold, with the hot spot because you are working outside the resolution limits of the camera. If you can move closer or use a telephoto lens, you may be able to improve the measurement. But if not, remember that the hot spot is actually hot even if you can’t accurately measure it.
8) b. False
While the interpretation of thermal signatures of rotating equipment can present challenges, especially to thermographers who aren’t qualified, the technology plays an invaluable role in many world-class CM programs. Thermography is best used to locate anomalies. Once detected, temperature values may be useful as well, but typically, information from other technologies (vibration, ultrasound and tribology) will also be important to the analysis.
How is your IR IQ?
Here is the suggested scoring scale:
8 correct = Red hot!
6-7 = Warm and learning
4-5 = Lukewarm and needs heating
Fewer than 4 = Too cold for comfort
Now your real-life test continues. There are a number of sources of information about thermography available, including this magazine. But you must learn to think critically and constantly challenge yourself to do better if you ever hope to make thermography pay its full return.
John Snell is the founder and president of Snell Infrared. The company works with all major suppliers of IR systems to provide their customers with training, while remaining independent of the sale of any particular product. To learn more, call 800-636-9820 or visit www.snellinfrared.com.