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Application Notes:
Infrared Thermal Imagers: A Primer for HVAC Technicians |
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Have you ever taken heat for an expensive failure after having performed routine maintenance? Have you ever wished for a crystal ball to see into the future or just to see things your traditional test equipment and eyes couldn't reveal? Compared to traditional test equipment, thermal imagers just may seem to have the predictive qualities of a crystal ball. By taking our visual sight capabilities beyond the visible spectrum to the radiated heat spectrum, we begin to see predictive qualities that do indeed foretell future likelihoods. |
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Thermal Imagers operate in the infrared spectrum and follow the same laws as infrared (IR) thermometers, but the similarity ends there. IR thermometers report an average "spot" temperature. IR Thermal imagers are similar to digital cameras and, in practical terms, may be thought of as infrared cameras. An LCD display shows a "movie" of the IR image as the user selects the image, focuses, then pulls the trigger to record the image. In addition to highlighting temperature variations and hotspots in real time through the LCD display, Fluke thermal imagers now include IR-Fusion®*, a technology that fuses a visual, or visible light, image with an infrared image for better identification, analysis and image management. Even with that, the power of thermal technology is not fully realized until the recorded image is downloaded to a computer, analyzed with the provided software, and saved in a comparative database along with descriptions, notes and an actual photograph to compare the IR with the visual image. Within the computer program, the image can reveal specific spot temperatures, a grid of temperature readings, minimum-maximum-average temperatures of a selected area of the image, emissivity and reflectivity can be adjusted, level and gain can be adjusted, the palette can be changed (color, grayscale, or ironbow which is a smoother edged color palette), and more.
Thermal imaging is gaining an invaluable predictive and diagnostic reputation in industries such as power distribution, plant maintenance, petro-chemical plants and process applications, to name a few. What industry is more suited for thermal imaging than the thermal dynamic industry of HVACR? Dynamic heat analysis of moving parts (Motors, bearings, sheaves, belts), electrical circuit quality (starters and contactors, disconnects, fuses and busses, electrical connections), duct heat loss or gain, conditioned envelope heat loss or gain, tracing membrane roof leaks, compressor operating condition (relative head, sump, suction, discharge temperatures and unloader or hot gas bypass operation), analysis of steam traps, radiators and convectors, radiant loops, or any process that can reveal the integrity of the process by comparative temperatures. The full range of HVACR applications for thermal imagers will only be realized once they are in the creative hands of HVACR technicians.
Using a Thermal ImagerAn HVAC technician interpreting a thermal image is similar to a doctor interpreting X-Rays or MRI's. This may sound ominous, but you already have the HVAC knowledge and experience to know what you are looking for. Just add a few facts about the nature of thermal imaging, and you're home free.
IR radiation is just beyond the visible radiation spectrum. Radiated light is reflected off surfaces or emitted from sources that our eyes receive and our brain interprets. IR radiation is heat radiated by or reflected from a material; radiation that our eyes cannot see. Our skin is the best sensor of IR radiation. We feel the radiation from a fire. We feel the radiation loss when standing close to a cold wall. A thermal imager interprets IR radiated or reflected heat by assigning a visible graduated color or gray scale to a radiated portrait of the scene. The color palette displays hot spots as white with diminishing temperatures through red-orange-yellow-green-blue-indigo-violet to black being cold. The gray scale palette also shows hot spots as white with diminishing temperatures through progressively darkening shades of gray to black being cold. This allows us to see a visible representation of the unseen IR spectrum. What visibly looks like a disconnect in good operating condition may be revealed as the L2 pole is operating 35 °F hotter (red) than L1 pole (blue). Equal loads, but different temperatures. This disconnect has a problem that couldn't be seen. Yet the thermal imager takes a "picture" of the entire device and its electrical connections with comparative temperatures. All real world materials absorb, reflect and transmit IR radiation depending on their physical properties.
IR radiation = Absorption + Reflection + TransmissionWhatever IR radiation is absorbed will be equally emitted. We do not encounter materials in the field that perfectly absorb and emit all IR radiation. A material that absorbs all IR radiation is called a "Black Body" and has an emissivity of unity (1). Most materials of interest that we encounter are called "Gray Bodies" since they are not perfect emitters, close maybe, but not perfect. Transmission through solids can usually be ignored in field work, with the exception being glass and plastic films which are referred to as "Non-Gray Bodies". This simplifies our working formula to:
Emissivity = 1 - ReflectivityReflectivity is inversely proportional to emissivity. The more an object reflects IR radiation, the less it emits. Reflectivity can be relatively judged according to our sight determinations of reflectivity. Polished chrome has a very high reflectivity and low emissivity. Brushed stainless steel has less reflectivity and more emissivity. Tarnished brass and copper have even less reflectivity with proportionately more emissivity. Most painted surfaces have very high emissivity and negligible reflectivity.
"Qualitative vs. quantitative"Most thermal imaging tasks are qualitative as opposed to quantitative. Quantitative is accuracy of temperature, while qualitative is relativity of temperature. When viewing a contactor for instance, the interest is in the temperature difference of the 12 contact points. Are the electrical connections all the same temperature (T1-L1, T2-L2, T3-L3)? Are the temperatures consistent between the fixed and movable contacts (T1C-L1C, T2C-L2C, T3C-L3C)? Seeing one point of elevated temperature directs us to a poor electrical connection or failing contactor points without being concerned that the reported temperature is off by some percentage. Painted surfaces have high emissivity and a very small margin of quantitative error. So our thermal image of a compressor, motor, bearings, steam traps, transformers, etc. will tend to be fairly accurate without taking the steps to fine tune imager emissivity.
Adjusting emissivityThermal imagers have adjustments for both emissivity and reflectivity. Both are easy to measure and compensate for when the need for quantitative readings outweighs qualitative readings.
For emissivity adjustments, a strip of black electrical tape can be fastened to a surface and the taped and untaped surfaces can be measured with the imager. The emissivity is adjusted until the untaped surface temperature equals the taped surface temperature. For high temperature surfaces, a contact temperature probe can be used to measure surface temperature, then the emissivity can be adjusted until the IR temperature equals the contact temperature. Charts are also available which list the emissivity ...
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