Technical Information |
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Advantage Online: 2003 Archives
CONSIDERING EMISSIVITY WHEN USING A NONCONTACT THERMOMETER
September 29, 2003 -Applying heat to temporarily soften metal structural parts is quite common, especially when repairing aluminum vehicles. When heating the metal, it is important to keep the temperature within the vehicle maker’s recommendations. There are several methods of monitoring heat, including heat detection crayons and paint, temperature indicating strips, and using a noncontact or IR (infrared) thermometer or a thermocouple (see Figure 1). Noncontact thermometers detect the amount of heat that radiates from an object. Different materials emit heat differently, depending on the amount of reflectivity the object has. This, known as emissivity, should be considered when using a noncontact thermometer. Emissivity is the ability of a surface to emit radiation. Noncontact thermometers read thermal radiation, or the amount of heat that is coming off of the heated part. Noncontact thermometers work well for a very accurate heat monitor, provided that the part being heated has a coating on it. Shiny metals such as aluminum or bare steel have a low emissivity and will not provide an accurate reading. The I-CAR Tech Centre has researched emissivity with the use of a thermal imaging camera to compare the accuracy of a noncontact thermometer on shiny metal and painted surfaces. An aluminum panel was painted semi-gloss black, although color is not important, at two-inch intervals, leaving two inches of bare aluminum between the painted areas (see Figure 2). Heat monitoring strips were applied in the center of the panel on both the painted and bare surfaces. Heat was applied to the back of the panel using an infrared heat lamp. Both the shiny bare aluminum and painted areas were checked with a thermocouple. The temperature readings are within 1° between both areas (see Figures 3 and 4). On the heat monitoring strips, the temperature reads approximately 160° F (71° C) (see Figure 5). Next, a noncontact thermometer is used to check the temperatures of both areas. The bare aluminum area measures only 87° F (30° C) (see Figure 6), whereas the painted area measures 161° F (71° C) (see Figure 7). This is because the noncontact thermometer can only read the amount of heat that is radiated from the panel. Shiny metal has a lower amount of heat radiating from it, or a lower emissivity. To make this more visible, the panel is checked through the thermal imaging camera. The panel appears in red and dark blue. The blue areas are where the panel is not painted and the red areas are the areas that are painted. These readings show the amount of heat radiating off of the panel, the same as a noncontact thermometer (see Figure 8). The bare aluminum shows that the temperature is near 83° F (28° C), and the painted area is 162° F (72° C). Also notice how reflections like the overhead lighting change the readings. The reflection changes the amount of heat that can be emitted, thus changing the temperature reading (see Figure 9). Some noncontact thermometers have an adjustment for emissivity, but there are many variables including the texture of the part being heated and the amount of reflection from the background that make adjustments difficult. It is nearly impossible to adjust the emissivity of the thermometer to achieve an accurate reading on a bare aluminum panel. Noncontact thermometers work well to monitor heat as long as there is a coating on the metal. Even a coating of rust will provide emissivity to obtain an accurate reading. If no coating exists, applying a coating of high-temperature spray paint and allowing it to dry before heating will ensure that the readings are accurate.
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