Emissivity and Its Application in the Coatings Inspection Industry
Dominic Pasquarelli and William Corbett, PCS
Introduction
Coating inspectors, engineers and others use a variety of inspection instruments to verify surface preparation and coating application outcomes conform the project specification or coating manufacturers instructions. Most of these instruments require contact with the prepared and/or coated surfaces. One exception is obtaining surface temperature using a non-contact infrared pyrometer that may or may not be equipped with a laser pointer. Depending on the manufacturer of the instrument, the emissivity may be adjusted based on the type of surface being measured. This brief article includes a description of emissivity, how it varies by surface type,how surface roughness and distance (from the surface) effects emissivity, the equipment that uses emissivity to measure surface temperature, the advantages and limitations of non-contact IR pyrometers, and the importance of annual calibration.
What is Emissivity
Let’s start by defining the term Emissivity. Emissivity is the measure of an object’s ability to emit infrared energy; the emitted energy indicates the temperature of the object. For example, an infrared pyrometer (“IR Gun”) emits infrared energy when the trigger is pulled; the energy reflected off the surface is read by the pyrometer as temperature (in °F or °C). Emissivity values range from 0 (mirror) to 1.0 (flat black surface); most painted surfaces have emissivity values close to 0.95.
How Emissivity Varies by Surface Type
During 2020 and 2021 most of us had our forehead temperature measured using a non-contact pyrometer, since elevated body temperature (above 100.4F) was a symptom of the COVID-19 virus. The emissivity of human skin ranges from 0.97 to 0.999, while the emissivity of most paints/coatings is 0.90. Therefore, an IR pyrometer pre-set to an emissivity of 0.90 will not provide an accurate skin temperature (and vice-versa). Other common applications of non-contact pyrometers are asphalt temperature during repaving (emissivity of 0.88) and food temperature, i.e., on a hot self-serve buffet (0.95-0.97). For contrast, the emissivity of aluminum foil is 0.03. The emissivity of common materials[1] are shown in the table below.
Effects of Emissivity when Measuring Surface Temperature
There are a few conditions that can affect emissivity and subsequently the temperature readings from the surface, including roughness/texture and distance from the measuring device to the surface. Each of these is explored below.
How Surface Roughness/Texture Affects Emissivity With metallic materials like steel, the emissivity will generally increase with surface roughness, but the amount of increase is unknown. So, measuring the surface temperature of an abrasive blast cleaned steel surface (nominal 2-3 mil surface profile) using an infrared pyrometer set to a specific emissivity value can produce false readings and is not recommended.
How Distance from a Surface Affects Temperature Measurement. Most non-contact infrared pyrometers have a distance-to-spot ratio (D:S), which in effect means the further away from the surface the pyrometer is positioned, the larger the diameter of the spot temperature measurement. In effect, the D:S is the ratio of the distance to the measurement surface and the diameter of the temperature measurement area. For example, a Fluke 64 MAX IR Thermometer has a 20:1 D:S ratio, which means the diameter of the measurement area is one-twentieth of the distance to the object. At 12 inches, the size of the spot is 0.6 inch (1/20th of 12”); at 24 inches away the diameter of the spot increases to 1.2-inches (1/20th of 24”). For contrast, at 10 feet away (120 inches), the diameter of the spot measurement is 6 inches. Conversely, the PosiTector IRT has a 5.7:1 D:S, which means when the probe is held 120 inches from the surface the diameter of the spot measurement is 21 inches, so the PosiTector IRT is better for large, uniform surfaces and the Fluke 64 MAX is better for smaller, more intricate surfaces where the temperature may vary.
Advantages and Limitations of Non-contact IR Pyrometers
The primary advantage of non-contact IR pyrometers is their ability to take readings at a distance. This allows for high temperature readings to be acquired at a safer distance than traditional contact thermometers. Another advantage of infrared thermometers is the ability to take readings in hard-to-reach areas. Since they are non-contact if line of site can be obtained for the desired area and the distance to spot ratio is small enough an accurate reading can be captured.
The primary limitation of non-contact IR pyrometers is maintaining accuracy on varying surfaces. A lot of lower cost models do not have an adjustable emissivity but rather a fixed one typically at 0.95. This gives the impression that the instrument is accurate on all surfaces it reads but in reality, a fixed emissivity will only be accurate on the materials that match the emissivity. There is no indicator within the instrument that will show if the reading being obtained is accurate or not, leaving it entirely up to the operator to decipher. Another limitation is that non-contact IR pyrometers only read surface temperature. When pointing them at an open container of paint or food only the surface is being measured, which could vary significantly from the internal temperature.
Calibration Processes and Importance of Annual Calibration
Calibration is important on non-contact IR pyrometers since there is no effective way to verify their accuracy outside of a laboratory. While the internal electronics do not typically drift, dirt, fingerprints, and other obstructions could build up on the lens during use. When this occurs temperature readings are inaccurate.
There are two primary methods for calibration. The first is using a flat plate calibrator. These devices utilize a fixed surface designed for a specific emissivity. This method is the lowest cost option but comes with the limitation of poor temperature uniformity across the surface and limited ability to test varying types of non-contact IR pyrometers. The other primary method is using a blackbody calibrator. These use a high emissivity black plate with a uniform heat source behind it to heat or cool the entire surface evenly. These can also allow for a variable emissivity to be generated so more fixed emissivity gages can be accommodated.
Summary
Coating inspectors, engineers, and others use infrared pyrometers to measure the surface temperature without having to physically contact the surface. However, the accuracy of the temperature measurement is dependent on having the pyrometer set to the correct emissivity based on the type of surface being measured. Emissivity is the measure of an object’s ability to emit infrared energy; the emitted energy indicates the temperature of the object. This article described emissivity, how it varies by surface type, how surface roughness and distance (from the surface) effects emissivity, the equipment that uses emissivity to measure surface temperature, the advantages and limitations of non-contact IR pyrometers, and the importance of annual calibration.
About the Authors
[1] https://en.wikipedia.org/wiki/Emissivity