Temperature is by far the most commonly measured industrial process parameter. Most temperature measurements are made using thermocouples or RTDs; however, for many applications involving a moving or batch-heated product, these contact devices provide only an indirect measure of temperature. Infrared thermometers often are able to provide a direct measurement of product temperature, but these devices sometimes are difficult to apply without significant measurement errors. Recent developments in infrared thermometer technology have eliminated many of the obstacles to accurate temperature measurement for this type of challenging application (figure 1).
- Low emissivity (high reflectivity) materials, especially metals.
- Variable-emissivity (variable-reflectivity) materials.
- Small or moving parts.
- Hostile operating conditions involving high ambient temperatures or optical obstructions such as dust, smoke and steam.
For applications with characteristics such as these, a more advanced infrared thermometer may be required. Advanced infrared thermometers can be divided into three distinct types: short-wavelength, dual-wavelength (also known as ratio or two-color) and multi-wavelength. Optimum temperature measurement results are obtained by selecting the most appropriate sensor technology for the process application. Table 1 summarizes some of the application characteristics appropriate for each sensor type.
Typical Short Wavelength Application
Short-wavelength, single-wavelength infrared thermometers offer good performance when measuring low emissivity materials at low temperatures. For example, a large Midwestern automotive component supplier had been sandblasting an aluminum part before painting. The sandblasting was done specifically to increase the emissivity value in an attempt to improve the performance of a long-wavelength infrared thermometer. The thermometer was used to confirm that the part had reached the proper preheat temperature of about 220oF (104oC). By changing from a long-wavelength sensor to a short-wavelength sensor, the measurement accuracy for the sandblasted part improved from +/-20oF (+/-11oC) to +/-3oF (+/-1.7oC). In addition, parts that had not been sandblasted could be measured with an accuracy of +/-6oF (+/-3.3oC). Eliminating the sandblasting step lowered the aluminum part emissivity from 0.35 to 0.10 and resulted in an estimated cost savings of about $1 million per year.
The accuracy improvement associated with the shorter wavelength sensor is shown in figure 2. The shorter wavelength sensor shows an improvement in measurement from about 18oF (10oC) to about 5oF (2.8oC) when measuring at 220oF (104oC).
This alone does not explain the total improvement in accuracy achieved by the component supplier. Indeed, the short-wavelength measurement benefits from the fact that most metals have a higher emissivity value at shorter wavelengths. Therefore, any emissivity variation represents a smaller emissivity change as a percent of emissivity. A change in emissivity affects a short- wavelength thermometer less than a long wavelength thermometer. At a short wavelength, a change in signal is less of a change in temperature error.
Typical Dual-Wavelength ApplicationOne of the greatest advantages of a dual-wavelength infrared thermometer is that the target can be smaller than the optical resolution of the instrument. This feature is particularly appropriate when viewing past an optical obstruction or when viewing a small, wandering target such as a thin wire (figure 3).
A wire may be heated for a number of reasons, including wire preheat for plastic coating of electrical wire, metallic diffusion coating used for steel belted tires or wire drawing for light bulbs. For each process, accurate temperature measurement and control are critical for efficient production and consistent quality. At a well-known light bulb filament plant, for example, it had been impossible to measure wires in diameters of less than about 0.1" using traditional infrared thermometers because the small diameter combined with physical movement of the wire created impossible optical alignment challenges.
For this application, the dual-wavelength sensors are able to provide a relatively large target area of 0.5" dia. while measuring the significantly smaller wire. Using a dual-wavelength sensor, it is possible to measure a wire that is less than 0.001" dia. and with a wander from side to side of as much as 0.2".
Typical Multi-Wavelength Application
Multi-wavelength infrared thermometers are used for applications where single- and dual-wavelength sensors are not able to provide the required accuracy. No matter how improved single- and dual-wavelength technologies have become, there will always be a place for the multi-wavelength technology. This technology is used to provide an accurate temperature reading when the traditional single- and dual-wavelength technologies fall short.
Advances in infrared technology have opened the door to new and difficult applications. In many heating processes, direct temperature measurement is a critical control paramter for the optimization of quality and productivity. Today's infrared thermometers are used for both simple as well as difficult applications.
Direct temperature measurement of critical process parameters is critical for the optimization of quality and productivity. Infrared thermometers are useful tools for obtaining the desired direct measurement but may sometimes be difficult to apply. Short-, dual and multi-wavelength infrared thermometers can help processors overcome many of these difficulties.