There are two main categories of non contact thermal detection devices: Thermal Measurement and Thermal Imaging. Thermal measurement devices (also known as the spot pyrometer) allow the user to measure the spot temperature on a point of the target that can be seen. These devices use a photodetector sensitive to a particular IR wavelength range. IR wavelength varies with temperature. As matter heats up, the radiation wavelength shortens. Long wavelength (far IR) detectors have a limited temperature range to 1000°F. Medium wavelength (middle IR) detectors are used for temperatures to 2000°F. Short wavelength (near IR) detectors are used for temperatures over 2000°F. Attenuation and bandpass filters can be used for special applications such as temperature measurement of objects through flames. Medium or short wavelength devices of this type with flame filters are widely used for on line temperature measurement of Steam Methane Reforming furnace tubes.
The accuracy of these devices can be as good as +/- 1% at reading distances to 15 ft. The target area measured is a function of distance, typically 0.1 – 0.5 in. at ~5 ft. to 1.5 – 6 in. at 30 ft. Resolution can be improved with higher magnification lenses. It is important to remember that if the size of the target to be measured is smaller than the measurement zone of the camera, the temperature reading will be the average of the target’s and the surrounding area’s temperature. Measuring temperatures at distances of 15 ft. or more with accuracy approaching +/- 1% sounds quite good, but this accuracy assumes that the actual emissivity is known and there is minimal background reflection.
Thermal imaging devices fall into two major types: Mechanical Scanning and Electronic Scanning. For the mechanical scanning device, the detector design is similar to the spot pyrometer, but the increased complexity associated with generating an image requires alterations in how the scene is presented to the detector and certain alterations in detector configuration. The detector may consist of a single element, a row or a column of elements. External mirrors are used to scan the image across the detector. The detector’s small size relative to the image permits very high resolution.
The electronic scanning device reduces mechanical complexity and the physical size of the system. Detector arrays have been developed that give each pixel its own detector element (focal plane arrays). The detectors are small with as many as 78,000 individual detectors in a space of 0.08 square inches. The current generation of detectors allows for the production of small, light, mechanically simple cameras capable of very high video quality and thermal resolution.
Most of the thermal imaging systems available allow the operator to adjust emissivity and enter the background temperature for use in the radiometric calculations performed by the camera. These systems are available with a variety of lenses and filters to fit most any application. In addition to the camera, most of the manufactures offer a software analysis program. The camera stores the image on an 8, 12, or 16 bit data card for transfer to the PC analysis program. These programs are usually PC/Windows based with a variety of features for data analysis and presentation. Of particular interest are those systems, such as the INFRAMETRICS THERMACAM, that allow image settings to be manipulated in the analysis program independent of the camera settings at the time the image was taken. The following is a listing of Thermal Imaging systems available that employ the latest in IR technology:
1. INFRAMETRICS THERMACAM – a digital, focal plane array type with background temperature correction capability. PC based analysis program available.
2. FLIR PRISM DS – similar to the THERMACAM.
3. AMBER RADIANCE PM – similar to the THERMACAM
Quantum Logic makes an infrared pyrometer that is equipped with a laser which enables you to determine tube emissivity. It also has a background temperature correction feature.