Chapter 3: IR Thermometers and Pyrometers
optical devices which intercepted and evaluated the visible
radiation emitted by glowing objects. A modern and more correct definition would be any non-contacting device intercepting and measuring thermal radiation emitted from an object to determine surface temperature.
Thermometer, also from a Greek root thermos, signifying hot, is used to describe a wide assortment of devices used to measure temperature. Thus a pyrometer is a type of
thermometer. The designation radiation thermometer has evolved over the past decade as an alternative to pyrometer. Therefore the terms pyrometer and radiation thermometer are used interchangeably by many references.
A radiation thermometer, in very simple terms, consists of an optical system and detector. The optical system focuses the energy emitted by an object onto the detector, which is
sensitive to the radiation. The output of the detector is proportional to the amount of energy radiated by the target object (less the amount absorbed by the optical system), and the response of the detector to the specific radiation wavelengths. This output can be used to infer the objects temperature. The emittivity, or emittance, of the object is an important variable in converting the detector output into an accurate temperature signal.
Infrared radiation thermometers/pyrometers, by specifically measuring the energy being radiated from an object in the 0.7 to 20 micron wavelength range, are a subset of radiation thermometers. These devices can measure this radiation from a distance. There is no need for direct contact between the radiation thermometer and the object, as there is with thermocouples and resistance temperature detectors (RTDs). Radiation thermometers are
suited especially to the measurement of moving objects or any surfaces that can not be reached or can not be touched.
But the benefits of radiation thermometry have a price. Even the simplest of devices is more expensive than a standard thermocouple or resistance temperature detector (RTD) assembly, and installation cost can exceed that of a standard thermowell. The devices are rugged, but do require routine maintenance to keep the sighting path clear, and to keep the optical elements clean. Radiation thermometers used for more difficult applications may have more complicated optics, possibly rotating or moving parts, and microprocessor-based electronics. There are no industry accepted calibration curves for radiation thermometers, as there are for
thermocouples and RTDs. In addition, the user may need to seriously investigate the application, to select the optimum technology, method of installation, and compensation needed for the measured signal, to achieve the performance desired.
Topics covered include:
1. The N Factor
2. Types of Radiation Thermometers
3. Design and Engineering
Other chapters of Transactions will systematically cover other aspects of temperature, humidity, pressure, strain, flow, level, pH, and conductivity instrumentation as well as other measurement, data acquisition and control topics. Click here to view the other chapters of Volume I, or to download Non-Contact Temperature Measurement in its entirety.
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