Turning a glow into a temperature
Any hot object gives off thermal radiation: energy carried away as electromagnetic waves. As its temperature rises, the radiation changes in intensity and, for some instruments, shifts toward shorter wavelengths. A pyrometer collects this light and uses the signal to estimate the surface temperature. It can do this from a distance, without putting a probe into the melt.
Some pyrometers measure infrared radiation; others compare the brightness of the object at selected wavelengths. The instrument’s sensor converts the incoming light into an electrical signal, and its calibration translates that signal into a temperature reading. The key is that it measures the surface’s radiation—not the temperature deep inside a piece of metal.
Why the surface matters
The estimate depends partly on emissivity: how effectively a surface emits radiation compared with an ideal radiator. Metal’s emissivity can vary with its composition, finish, oxidation, and temperature. A shiny surface may also reflect radiation from nearby furnaces or other hot objects, making the reading less reliable. Operators choose an appropriate instrument and aim it at a clear, suitable spot to reduce these errors.
That tradeoff is useful in a furnace or foundry, where a contact sensor could be damaged, contaminate the melt, or simply be impractical to place. A pyrometer lets workers monitor a fiercely hot surface from outside the process—but its reading is an informed estimate shaped by what the instrument can see and by the condition of the metal’s surface.
