Silicon grown by Czochralski process 1956

The disappearing-filament pyrometer is an optical pyrometer, in which the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

of a glowing incandescent object is measured by comparing it to the light of a heated

filament The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: Astronomy * Galaxy filament, the largest known cosmic structures in the universe * Solar filament ...

. Invented independently in 1901 by Ludwig Holborn and

Ferdinand Kurlbaum Ferdinand Kurlbaum (October 4, 1857 in Burg bei Magdeburg – July 29, 1927 in Berlin) was a German physicist. He was a son of a judicial officer. Until 23 he made his Abitur. He studied mathematics and physics in Heidelberg and Berlin from ...

in Germany and Everett Fleet Morse in the United States, it was the first device which could measure temperatures above 1000 °C. Disappearing filament pyrometers have been used to measure temperatures between about 600 °C and 3000 °C. Like other optical pyrometers they are used to measure the temperature of objects too hot for contact thermometers, such as molten metals. Widely used in the steel and

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...

s industries as well as for research, they have been almost totally superseded by electronic spectral-band pyrometers. The simplest design has optics like a

Keplerian telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and a ...

. A thin wire (

), placed at the focal plane of the objective lens, is heated by electric current. When seen through the eyepiece, the wire appears silhouetted in front of the hot luminous object under investigation. The user compares the brightness of the glowing filament with the object behind, and adjusts the current through the filament until it seems to "disappear" in front of the glowing object. At that point the filament and object are at the same temperature. The user then reads the temperature off the filament current control dial, which is calibrated by the filament's current-vs-temperature curve. or in some instruments from a current-vs-temperature table. The filament seems to "disappear" against the background of the object because two objects at the same temperature have the same

black-body spectrum A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...

. In other designs the current through the filament is kept constant, and the radiation allowed through from the target object is varied with calibrated attenuating wedges in the optical path, or a prism is used to place the images of the target object and a calibrated glowing surface next to each other, e.g. as a disk inside a ring. Many disappearing-filament pyrometers use a red filter. The combination of the filter and the human eye's response only allows through a narrow band of red wavelengths, so the luminosity comparison is made over only a narrow band of wavelengths. This reduces errors due to the target and filament not having identical emission spectra. For very hot objects, additional filters can be used to protect the eye from excessive light. The resolution of the instrument depends somewhat on the operator, but with a skilled operator a resolution of 10 °C for temperatures up to 2000 °C can be achieved. Disappearing-filament pyrometers can be used only if the object under study emits visible light similar to a hot

black body A black body or blackbody is an idealized physical object, physical body that absorption (electromagnetic radiation), absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence (optics), angle of incidence. T ...

; this means that its temperature must be high enough (around 600 °C and up) and the object must not be fully transparent or highly reflective. For good accuracy, the object should appear dark gray or black when cold.

References

{{reflist Measuring instruments Optical instruments

See also

References