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a group of methods for measuring temperature. At one time, the term “pyrometry” encompassed all methods employed to measure temperatures higher than limit-temperatures for mercury thermometers. However, since the 1960’s “pyrometry” has come into increasingly widespread use to refer exclusively to optical methods, particularly those that use pyrometers. This modern usage excludes many nonoptical techniques, for example, those that use resistance thermometers or thermoelectric thermometers with thermocouples.
Almost all optical methods are based on measurements of intensity of thermal radiation (or, sometimes, absorption) of bodies. The intensity of thermal radiation strongly depends on the temperature of the body under study and decreases sharply with decreasing temperature. Thus, pyrometric methods are used for measuring relatively high temperatures; a batch-produced radiation pyrometer, for example, is used for temperatures 200°C and above. Pyrometry is out of secondary importance at temperatures below 1000°C; it is the principal method used for temperatures above 1000°C and practically the only method that can measure temperatures above 3000°C. Pyrometry is used under industrial and laboratory conditions to determine the temperatures of furnaces and other heating devices and temperatures of molten metals and materials that are produced from them, for example, rolled stock. Pyrometric methods can also measure temperatures of flames, heated gases, and plasma. They can be used to measure very high temperatures, since direct contact is not required between the sensor of the pyrometer and the surface of the body whose temperature is being determined.
The basic requirement for the use of pyrometric methods is that the body must emit purely thermal radiation; that is, it must obey Kirchhoff’s radiation law. Solid bodies and liquids at high temperatures usually satisfy this requirement, while with gases and plasma the applicability of Kirchhoff’s law must be verified in each new case or under each new physical condition. Thus, radiation from a homogeneous plasma layer obeys Kirchhoff’s law if the velocity distribution of the plasma’s molecules, atoms, ions, and electrons conforms to the Maxwell distribution law, if the population of excited energy levels conforms to Boltzmann’s law, if dissociation and ionization are governed by the law of mass action and if in all these equations the same value of temperature is used (seeMAXWELLIAN DISTRIBUTION; BOLTZMANN STATISTICS; MASS ACTION, LAW OF).
Such a state of plasma is called a thermal equilibrium state. The radiation intensity of equilibrium homogeneous plasma in both line and continuous spectra is unambiguously defined by the plasma’s chemical composition, pressure, atomic constants, and equilibrium temperature. If a plasma is nonhomogeneous, its radiation does not obey Kirchhoff’s law even if conditions of thermodynamic equilibrium are satisfied everywhere. With non-homogeneous plasmas, pyrometric methods are applicable only to sources of light that have axial symmetry.
The temperature measurements are simplest for solids and liquids, where emission spectra are purely continuous. In this case, the temperatures are measured with pyrometers that function on the basis of blackbody radiation laws. The surface of the body under investigation is usually concave so as to allow the coefficient of absorption to approach unity; the optical properties of such a body approximate those of an ideal blackbody.
The pyrometric techniques based on the measurement of the spectral line intensities are the most universal. These methods have maximum accuracy when the absolute probability of corresponding transition and the concentration of a given kind of atom are known. If the concentration of atoms is now known with sufficient accuracy, a method of relative intensities is used in which the temperature is calculated from the ratio of the intensities of two or more spectral lines. Variations on these methods have been developed in order to measure temperatures of both optically thin and optically thick plasma layers.
Another group of pyrometric techniques determines temperature from the shape or width of spectral lines, which are features that are influenced by temperature either directly, through the Doppler effect, or indirectly, through the Stark effect, and by the dependence of plasma density on temperature. Certain methods measure temperature from the absolute or relative intensity of the continuous spectrum. Especially important are methods that determine temperature from the spectrum of laser radiation that is scattered by the plasma; such methods make it possible to study nonhomogeneous plasma.
Pyrometric methods have the following drawbacks: pyrometric measurement is a laborious process, the results are complex to interpret, and the degree of accuracy is only moderate; for example, errors in measurements on plasma at best range from 3 to 10 percent. Valuable information concerning the state of nonequilibrium plasma can be obtained using pyrometry, although the concept of temperature is not applicable in this case.
REFERENCESOplicheskaia pirometriia plazmy. Collection of articles translated from English and edited by N. N. Sobolev. Moscow, 1960.
Griem, H. Spektroskipiia plazmy. Moscow, 1969. (Translated from English.)
Metody issledovaniia plazmy: spektroskipiia, lazery, zondy. Translated from English and edited by S. Iu. Luk’ianov. Moscow, 1971.
V. N. KOLESKNIKOV