ASTM E423-71(2014)
(Test Method)Standard Test Method for Normal Spectral Emittance at Elevated Temperatures of Nonconducting Specimens
Standard Test Method for Normal Spectral Emittance at Elevated Temperatures of Nonconducting Specimens
SIGNIFICANCE AND USE
5.1 The significant features are typified by a discussion of the limitations of the technique. With the description and arrangement given in the following portions of this test method, the instrument will record directly the normal spectral emittance of a specimen. However, the following conditions must be met within acceptable tolerance, or corrections must be made for the specified conditions.
5.1.1 The effective temperatures of the specimen and blackbody must be within 1 K of each other. Practical limitations arise, however, because the temperature uniformities are often not better than a few kelvins.
5.1.2 The optical path length in the two beams must be equal, or, preferably, the instrument should operate in a nonabsorbing atmosphere, in order to eliminate the effects of differential atmospheric absorption in the two beams. Measurements in air are in many cases important, and will not necessarily give the same results as in a vacuum, thus the equality of the optical paths for dual-beam instruments becomes very critical.Note 4—Very careful optical alignment of the spectrophotometer is required to minimize differences in absorptance along the two paths of the instrument, and careful adjustment of the chopper timing to reduce “cross-talk” (the overlap of the reference and sample signals) as well as precautions to reduce stray radiation in the spectrophotometer are required to keep the zero line flat. With the best adjustment, the “100 % line” will be flat to within 3 %.
5.1.3 Front-surface mirror optics must be used throughout, except for the prism in prism monochromators, and it should be emphasized that equivalent optical elements must be used in the two beams in order to reduce and balance attenuation of the beams by absorption in the optical elements. It is recommended that optical surfaces be free of SiO2 and SiO coatings: MgF2 may be used to stabilize mirror surfaces for extended periods of time. The optical characteristics of these coatings are cr...
SCOPE
1.1 This test method describes an accurate technique for measuring the normal spectral emittance of electrically nonconducting materials in the temperature range from 1000 to 1800 K, and at wavelengths from 1 to 35 μm. It is particularly suitable for measuring the normal spectral emittance of materials such as ceramic oxides, which have relatively low thermal conductivity and are translucent to appreciable depths (several millimetres) below the surface, but which become essentially opaque at thicknesses of 10 mm or less.
1.2 This test method requires expensive equipment and rather elaborate precautions, but produces data that are accurate to within a few percent. It is particularly suitable for research laboratories, where the highest precision and accuracy are desired, and is not recommended for routine production or acceptance testing. Because of its high accuracy, this test method may be used as a reference method to be applied to production and acceptance testing in case of dispute.
1.3 This test method requires the use of a specific specimen size and configuration, and a specific heating and viewing technique. The design details of the critical specimen furnace are presented in Ref (1),2 and the use of a furnace of this design is necessary to comply with this test method. The transfer optics and spectrophotometer are discussed in general terms.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E423 − 71 (Reapproved 2014)
Standard Test Method for
Normal Spectral Emittance at Elevated Temperatures of
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Nonconducting Specimens
This standard is issued under the fixed designation E423; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The general physical properties of ceramic materials combine to make thermal gradients a serious
problem in the evaluation and use of thermal emittance data for such materials. Ceramic materials in
general tend to be somewhat translucent, and hence emit and absorb thermal radiant energy within a
surface layer of appreciable thickness. Ceramic materials in general also tend to have low thermal
conductivity and high total emittance as compared to metals. These properties combine to produce
thermal gradients within a heated specimen unless careful precautions are taken to minimize such
gradients by minimizing heat flow in the specimen. The gradients tend to be normal to a surface that
is emitting or absorbing radiant energy.As a further complication, the gradients tend to be nonlinear
near such a surface.
When a specimen is emitting from a surface layer of appreciable thickness with a thermal gradient
normaltothesurface,ithasnouniquetemperature,anditisdifficulttodefineaneffectivetemperature
fortheemittinglayer.Emittanceisdefinedastheratioofthefluxemittedbyaspecimentothatemitted
by a blackbody radiator at the same temperature and under the same conditions. It is thus necessary
to define an effective temperature for the nonisothermal specimen before its emittance can be
evaluated. If the effective temperature is defined as that of the surface, a specimen with a positive
thermal gradient (surface cooler than interior) will emit at a greater rate than an isothermal specimen
at the same temperature, and in some cases may have an emittance greater than 1.0. If the thermal
gradient is negative (surface hotter than interior) it will emit at a lesser rate. If the “effective
temperature” is defined as that of an isothermal specimen that emits at the same rate as the
nonisothermal specimen, we find that the effective temperature is difficult to evaluate, even if the
extinctioncoefficientandthermalgradientareaccuratelyknown,whichisseldomthecase.Ifspectral
emittance is desired, the extinction coefficient, and hence the thickness of the emitting layer, changes
with wavelength, and we have the awkward situation of a specimen whose effective temperature is a
function of wavelength.
There is no completely satisfactory solution to the problem posed by thermal gradients in ceramic
specimens. The most satisfactory solution is to measure the emittance of essentially isothermal
specimens, and then consider the effect of thermal gradients on the emitted radiant flux when
attempting to use such thermal emittance data in any real situation where thermal gradients normal to
the emitting surface are present.
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E423 − 71 (2014)
1. Scope must be further qualified in order to convey a more precise
meaning. Thermal-radiant exitance that occurs in all possible
1.1 This test method describes an accurate technique for
directions is referred to as hemispherical thermal-radiant exi-
measuring the normal spectral emittance of electrically non-
tance. When limited directions of propagation or observation
conducting materials in the temperature range from 1000 to
are involved, the term directional thermal-radiant exitance is
1800 K, and at wavelengths from 1 to 35 µm. It is particularly
used.Thus,normalthermal-radiantexitanceisaspecialcaseof
suitable for measuring the normal spectral emittance of mate-
directional thermal-radiant exitance, and means in a direction
rialssuchasceramicoxides,whichhaverelativelylowthermal
perpendicular (normal) to the surface. Therefore, spectral
conductivity and are translucent to appreciable depths (several
normal emittance refers to the radiant flux emitted by a
millimetres) below the surface, but which become essentially
specimen within a narrow wavelength band and emitted into a
opaque at thicknesses of 10 mm or less.
small solid angle about a direction normal to the plane of an
1.2 This test method requires expensive equipment and
incremental area of a specimen’s surface. These restrictions in
rather elaborate precautions, but produces data that are accu-
angle occur usually by the method of measurement rather than
rate to within a few percent. It is particularly suitable for
by radiant
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