ASTM C835 – Standard Test Method for Total Hemispherical Emittance of Surfaces up to 1400°C
Significance and Use
5.1 The emittance as measured by this test method can be used in the calculation of radiant heat transfer from surfaces that are representative of the tested specimens, and that are within the temperature range of the tested specimens.
5.2 This test method can be used to determine the effect of service conditions on the emittance of materials. In particular, the use of this test method with furnace exposure (time at temperature) of the materials commonly used in all-metallic insulations can determine the effects of oxidation on emittance.
5.3 The measurements described in this test method are conducted in a vacuum environment. Usually this condition will provide emittance values that are applicable to materials used under other conditions, such as in an air environment. However, it must be recognized that surface properties of materials used in air or other atmospheres may be different. In addition, preconditioned surfaces, as described in 5.2, may be altered in a vacuum environment because of vacuum stripping of absorbed gases and other associated vacuum effects. Thus, emittances measured under vacuum may have values that differ from those that exist in air, and the user must be aware of this situation. With these qualifications in mind, emittance obtained by this test method may be applied to predictions of thermal transference.
5.4 Several assumptions are made in the derivation of the emittance calculation as described in this test method. They are that:
5.4.1 The enclosure is a blackbody emitter at a uniform temperature,
5.4.2 The total hemispherical absorptance of the completely diffuse blackbody radiation at the temperature of the enclosure is equal to the total hemispherical emittance of the specimen at its temperature, and
5.4.3 There is no heat loss from the test section by convection or conduction. For most materials tested by the procedures as described in this test method, the effects of these assumptions are small and either neglected or corrections are made to the measured emittance.
5.5 For satisfactory results in conformance with this test method, the principles governing the size, construction, and use of apparatus described in this test method should be followed. If these principles are followed, any measured value obtained by the use of this test method is expected to be accurate to within ±5 %. If the results are to be reported as having been obtained by this test method, all of the requirements prescribed in this test method shall be met.
5.6 It is not practical in a test method of this type to establish details of construction and procedure to cover all contingencies that might offer difficulties to a person without technical knowledge concerning the theory of heat transfer, temperature measurements, and general testing practices. Standardization of this test method does not reduce the need for such technical knowledge. It is recognized also that it would be unwise to restrict in any way the development of improved or new methods or procedures by research workers because of standardization of this test method.
1.1 This calorimetric test method covers the determination of total hemispherical emittance of metal and graphite surfaces and coated metal surfaces up to approximately 1400°C. The upper-use temperature is limited only by the characteristics (for example, melting temperature, vapor pressure) of the specimen and the design limits of the test facility. This test method has been demonstrated for use up to 1400 °C. The lower-use temperature is limited by the temperature of the bell jar.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.