ASTM E408 Standard Test Method for Determination of Total Normal Emittance of Surfaces Using Inspection-Meter Techniques 

ASTM E408 Introduction:

The ASTM E408 test method provides a standard practice that is adequately applicable for measuring the total normal emittance of surfaces using the inspection-meter techniques. The emittance is defined as the characteristic of a material that indicates the relative ability of a material to emit thermal radiation compared to a perfect black body at this temperature. There is a need for proper thermal management when the property must be adequately measured. The test procedure may only be necessary for the construction, aerospace, and manufacturing industries. 

What is the ASTM E408 Standard Test?

The ASTM E408 procedure sets the standard for measuring the total normal emittance with portable inspection meters to establish a convenient and reliable approach to characterizing surfaces in laboratory and field measurements. This will be most useful for appraisals of thermal performance in materials such as coatings, foils, and other surfaces encountered in connection with the purpose of energy conservation, heat dissipation, or thermal isolation. Therefore, ASTM E408 standards of the method will make way for emittance measurements with high consistency, accuracy, and contribution in critical areas. This will enable material selection, quality control, and legal regulations to be met.

ASTM E408 Test Scope:

The ASTM E408 is a test method for determining the total normal emittance of surfaces utilizing inspection-meter procedures. Commercially available equipment for this assessment includes at least two different types. The first method (method A) measures radiant energy emitted from the specimen, while the second method (method B) measures radiant energy reflected from it. These methods are designed for quick measurements on big surfaces that require a non-destructive test.

ASTM E408 Standard Test Procedure

ASTM E408 Method A:

The surface to be measured is placed against an opening (or aperture) on the instrument as per ASTM E408. The cavities are rotated alternately across the aperture by a driving mechanism. A vacuum thermocouple detects the cavity radiation reflected off the specimen while rectifying contacts coupled to the rotation converts the amplifier output to a DC signal. This signal is read with a millivolt meter. Next, the meter reading is calibrated with known reflectance standards to obtain reflectance values on the test surface. Finally, the data can be transformed into total normal emittance by subtracting the measured reflectance from unity.

ASTM E408 Method B:

The surface to be measured is positioned against the aperture on the portable sensing component according to ASTM E408. Radiant radiation emitted and reflected from the specimen illuminates a thermopile when it passes through a suitable transmitting vacuum window. We chill the thermopile and the hollow walls the specimen sees to reduce the quantity of energy reflected from the specimen. Subsequently, the thermopile’s output is amplified and measured by an appropriate meter. Finally, the meter reading must be calibrated to recognized emittance criteria.

ASTM E408 Specimen Size

Method A uses opaque or semi-transparent specimens in the wavelength area of interest. Furthermore, method B employs opaque specimens to infrared radiation in the wavelength range of interest.

Advantages & Disadvantages of ASTM E408 Standard Test Method

This standard test method has its advantages and disadvantages. Some of them are discussed below:

ASTM E408 Test Conclusion:

The ASTM E408 Standard Test Method presents a handy nondestructive tool for measuring total normal emittance with high accuracy from various surfaces. Its uniformity is essential to its applicability in thermal management and quality control. While this is so, surface conditions must be adequately considered by careful calibration if reliable results are to be achieved. In summary, ASTM E408 C-1371 is essential for evaluating the thermal properties of surfaces in any industry.

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