ASTM C1371-98

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: C 1371 – 98
Standard Test Method for
Determination of Emittance of Materials Near Room
Temperature Using Portable Emissometers
This standard is issued under the fixed designation C 1371; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 408 Test Method for Total Normal Emittance of Surfaces
Using Inspection-Meter Techniques
1.1 This test method covers a technique for determination of
E 691 Practice for Conducting an Interlaboratory Test Pro-
the emittance of typical materials using a portable differential
gram to Determine the Precision of a Test Method
thermopile emissometer. The purpose of the test method is to
provide a comparative means of quantifying the emittance of
3. Terminology
opaque, highly thermally conductive materials near room
3.1 Definitions—For definitions of some terms used in this
temperature as a parameter in evaluating temperatures, heat
test method, refer to Terminology C 168.
flows, and derived thermal resistances of materials.
3.2 Definitions of Terms Specific to This Standard:
1.2 This test method does not supplant Test Method C 835,
3.2.1 diffuse surface—a surface that emits or reflects equal
which is an absolute method for determination of total hemi-
radiation intensity, or both, into all directions (2).
spherical emittance, or Test Method E 408, which includes two
3.2.2 emissive power—the rate of radiative energy emission
comparative methods for determination of total normal emit-
per unit area from a surface (2).
tance. Because of the unique construction of the portable
3.2.3 emissometer—an instrument used for measurement of
emissometer, it can be calibrated to measure the total hemi-
emittance.
spherical emittance. This is supported by comparison of
3.2.4 Lambert’s cosine law—the mathematical relation de-
emissometer measurements with those of Test Method C 835
2 scribing the variation of emissive power from a diffuse surface
(1).
as varying with the cosine of the angle measured away from the
1.3 This standard does not purport to address all of the
normal of the surface (2).
safety concerns, if any, associated with its use. It is the
3.2.5 normal emittance—the directional emittance perpen-
responsibility of the user of this standard to establish appro-
dicular to the surface.
priate safety and health practices and determine the applica-
3.2.6 radiative intensity—radiative energy passing through
bility of regulatory limitations prior to use.
an area per unit solid angle, per unit of the area projected
2. Referenced Documents normal to the direction of passage, and per unit time (2).
3.2.7 spectral—having a dependence on wavelength; radia-
2.1 ASTM Standards:
tion within a narrow region of wavelength (2).
C 168 Terminology Relating to Thermal Insulating Materi-
3.2.8 specular surface—mirrorlike in reflection behavior
als
(2).
C 680 Practice for Determination of Heat Gain or Loss and
3.3 Symbols:Symbols:
the Surface Temperatures of Insulated Pipe and Equipment
3 3.3.1 For standard symbols used in this test method, see
Systems by the Use of a Computer Program
Terminology C 168. Additional symbols are listed here:
C 835 Test Method for Total Hemispherical Emittance of
3 a5 total absorptance, dimensionless
Surfaces from 20 to 1400°C
a 5 spectral absorptance, dimensionless
l
E 177 Practice for Use of the Terms Precision and Bias in
e 5 total emittance of the high-emittance calibration stan-
hi
ASTM Test Methods
dard, dimensionless
e 5 total emittance of the low-emittance calibration stan-
low
dard, dimensionless
This test method is under the jurisdiction of ASTM Committee C-16 on
e 5 apparent total emittance of the test specimen, dimen-
Thermal Insulation and is the direct responsibility of Subcommittee C16.30 on spec
Thermal Measurements.
sionless
Current edition approved November 10, 1998. Published March 1999. Originally
e5 apparent total emittance of the surface, dimensionless
published as C 1371 - 97. Last previous edition C 1371 - 97.
The boldface numbers in parentheses refer to the list of references at the end of
this standard.
Annual Book of ASTM Standards, Vol 04.06.
4 5
Annual Book of ASTM Standards, Vol 14.02. Annual Book of ASTM Standards, Vol 15.03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C 1371
e 5 apparent total emittance of the surface 1, dimensionless The detector thermopiles are heated in order to provide the
e 5 apparent total emittance of the surface 2, dimensionless necessary temperature difference between the detector and the
e 5 apparent total emittance of the surface of detector, surface. The differential thermopile consists of one thermopile
d
dimensionless that is covered with a black coating and one that is covered
e 5 apparent total emittance of the surface of specimen, with a reflective coating. The instrument is calibrated using
s
dimensionless two standards, one with a high emittance and the other with a
e 5 spectral emittance, dimensionless low emittance, which are placed on the flat surface of a heat
l
l5 wavelength, μm sink (the stage) as shown in Fig. 1. A specimen of the test
r5 total reflectance, dimensionless material is placed on the stage and its emittance is quantified by
−8 2
s5 Stefan-Boltzmann constant, 5.6696 3 10 W/m ·K comparison to the emittances of the standards. The calibration
t5 total transmittance, dimensionless shall be checked repeatedly during the test as prescribed in 7.2.
A 5 area of surface, m
k 5 proportionality constant, V·m /W 5. Significance and Use
Q 5 radiation heat transfer, W
rad
5.1 Surface Emittance Testing:
q 5 radiative heat flux, W/m
rad
5.1.1 Thermal radiation heat transfer is reduced if the
T 5 temperature of the test surface, K
surface of a material has a low emittance. Since the controlling
T 5 temperature of the radiant background, K
factor in the use of insulation may be condensation control or
T 5 temperature of the detector, K
d
personnel protection, it is important to note that a low
T 5 temperature of the surface of specimen, K
s
emittance will also change the surface temperature of a
V 5 voltage output of the detector when stabilized on
hi
material. One possible criterion in the selection of these
high-emittance calibration standard
materials is the question of the effect of aging on the surface
V 5 voltage output of the detector when stabilized on
low
emittance. If the initial low surface emittance of a material is
low-emittance calibration standard
V 5 voltage output of the detector when stabilized on test
spec
specimen
The sole source of supply of emissometers known to the committee at this time
is Devices & Services Co., 10024 Monroe Drive, Dallas, TX 75229. If you are
4. Summary of Test Method
aware of alternative suppliers, please provide this information to ASTM Headquar-
4.1 This test method employs a differential thermopile
ters. Your comments will receive careful consideration at a meeting of the
emissometer for total hemispherical emittance measurements. responsible technical committee, which you may attend.
NOTE 1—(a) Emissometer measuring head on high-emittance standard during calibration, showing heat sink and cable to readout device. (b) Bottom
view of emissometer measuring head showing high- and low-emittance detector elements. The diameter of the emissometer measuring head is about 50
mm and the detector elements are recessed about 3 mm into the measuring head.
FIG. 1 Schematic of Emissometer
C 1371
not maintained during service, then the long-term value of the about 0.06) and a blackened standard (emittance about 0.9).
material is diminished. The standards should be traceable to measurements made using
5.1.2 This test method provides a means for comparative an absolute test method (for example, Test Method C 835). It is
periodic testing of low emittance surfaces in the field. In this recommended that one set be used as working standards and
way the effects of aging on the reflective properties can be the other set be put aside and used for periodic checks of the
monitored. emittance of the working standards. The time period between
5.1.3 This test method can be used to measure the total checks of the working standards will depend upon the amount
hemispherical emittance with a precision of better than 60.02 that the working standards are used.
units, if some care is taken to avoid potential misapplications. 6.1.6 Sample of the Surface to be Tested, collected carefully
(1) The emittances of the calibration standards shall have been so as to preserve the in-situ surface condition. A specimen
obtained from accurate independent measurements of total slightly larger than the outer dimensions of the emissometer
hemispherical emittance. This test method should not be used measuring head is carefully cut from the sample.
for specimens that are highly anisotropic or transparent to
7. Procedure
infrared radiation. This test method also should not be used for
specimens with significant thermal resistance (see 7.3.4).
7.1 Set-up—A sample of the material to be tested should be
5.1.4 Once a reliable emittance measurement has been
collected as near as possible to the time of the test, to control
determined, the value can be applied to calculate radiative heat
sample conditioning history. The emissometer should be set up
flow from the subject surface. For example, if the temperature
as near as possible to the sample site, and allowed to equili-
of the surface, T , and the temperature of the surroundings, T ,
brate until the calibrations remain stable, with no drift.
1 2
are known, then the radiative heat flow, Q , is given by:
rad
NOTE 2—For the emissometer a warm-up time of 1 h has been found
4 4
Q 5 Aes ~T 2 T ! (1) to be acceptable.
rad 1 2
where A is the area of the surface, and either A is assumed to 7.2 Instrument Calibration:
7.2.1 Place the high- and low-emittance standards on the
be much smaller than the area of the surroundings or the
emittance of the surroundings is assumed to be unity. If heat sink. Thermal contact between the standards and the heat
sink is improved by filling the air gaps between the standards
desired, this radiative heat flow can be combined with convec-
tive heat flow to calculate the total heat flow from the surface and the heat sink with distilled water or other high conductance
material.
(a method which may be appropriate is described in Practice
C 680). 7.2.2 Place the emissometer measuring head over the high-
emittance standard. Allow at least 90 s for the reading to
stabilize.
6. Apparatus
7.2.2.1 If a standard millivoltmeter is used as the readout
6.1 This test method applies only to emittance tests con-
device, record the output voltage.
ducted by means of a heated, differential thermopile emissom-
7.2.2.2 If the emittance is read out directly, use the variable
eter, such as that shown in Fig. 1. The following elements are
gain control on the readout device to adjust the readout to be
used:
equal to the emittance of the high-emittance standard.
6.1.1 Differential Thermopile Radiant Energy Detector—
7.2.3 Place the emissometer measuring head over the low-
The differential thermopile consists of elements with high and
emittance standard, and again allow at least 90 s for the reading
low emittance that produce an output voltage proportional to
to stabilize.
the temperature difference caused by different amounts of
7.2.3.1 If a standard millivoltmeter is used as the readout
thermal energy emitted and absorbed by each. The output
device, calculate the expected reading from the low-emittance
voltage is proportional to the emittance of the surface that the
standard by means of (Eq 2) (see Section 8). Then adjust the
detector faces.
offset trimmer on the emissometer until the readout value
6.1.2 Controlled Heater—Within the emissometer measur-
agrees with the calculated reading.
ing head that maintains the head at a temperature above that of
7.2.3.2 If the emittance is read out directly, use the offset
the specimen or calibration standard.
trimmer control on the emissometer to adjust the readout to be
6.1.3 Readout Device—Typically a digital millivoltmeter,
equal to the emittance of the low-emittance standard.
and may include a means of conditioning the thermopile output
7.2.4 Place the emissometer measuring head over the high-
signal so that the emittance can be read directly.
emittance standard again, and repeat the procedure in 7.2.1-
NOTE 1—The emissometer has a direct readout of emittance, with a
7.2.3, until the measuring head can be moved from one
resolution of 60.01 units. For the work described in Ref (1), the resolution
standard to the other without requiring any adjustment to
was increased to 60.001 units.
obtain the expected reading.
6.1.4 Heat Sink Stage—A heat sink with a flat surface or
NOTE 3—Devices & Services Co. suggests that for best accuracy when
stage upon which the reference standards and specimen are
evaluating low-emittance specimens (e below about 0.2), a high-emittance
placed, and which provides a means of maintaining the
standard should be used that is in about the same emittance range as the
standards and specimen at the same, stable temperature. specimen (3). In general, the closer the emittance of the high-emittance
reference standard is to that of the specimen, the less the detector will
6.1.5 Reference Standards—the manufacturer of the emis-
6 overshoot, and the accuracy of the measurements will be improved.
someter supplies two sets of reference standards, each set
consisting of a polished stainless steel standard (emittance 7.3 Specimen Collection:
C 1371
7.3.1 Since many different kinds of materials can be tested 9.1.1 Name and any other pertinent identification of the
by means of this technique, different specimen collection material, including a physical description.
procedures may be required, depending on the nature of the
9.1.2 Description of the specimen and its relationship to the
material. In general, the procedure should ensure minimum sample, including a brief history of the specimen, if known.
alteration of the specimen surface. For example, if the emit-
9.1.3 Thickness of the specimen as received and as tested.
tance of a dust-covered specimen is desired, the dust shall not
9.1.4 Temperature of the room in which the measurements
be removed.
were conducted, °C.
7.3.2 All contact with the specimen surface shall be
9.1.5 Source
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