ASTM E511-73(1994)e1
(Test Method)Standard Test Method for Measuring Heat Flux Using a Copper-Constantan Circular Foil, Heat-Flux Gage
Standard Test Method for Measuring Heat Flux Using a Copper-Constantan Circular Foil, Heat-Flux Gage
SCOPE
1.1 This test method describes the measurement of radiative or convective heat flux, or both, using a transducer whose sensing element (1, 2) is a thin circular metal foil. While benchmark calibration standards exist for radiative environments, no uniform agreement among practitioners or government entities exists for convective environments.
1.2 The values stated in SI units are to be regarded as the standard. The values stated in parentheses are provided for information only.
1.3 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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e1
Designation: E 511 – 73 (Reapproved 1994)
Standard Test Method for Measuring
Heat Flux Using a Copper-Constantan Circular Foil,
Heat-Flux Gage
This standard is issued under the fixed designation E 511; 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.
e NOTE—Section 10 was added editorially in December 1994.
1. Scope 3. Significance and Use
1.1 This test method describes the measurement of heat flux 3.1 The purpose of this test method is to measure the heat
absorbed by a thin circular foil of copper-constantan construc- flux at a location from a radiant or convective source, or both.
tion by either convection or radiation or a combination of both. In the case of radiant energy, the absorptivity of the surface of
1.2 The values stated in SI units are to be regarded as the the instrument should be known. In the case of convection
standard. The values given in parentheses are for information energy, particularly at high velocities, the shape and size of the
only. probe body in which the foil sensor is mounted should be the
1.3 This standard does not purport to address all of the same as the test specimen.
safety concerns, if any, associated with its use. It is the 3.2 The gage has certain limitations as follows:
responsibility of the user of this standard to establish appro- 3.2.1 The gage cannot measure conduction.
priate safety and health practices and determine the applica- 3.2.2 The body temperature must be in the range from 50 to
bility of regulatory limitations prior to use. 450°F (−45 to 235°C) in order for the calibration to be valid.
At lower or higher temperatures, the gage is no longer linear
2. Summary of Test Method
due to changes in thermoelectric output not compensated for by
2.1 The circular foil heat-flux gage provides a self-
changes in physical properties of the constantan foil.
generated millivolt output in response to the thermal energy 3.2.3 Foil diameters and thickness are limited. Maximum
absorbed. The sensing foil (see Fig. 1) is connected at its
optimum foil diameter to thickness ratio is 4 to 1 for sensors
perimeter to a heat sink having a thermoelectric potential less than 2.54-mm (0.100-in.) diameter. Foil diameters range
different from that of the foil material, thus forming a thermo-
from 25.4 to 0.254 mm (1.0 to 0.010 in.) with most gages
couple junction. Another thermocouple junction is made at the between 6.35 and 1.02 mm (0.250 and 0.040 in.).
center of the foil using a fine wire. When the sensor is exposed
3.2.4 Large-diameter foils in vacuum environment have
to a heat source, the heat flux absorbed by the circular foil is significantly different sensitivities than in air and should not be
transferred radially to the heat sink, and an equilibrium
so used unless calibrated in vacuum.
temperature difference is rapidly established between the 3.2.5 Response time is a function of the radius or diameter
center and edge of the foil. The equilibrium thermoelectric
of the foil squared. Range is from 0.001 s (0.25 mm (0.010 in.)
potential, E, between the center and edge of the foil will then in diameter) to 6 s (25.4 mm (1 in.) in diameter). Response
vary in proportion to the heat flux, q, absorbed by the foil. The time is defined as the time to sense 63 % of a step function.
body is normally made of copper and the foil of thermocouple-
3.2.6 The response time, t, is approximated by the formula
2 2
type constantan. If these metals are used in the gage, the t5 6D , where D is in inches (1) or t5 0.0094 D where D
thermoelectric potential will be directly proportional to heat
is in millimetres. The sensitivity of the gage may be expressed
flux absorbed such that by the equation E/q 5 0.03 D /S where D is the diameter of the
foil in inches, S is the thickness of the foil in inches, E is the
q 5 KE (1)
emf in millivolts and q is the heat flux in Btu/ft ·s. In SI, the
where K is a constant determined experimentally during
equation is E/q 5 0.0046 D /S, where D and S are in millime-
calibration. All further discussion will assume the use of these
2 2
tres, and q is in cal/cm ·s or E/q 5 19.3 D /S where D and S are
two metals in the gage since this construction is by far the most
in metres and q is in W/m .
common.
3.2.7 The field of uniform flux must exceed the area of the
sensor.
This test method is under the jurisdiction of ASTM Committee E-21 on Space
Simulation and Applications of Space Technology and is the direct responsibility of
Subcommittee E21.08 on Thermal Protection. The boldface numbers in parentheses refer to the list of references at the end of
Current edition approved Nov. 27, 1973. Published January 1974. this test method.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E511
FIG. 1 Heat Drain—Either by Water Cooling the Body with a Surrounding Water Jacket or Conducting the Heat Away with Sufficient
Thermal Mass
3.3 The temperature of the gage is normally low in com- 4.3.1 Certain conditions of very low heat-transfer rate exist
parison to the heat source. The resulting heat flux measured by under which convection will not be correctly measured using a
the gage is known as a “cold-wall” heat flux. circular foil heat-flux gage. This complex subject is discussed
in the literature (3).
4. Apparatus
4.4 Water-Cooled Sensors—Water-cooled sensors should be
4.1 Gage—The gage shall consist of a circular foil sensor,
used in any application in which the sensor body would
as shown in Fig. 1, connected to a heat sink. The output leads
otherwise rise above 450°F (235°C). Typical cooled assemblies
from the calorimeter shall be connected to instrumentation
are shown in Fig. 4.
capable of readout in millivolts. This instrumentation shall be
4.4.1 Amount of Coolant Flow—Whatever coolant flow will
potentiometric or have an impedance of 100 000 V or greater.
prevent local boiling of the coolant at the face of the gage is
(Sensor impedance is usually less than 1 V).
adequate. This phenomenon can be detected by observing the
4.2 Sensor—The sensor, constructed using a copper body,
outlet flow. If the outlet flow develops a pulsating output,
copper leads, and a constantan (thermocouple-type) foil, pro-
boiling is occurring. The exact pressure required for a given
duces an emf output in millivolts which is a linear function of
design to achieve the desired flow varies according to the
heat flux. Other metal combinations are used but most are
resistance to flow, which is dependent upon the design of the
nonlinear (2).
water-flow path. Rarely is a gage designed to require more than
4.2.1 The wire leads used to convey the signal from the
a few gallons of water per minute and most require only a
sensor to the readout device are usually made of stranded,
fraction of a gallon per minute. Exposure time will have no
tinned copper. The wires are usually TFE-fluorocarbon-coated
effect upon gage performance as long as adequate cooling is
and shielded with a braid overwrap which is also TFE-
provided.
fluorocarbon-covered. The leads are color coded to distinguish
4.5 Mount Materials of Construction—Mount bodies are
the positive lead from the negative lead. It is common practice
normally made of oxygen-free high-conductivity (OFHC)
to use the color black on the negative lead.
copper. The sides of the body may be made of brass, but copper
4.3 Circular Foil—Figs. 2 and 3 may be used as a guide for
is frequently used throughout except for water inlet stems,
the dimensions of the circular foil. As can be seen from Figs.
which for support purposes, are usually brass or stainless steel.
2 and 3, a variety of different thicknesses and diameters will
result in the same sensitivity. Most units are designed for a 4.5.1 Special Case: Heat fluxes in excess of 34 050 kW/
2 2
maximum output of 10 mV. At this output, the center of the m (3000 Btu/ft ·s)—Such high fluxes require thin external
gage is about 400°F (205°C) higher than the edge temperature. shells for quick transfer of heat into high velocity (15 to 30 m/s
E511
FIG. 2 Chart for Design of Copper-Constantan Circular Foil Heat-Flow Meters (SI Units)
(50 to 100 ft/s)) water channels. The high velocity is produced soot (total normal emittance, e 5 0.99) (4), and camphor
TN
by high-pressure water 3.4 to 6.9 MPa (500 to 1000 psi). For soot (e 5 0.98) (4). The soots all have the disadvantage of
TN
such high pressure shells, zirconium-copper is used since its low oxidation resistance and poor adhesion to the gage surface.
yield strength is much larger than OFHC copper. Colloidal graphite coatings (e 5 0.83) (5) are commonly
TN
4.6 Sensor Surface—Gage performance is highly sensitive used since they are readily dried from acetone or alcohol
to surface condition. Gages are coated with thin layers of solutions and tenaciously adhere to the gage surface. However,
metallic and nonmetallic materials for special applications. the coatings can be quickly removed from solvents. Spray
Coatings are used to affect the radiant or convective heat black lacquer paints (e 5 up to 0.98), some of which may
TN
absorbing qualities of the gage, or both. Basic surface condi- require baking, are also used and are intermediate between the
tions are: colloidal graphites and soots in oxidation resistance and
(a) No coating; adherence.
(b) High emissivity coatings, high absorption; 4.6.2.1 The emissivity or absorbance of the coating should
(c) Low emissivity coatings, reflection; and be determined to the required accuracy if a coating of unknown
(d) Coatings that catalyze recombination reactions in non- emittance is used. If the emittance of the coating is altered
equilibrium flow. during the test, it should be redetermined at the end of the test.
4.6.1 A gage with no coating is recommended for use in 4.6.3 Reflection—Low emissivity metallic coatings such as
most convective applications. highly polished gold and nickel are also used in special cases
4.6.2 High Absorption—High-emissivity coatings are used where the reflection of radiant heat is desired. The coatings are
when radiant energy is to be measured. Ideally the coating usually only a fraction of a mil thick. Such coatings decrease
should provide a nearly diffuse absorbing surface. A diffuse the sensitivity of the gage. The gold coating also causes the
coating is defined as one that has no change in ab
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