ASTM C1046-95(2021)

Standard Practice for In-Situ Measurement of Heat Flux and Temperature on Building Envelope Components

SIGNIFICANCE AND USE
5.1 Traditionally, HFTs have been incorporated into laboratory testing devices, such as the heat flow meter apparatus (Test Method C518), that employ controlled temperatures and heat flow paths to effect a thermal measurement. The application of heat flux transducers and temperature transducers to building components in situ can produce quantitative information about building thermal performance that reflects the existing properties of the building under actual thermal conditions. The literature contains a sample of reports on how these measurements have been used (1-8).3  
5.2 The major advantage of this practice is the potential simplicity and ease of application of the sensors. To avoid spurious information, users of HFTs shall: (1) employ an appropriate S, (2) mask the sensors properly, (3) accommodate the time constants of the sensors and the building components, and (4) account for possible distortions of any heat flow paths attributable to the nature of the building construction or the location, size, and thermal resistance of the transducers.  
5.3 The user of HFTs and TTs for measurements on buildings shall understand principles of heat flux in building components and have competence to accommodate the following:  
5.3.1 Choose sensor sites using building plans, specifications and thermography to determine that the measurement represents the required conditions.  
5.3.2 A single HFT site is not representative of a building component. The measurement at an HFT site represents the conditions at the sensing location of the HFT. Use thermography appropriately to identify average and extreme conditions and large surface areas for integration. Use multiple sensor sites to assess overall performance of a building component.  
5.3.3 A given HFT calibration is not applicable for all measurements. The HFT disturbs heat flow at the measurement site in a manner unique to the surrounding materials (9, 10); this affects the conversion constant, S, to be used. The us...
SCOPE
1.1 This practice covers a technique for using heat flux transducers (HFTs) and temperature transducers (TTs) in measurements of the in-situ dynamic or steady-state thermal behavior of opaque components of building envelopes. The applications for such data include determination of thermal resistances or of thermal time constants. However, such uses are beyond the scope of this practice (for information on determining thermal resistances, see Practice C1155).  
1.2 Use infrared thermography with this technique to locate appropriate sites for HFTs and TTs (hereafter called sensors), unless subsurface conditions are known.  
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.  
1.4 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Sep-2021
• ICS: 91.100.99 - Other construction materials
• Technical Committee: C16 - Thermal Insulation
• Drafting Committee: C16.30 - Thermal Measurement

Relations

• Refers: ASTM C168-24 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 15-Apr-2024
• Refers: ASTM C1130-24 - Standard Practice for Calibration of Thin Heat Flux Transducers
• Effective Date: 15-Mar-2024
• Refers: ASTM C1153-23 - Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging
• Effective Date: 01-Sep-2023
• Refers: ASTM C168-18 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 15-Apr-2018
• Refers: ASTM C168-17 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 01-Jun-2017
• Refers: ASTM C168-15a - Standard Terminology Relating to Thermal Insulation
• Effective Date: 15-Oct-2015
• Refers: ASTM C1153-10(2015) - Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging
• Effective Date: 01-Sep-2015
• Refers: ASTM C518-15 - Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
• Effective Date: 01-Sep-2015
• Refers: ASTM C168-15 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 01-Jun-2015
• Refers: ASTM C168-13 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 01-Apr-2013
• Refers: ASTM C1130-07(2012) - Standard Practice for Calibrating Thin Heat Flux Transducers
• Effective Date: 01-Sep-2012
• Refers: ASTM C1060-11a - Standard Practice for Thermographic Inspection of Insulation Installations in Envelope Cavities of Frame Buildings
• Effective Date: 15-Mar-2011
• Refers: ASTM C1060-11 - Standard Practice for Thermographic Inspection of Insulation Installations in Envelope Cavities of Frame Buildings
• Effective Date: 01-Mar-2011
• Refers: ASTM C1153-10 - Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging
• Effective Date: 01-Sep-2010
• Refers: ASTM C518-10 - Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
• Effective Date: 01-May-2010

Overview

ASTM C1046-95(2021), published by ASTM International, details standard practice for the in-situ measurement of heat flux and temperature on building envelope components. This standard outlines a method for using heat flux transducers (HFTs) and temperature transducers (TTs) to evaluate the real-world thermal behavior of opaque building envelope elements-such as walls, roofs, and floors-under actual service conditions. Data derived using this standard are essential in understanding building envelope performance, improving energy efficiency, and supporting diagnostics in both new construction and retrofit scenarios.

Key Topics

• Heat Flux Measurement: The use of HFTs enables direct measurement of energy transfer through building envelope components.
• Temperature Measurement: TTs, including thermocouples and RTDs, are used alongside HFTs to capture precise temperature gradients.
• Sensor Placement: Guidance is provided on selecting optimal sensor sites using building documentation and infrared thermography, ensuring measurements represent realistic conditions.
• Calibration and Accuracy: The standard stresses the importance of calibrating HFTs according to ASTM C1130 to avoid misinterpretation due to environmental or installation variations.
• Data Integrity: Recommendations address minimizing distortions and errors in heat flow measurements, including sensor masking, thermal contact enhancement, and consideration of multidimensional flows.
• Reporting and Analysis: Specifies essential elements for documenting and interpreting measurement results, contributing to repeatability and comparability across projects.

Applications

Applying ASTM C1046-95(2021) offers significant value in various sectors of the building and construction industry:

• Building Energy Assessments: Allows building professionals to quantify the actual thermal performance of walls, roofs, and other envelope elements in their operational state.
• Retrofit Optimization: Facilitates identification of thermal bridges, insulation performance, and air leakage points to target efficiency upgrades and improvements.
• Quality Assurance during Construction: Assists in verifying compliance with thermal performance specifications by measuring as-built characteristics.
• Research and Development: Provides a framework for experimental analysis in building science, supporting the advancement of new materials and assemblies.
• Commissioning and Diagnostics: Supports post-occupancy evaluations and forensic diagnosis of building envelope failures or inefficiencies.

Related Standards

For comprehensive evaluation and measurement, ASTM C1046-95(2021) is frequently used and referenced alongside the following standards:

• ASTM C168 – Terminology Relating to Thermal Insulation
• ASTM C518 – Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
• ASTM C1060 – Practice for Thermographic Inspection of Insulation Installations in Envelope Cavities of Frame Buildings
• ASTM C1130 – Practice for Calibration of Thin Heat Flux Transducers
• ASTM C1153 – Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging
• ASTM C1155 – Practice for Determining Thermal Resistance of Building Envelope Components from In-Situ Data

Practical Value

Implementing ASTM C1046-95(2021) enables accurate, repeatable in-situ thermal performance measurements that reflect true building envelope conditions. This practice supports energy consultants, facilities managers, architects, and engineers in making informed decisions to enhance thermal efficiency, ensure sustainability goals, and validate the effectiveness of construction or retrofit interventions. Proper use leads to better energy conservation strategies and helps comply with evolving building codes and sustainability certifications.

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Keywords: ASTM C1046, in-situ heat flux measurement, temperature measurement, building envelope components, energy efficiency, heat flux transducers, thermal performance, building diagnostics, construction quality assurance, infrared thermography.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C1046 − 95 (Reapproved 2021)
Standard Practice for
In-Situ Measurement of Heat Flux and Temperature on
Building Envelope Components
This standard is issued under the fixed designation C1046; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This practice covers a technique for using heat flux
C168 Terminology Relating to Thermal Insulation
transducers (HFTs) and temperature transducers (TTs) in mea-
C518 Test Method for Steady-State Thermal Transmission
surements of the in-situ dynamic or steady-state thermal
Properties by Means of the Heat Flow Meter Apparatus
behavior of opaque components of building envelopes. The
C1060 Practice for Thermographic Inspection of Insulation
applications for such data include determination of thermal
Installations in Envelope Cavities of Frame Buildings
resistances or of thermal time constants. However, such uses
C1130 Practice for Calibration of Thin Heat Flux Transduc-
are beyond the scope of this practice (for information on
ers
determining thermal resistances, see Practice C1155).
C1153 Practice for Location of Wet Insulation in Roofing
Systems Using Infrared Imaging
1.2 Use infrared thermography with this technique to locate
C1155 Practice for Determining Thermal Resistance of
appropriate sites for HFTs and TTs (hereafter called sensors),
Building Envelope Components from the In-Situ Data
unless subsurface conditions are known.
1.3 The values stated in SI units are to be regarded as the
3. Terminology
standard. The values given in parentheses are for information
3.1 Definitions—For definition of terms relating to thermal
only.
insulating materials, see Terminology C168.
1.4 This standard does not purport to address all of the 3.2 Definitions of Terms Specific to This Standard:
3.2.1 building envelope component—a portion of the build-
safety concerns, if any, associated with its use. It is the
ing envelope, such as a wall, roof, floor, window, or door, that
responsibility of the user of this standard to establish appro-
has consistent construction.
priate safety, health, and environmental practices and deter-
3.2.1.1 Discussion—For example, an exterior stud wall
mine the applicability of regulatory limitations prior to use.
would be a building envelope component, whereas a layer
1.5 This international standard was developed in accor-
thereof would not be.
dance with internationally recognized principles on standard-
3.2.2 thermal time constant—the time necessary for a step
ization established in the Decision on Principles for the
change in temperature on one side of an item (for example, an
Development of International Standards, Guides and Recom-
HFT or building component) to cause the corresponding
mendations issued by the World Trade Organization Technical
change in heat flux on the other side to reach 63.2 % of its new
Barriers to Trade (TBT) Committee.
equilibrium value where one-dimensional heat flow occurs. It
is a function of the thickness, placement, and thermal diffusiv-
ity (see Appendix X1) of each constituent layer of the item.
3.2.2.1 Discussion—
t/τ
t 5 τ when q t 5 q 1 q 2 q l 2 e
~ ! ~ !~ !
1 2 1
This practice is under the jurisdiction of ASTM Committee C16 on Thermal
Insulation and is the direct responsibility of Subcommittee C16.30 on Thermal
Measurement. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Oct. 1, 2021. Published October 2021. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 1985. Last previous edition approved in 2013 as C1046 – 95 (2013). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/C1046-95R21. the ASTM website.
Copyright ©ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA19428-2959. United States
C1046 − 95 (2021)
where: 5.3.2 A single HFT site is not representative of a building
component. The measurement at an HFT site represents the
q = is the previous equilibrium heat flux, and
conditions at the sensing location of the HFT. Use thermogra-
q = is the new heat flux after the step change.
phy appropriately to identify average and extreme conditions
3.3 Symbols Applied to the Terms Used in This Standard:
and large surface areas for integration. Use multiple sensor
sites to assess overall performance of a building component.
E = measured voltage from the HFT, typically in mV,
2 2
q = heat flux, W/m (Btu/h·ft ),
5.3.3 A given HFT calibration is not applicable for all
S = heat-flux transducer conversion factor that relates the
measurements.The HFTdisturbs heat flow at the measurement
output of the HFT, E,to q through the HFT for the
site in a manner unique to the surrounding materials (9, 10);
2 2
conditions of the test, W/m ·V (Btu/h·ft ·mV). This
this affects the conversion constant, S, to be used. The user
may be a function of temperature, heat flux, and other
shall take into account the conditions of measurement as
factors in the environment as discussed in Section 7.
outlined in 7.1.1. In extreme cases, the sensor is the most
This may also be expressed as S(T) to connote a
significant thermal feature at the location where it has been
function of temperature,
placed, for example, on a sheet metal component. In such a
T = temperature, K (°C, °R, or °F),
case, meaningful measurements are difficult to achieve. The
t = time, s (hours, days), and
user shall confirm the conversion factor, S, prior to use of the
τ = thermal time constant, s (hours, days).
HFT to avoid calibration errors. See Section 7.
4. Summary of Practice
5.3.4 The user shall be prepared to accommodate non-
steady-state thermal conditions in employing the measurement
4.1 Heat flux transducers are installed on or within a
technique described in this practice. This requires obtaining
building envelope component in conjunction with temperature
data over long periods, perhaps several days, depending on the
transducers, as required. Heat flux through a surface is influ-
type of building component and on temperature changes.
enced by temperature gradients, thermal conductance, heat
capacity, density and geometry of the test section, and by 5.3.5 Heat flux has a component parallel to the plane of the
convective and radiative coefficients. The resultant heat fluxes HFT. The user shall be able to minimize or accommodate this
aredeterminedbymultiplyingaconversionfactorSoftheHFT factor.
by its electrical output. The S values shall have been obtained
according to Practice C1130.
6. Apparatus
6.1 Essential equipment for measuring heat flux and tem-
5. Significance and Use
perature includes the following:
5.1 Traditionally, HFTs have been incorporated into labora-
6.1.1 Heat Flux Transducer—Arigid or flexible device (see
torytestingdevices,suchastheheatflowmeterapparatus(Test
Appendix X2) in a durable housing, composed of a thermopile
Method C518), that employ controlled temperatures and heat
(or equivalent) for sensing the temperature difference across a
flow paths to effect a thermal measurement. The application of
thin thermal resistive layer, which produces a voltage output
heat flux transducers and temperature transducers to building
that is a function of the corresponding heat flux and the
components in situ can produce quantitative information about
geometry and material properties of the HFT.
building thermal performance that reflects the existing proper-
ties of the building under actual thermal conditions. The
NOTE 1—All calibrations relating output voltage to heat flux shall
literature contains a sample of reports on how these measure-
conform to Practice C1130 and pertain to the measurement at hand.
ments have been used (1-8).
Manufacturers’ calibrations supplied with HFTs often do not conform
with Practice C1130. Obtain the HFT conversion factor as described in
5.2 The major advantage of this practice is the potential
Section 8 of Practice C1130.
simplicity and ease of application of the sensors. To avoid
6.1.2 Temperature Transducer—A thermocouple, resistance
spurious information, users of HFTs shall: (1) employ an
thermal device (RTD), or thermistor for measuring tempera-
appropriate S,(2) mask the sensors properly, (3) accommodate
tures on or within the construction, or for measuring air
the time constants of the sensors and the building components,
temperatures. Some HFTs incorporate thermocouples.
and (4) account for possible distortions of any heat flow paths
attributable to the nature of the building construction or the
6.1.3 Recorder—An instrument that reads sensor output
location, size, and thermal resistance of the transducers.
voltageandrecordseitherthevoltage,heatflux,ortemperature
values calculated from appropriate formulas, with durable
5.3 The user of HFTs and TTs for measurements on build-
output (for example, magnetic tape, magnetic disk, punch tape,
ings shall understand principles of heat flux in building
printer, or plotter).
components and have competence to accommodate the follow-
ing: 6.1.4 Attachment Materials—Pressure-sensitive tape,
adhesive, or other means for holding heat flux and temperature
5.3.1 Choose sensor sites using building plans, specifica-
tions and thermography to determine that the measurement transducers in place on the test surface or within the construc-
tion.
represents the required conditions.
6.1.5 Thermal Contact Materials—Gel toothpaste, heat sink
grease, petroleum jelly, or other means to improve thermal
The boldface numbers in parentheses refer to the list of references at the end of
this practice. contact between an irregular surface and a smooth HFT.
C1046 − 95 (2021)
6.1.6 Absorptance and Emittance Control Supplies— 8.4 Install HFTs either on an indoor surface of the compo-
Coatings or sheet material to match the radiative absorptance nent if the construction is complete or within a building
and emittance of the sensor with that of the surrounding component when the component is being constructed and
surfaces. retrieval is not required. Infrared thermography is required
when the internal configuration of the component is poorly
7. HFT Signal Conversion known. Seek perpendicular flow, and avoid unforeseen thermal
anomalies.
7.1 The conversion factor (S) is a function of the HFT
8.5 Use infrared thermography to determine the character-
design and the thermal environment surrounding the HFT (8,
isticsofcandidatesensorsitesonthebuildingcomponentwhen
9).Adifference between thermal conductivities of the HFTand
the internal configuration of the component is poorly known
its surroundings causes it to act either as a partial blockade or
(see Practices C1060 and C1153).
conduit for heat flux. Radiative heat passes into the HFT at a
different rate than it does into the surrounding surface, depend-
NOTE 3—Close visual inspection of a stud wall can often reveal the
ing on the mismatch between the absorptivities of HFT and
locations of framing members when there are slight imperfections above
surface.The presence of air moving across an HFTcan change nailheads,butthermographycanrevealwhetherornotthereisunexpected
cross blocking, air leakage, or convection owing to missing, incorrectly
the conductance of the air film at the HFT and cause the heat
applied, or shifted insulation.
flux through the HFT to differ from that through the surround-
NOTE4—Thermographicinstrumentsproduceatwo-dimensionalimage
ing surface.
of a surface by measuring thermal radiation emanating from that surface.
7.1.1 Determine S according to the procedure outlined in
Atemperature gradient on the surface is seen as a variation in contrast or
in pseudocolor on a viewer screen. If the radiation gradients are caused by
Practice C1130, as appropriate to the conditions of use, that is,
heat transfer variations in the wall because of thermal anomalies, these
surface-mounted or embedded and surrounded by materials
anomalies and their locations are made visible. Certain thermographic
that will be present.
patterns can be recognized as framing, air leakage, or convection.
7.2 Confirm that the time constant of the HFT is much less
8.6 Determine whether to deploy sensors in a line or in
than the time constant of the building component to be
some other arrangement, based on knowledge of the compo-
measured if the temperatures throughout the HFT and the
nent’s internal configuration. Note that a wall with suspected
construction will not be steady state. If the mass of an HFT of
internal convection requires, at a minimum, sensors at the top,
a certain area is less than one fiftieth of the mass of the same
bottom, and center of the suspected convective area.
area of building component, then its time constant is small
enough. If not, then estimate the thicknesses and thermal
9. Test Procedures
diffusivities of the constituent layers of the HFT and the
9.1 Sensor Site Selection—Select appropriate sensor sites
building component, using Appendix X1 or other recognized
according to Section 8. The HFT shall cover a region of
technique, to determine whether the time constant of the HFT
uniformheatfluxonthechosensite.IftheHFTcoversaregion
is less than one fiftieth of that of the component’s time
with significantly nonuniform heat flux, then demonstrate that
constant.
the HFT correctly averages the input it receives.
8. Selection of Sensor Sites
9.2 Permanent Sensor Installation:
9.2.1 Sensors built into the construction offer more reliable
8.1 The user shall choose a place in the construction for
results than sensors mounted on an exterior surface, because
siting the HFTs where one-dimensional heat flow perpendicu-
they are usually protected from radiant heat sources and
lartotheexteriorsurfacesoccurs,unlesstheuserispreparedto
convection, which may affect the sensor differently than the
deal with multidimensional heat flow in the analysis of the
surrounding building material. The measurement is also likely
data.
to have less variance.
NOTE 2—For example, a sensor site in the center of a fully insulated
9.2.2 Tape or glue the HFTs to a smooth surface within the
stud cavity represents heat flow perpendicular to the wall surface, whereas
construction to ensure good thermal contact.
a location near a stud or blocking does not.Awall incorporating concrete
9.2.3 Position temperature transducers on and within the
masonry units has significant multidimensional heat flow through the
construction, as required, to obtain temperature gradients
concrete webs and possible air convection cells in the block cores.
(Experience indicates, however, that the face of a concrete masonry unit across its thickness. Place sensors at the exterior surfaces and
distributes heat flux sufficiently that HFT placement is insensitive to
at interfaces between materials within the construction. Install
location on the block.) Similarly, an empty stud cavity has convection as
sensors at the exterior surfaces in one of the following two
a potential lateral heat flow mechanism and a masonry or stone wall has
ways:
vertical heat conduction near the ground level. Air leakage can also be a
source of multidimensional heat flow. 9.2.3.1 Surface mount temperature transducers with tape or
adhesive. Cover surface-mounted sensors with an opaque
8.2 Do not place the HFTs where they contribute more than
coating of the same surface absorptance as the surrounding
1 % additional resistance to the construction subject to thermal
material.
measurement, unless the thermal properties of the HFTs are
well known and the analysis technique is appropriate.
NOTE 5—Be aware that some visually opaque materials are transparent
in the infrared spectrum.
8.3 Do not place HFTs on surfaces with high lateral
NOTE 6—Surface mounting results in a slightly lower temperature
conductance, unless the S has been confirmed for the precise
reading in cool ambient conditions and a slightly higher reading in warm
condition. ambient conditions than the surface temperature, since the protruding
C1046 − 95 (2021)
sensor is more affected by air film temperature.
9.3.6 Connect HFT and TTs for each location to the re-
corder.
9.2.3.2 Flush mount temperature transducers by burying
them at the same depth that the sensor is thick. Use the same
9.4 Data Acquisition and Analysis:
paint, or in the case of a natural finish, such as brick or wood,
9.4.1 Establish the frequency of reading heat fluxes and
a powder of that finish material made into a paste and glued
temperatures required (for measuring thermal resistances, see
aroundthesensor.Formostnonmetallicmaterials(seeRef (11)
Practice C1155). Monitor the fluctuations in temperature and
or (12)), the absorptance is in the range of 0.85 to 0.90.
heat flux to confirm that they are consistent with expectations.
9.2.4 Check the uniformity of surface absorptance with an Adjust the frequency of readings, if required.
9.4.2 Establish the frequency for recording heat fluxes and
infrared imager or single-point radiometer. Check the match of
absorptance of the covered HFT with that of the surrounding temperatures with a data acquisition system or an integrating
area by comparing the image or radiometer output of each area voltmeter appropriate for the required calculation or graphic
after a stabilization time of at least 15 min. representation. Average the data obtained between recording
intervalswithanelectronicaveragingfunctionor,inthecaseof
NOTE 7—Infrared imagers and single-point radiometers sense the
discrete readings, using an appropriate, recognized method.
radiation leaving a surface; they provide a direct relationship of visual or
numerical output to surface absorptance for a given temperature.An HFT
9.5 Duration of Measurement:
that changes the thermal resistance of the envelope component or diverts
9.5.1 For determining the thermal resistance of building
heat flux significantly will not be representative of its surroundings. Be
envelope components, follow the guidance given in Practice
aware that infrared devices are spectral in nature, so that the comparison
C1155.
is made for specific wavelength bands in the infrared, not for the total
spectrum. 9.5.2 For other measurements, obtain the required number
of temperature and heat flux readings.
9.3 Temporary Sensor Installation:
9.3.1 Where the interior of the building construction is NOTE 9—The thermal time constant of a component, the presence of
insulation, and the variation and average value of the temperature
inaccessible or the sensor shall be removed nondestructively,
difference (∆T) across a component all influence how long it takes to have
mount the HFT on an accessible indoor surface of the con-
a change in temperature at one location in the section affect heat flow
struction. Place a layer of material over the entire exposed
elsewhere. In most cases ∆T is an important variable. Refer to the
surface of the HFT that matches the HFT surface absorptance
literature (1-8).
to that of the surrounding surface and creates a smooth
10. Calculation
transition for air flow.
10.1 Calculation of Heat Flux—Calculate heat flux, q,
NOTE8—Alayerofmaskingtape,orsomeotherthinmaterial,willboth
according to the following equation, the time average of the
match the HFT absorptance with that of most nonmetallic finishes and
provide a smooth transition for air flow. If the surface is metallic, refer to HFT output:
atableofabsorptivitiesoremissivities (11, 12)forguidanceconcerningan
q 5 S T ·E (1)
~ !
i i
appropriate material, such as aluminum foil (shiny or dull side out).
Furthermore, measurements on metallic surfaces are more sensitive to
where:
whether or not S represents field conditions. To test the match of HFT
E = the averaged voltage reading, of the ith measurement,
surface absorptance to the surrounding surface, confirm that the sensors
i
are invisible to an infrared imager of sufficient spatial resolution to view and
objects one fifth the size of the HFT.
T = the corresponding temperature of the ith measurement.
i
NOTE 10—S can also be a function of other thermal factors. See Section
9.3.2 On smooth, flat surfaces, apply masking tape around
7.
the perimeter of the HFTand press it onto the surface to ensure
10.2 Calculation of Temperature—Calculate temperature
good contact on the entire interface.
for each averaged temperature transducer output according to
9.3.3 On rough surfaces, apply the HFTin the same manner
the calibration values or formulas for the sensor.
as 9.3.2, except also apply a heat conductive material, such as
gel toothpaste or petroleum jelly, between the sensor and the
11. Interpretation of Results
surface in a thin layer. Note that air gaps greater than 0.5 mm
11.1 Corroboration of Results—Assess the efficacy of mea-
(0.02 in.) can cause errors from 2 to 10 % because of
surements with reference to independent forms of information,
convection (13).
such as as-built drawings or thermograms. If the results appear
9.3.4 As an alternative, place the HFT under a rectangular
contrary to expectations, inspect the interior of the component,
cover of gypsum wallboard or plywood with a recessed area in
as required.
the center for the HFT and provision for the wires to exit from
under the cover. Choose this method if rapid fluctuations in
11.2 Generalization of Results—Consider the possible
HFT output a
...