Hukseflux FHF03 User manual

Copyright by Hukseflux | manual www.hukseflux.com | [e-mail]
USER MANUAL FHF03
Economical foil heat flux sensor with thermal spreaders,
flexible, 30 x 15 mm, with temperature sensor
Copyright by Hukseflux Thermal Sensors B.V. | www.huksefluxusa.com | [e-mail]

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Warning statements
Putting more than 12 Volt across the sensor wiring
can lead to permanent damage to the sensor.
Do not use “open circuit detection” when measuring
the sensor output.

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Contents
Warning statements 2
Contents 3
List of symbols 4
Introduction 5
1Ordering and checking at delivery 8
1.1 Ordering FHF03 8
1.2 Included items 8
1.3 Quick instrument check 9
2Instrument principle and theory 10
3Specifications of FHF03 13
3.1 Specifications of FHF03 13
3.2 Dimensions of FHF03 16
4Standards and recommended practices for use 17
4.1 Heat flux measurement in industry 17
5Installation of FHF03 18
5.1 Site selection and installation 18
5.2 Installation on curved surfaces 20
5.3 Electrical connection 22
5.4 Requirements for data acquisition / amplification 25
6Maintenance and trouble shooting 26
6.1 Recommended maintenance and quality assurance 26
6.2 Trouble shooting 27
6.3 Calibration and checks in the field 28
7Appendices 29
7.1 Appendix on cable extension 29
7.2 Appendix on standards for calibration 30
7.3 Appendix on calibration hierarchy 30
7.4 Appendix on correction for temperature dependence 31
7.5 Appendix on measurement range for different temperatures 32
7.6 Appendix on using FHF03 with BLK –GLD sticker series 33
7.7 EU declaration of conformity 34

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List of symbols
Quantities Symbol Unit
Heat flux ΦW/m²
Voltage output U V
Sensitivity S V/(W/m2)
Temperature T °C
Thermal resistance per unit area Rthermal,A K/(W/m²)
subscripts
property of heatsink heatsink
maximum value, specification limit maximum

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Introduction
FHF03 is an economical sensor for general-purpose heat flux measurement. It is small,
thin and versatile. FHF03 has an integrated temperature sensor and thermal spreaders to
reduce thermal conductivity dependence. It is applicable over a temperature range from
–40 to +150 °C. FHF03 is often applied as part of a larger test- or measuring system.
FHF03 measures heat flux through the object in which it is incorporated or on which it is
mounted, in W/m2. The sensor in FHF03 is a thermopile. This thermopile measures the
temperature difference across FHF03’s flexible body. A type T thermocouple is integrated
as well. The thermopile and thermocouple are passive sensors; they do not require
power. A thermal spreader, which is a conductive layer covering the sensor, helps reduce
the thermal conductivity dependence of the measurement. With its incorporated
spreaders, the sensitivity of FHF03 is independent of its environment. Many competing
sensors do not have thermal spreaders. Equipped with well-protected wire connections
and a sturdy, shielded cable, FHF03 is designed for robustness. Qualities like these are
unmatched at this price level.
Figure 0.1 FHF03 foil heat flux sensor with thermal spreaders: small, thin and versatile
The economical FHF03 foil heat flux sensor has unique features and benefits:
•flexible (bending radius ≥ 25 x 10-3 m)
•low thermal resistance
•wide temperature range
•fast response time
•integrated type T thermocouple
•robust: well-protected wire connections and a sturdy, shielded cable
•IP protection class: IP67 (essential for outdoor application)
•thermal spreader included, low thermal conductivity dependence

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Using FHF03 is easy. It can be connected directly to commonly used data logging systems.
The heat flux in W/m2is calculated by dividing the FHF03 output, a small voltage, by the
sensitivity. The sensitivity is provided with FHF03 on its product certificate. For increased
sensitivity, robustness and a larger sensing area, consider using model FHF04 and, in
particular for building physics and soil heat flux, model HFP01, the world’s most popular
heat flux sensor.
FHF03 calibration is traceable to international standards. The factory calibration method
follows the recommended practice of ASTM C1130-17. When used under conditions that
differ from the calibration reference conditions, the FHF03 sensitivity to heat flux may be
different than stated on its certificate. See Chapter 2 in this manual for suggested
solutions.
Figure 0.2 FHF03 foil heat flux sensor being installed to measure heat flux on a computer
processor

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Would you like to study energy transport / heat flux in detail? Hukseflux helps taking this
measurement to the next level: order FHF03 with radiation-absorbing black and radiation-
reflecting gold stickers. You can then measure convective + radiative flux with one, and
convective flux only with the other. Subtract the 2 measurements and you have radiative
flux. They can be applied to the sensor by the user or ordered pre-applied at the factory;
see the BLK –GLD sticker series user manual and installation video for instructions.
Figure 0.3 FHF03: with BLK-3015 and GLD-3015 stickers to measure radiative and
convective heat fluxes seperately
See also:
•model FHF04 for increased sensing area and sensitivity
•model FHF04SC for a self-calibrating version of FHF04
•model HFP01 for increased sensitivity (also consider putting two or more FHF02s in
series)
•BLK - GLD sticker series to separate radiative and convective heat fluxes
•Hukseflux offers a complete range of heat flux sensors with the highest quality for
any budget

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1Ordering and checking at delivery
1.1 Ordering FHF03
The standard configuration of FHF03 is with 2 metres of cable.
Common options are:
•with 5 metres of cable
•with LI19 hand-held read-out unit / datalogger
•BLK-3015 black sticker (to measure radiative as well as conventive heat flux)
•GLD-3015 gold sticker (to measure convective heat flux only)
•BLK - GLD sticker series can also be ordered pre-applied at the factory
1.2 Included items
Arriving at the customer, the delivery should include:
•heat flux sensor FHF03 with cable of the length as ordered
•product certificate matching the instrument serial number
Figure 1.2.1 FHF03’s serial number and sensitivity are visible on the sticker close to
FHF03’s strain relief. The sensor is delivered with bundled cable and wire ends.

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1.3 Quick instrument check
A quick test of the instrument can be done by connecting it to a multimeter.
1. Check the sensor serial number and sensitivity on the sticker against the product
certificate provided with the sensor.
2. Inspect the instrument for any damage.
3. Check the electrical resistance of the sensor between the red [+] and black [-] wires.
Use a multimeter at the 100 Ω range. Measure the sensor resistance first with one
polarity, then reverse the polarity. Take the average value. Measured resistance should
be the nominal sensor resistance of 25 Ω plus 0.2 Ω/m. Infinite resistance indicates a
broken circuit; zero or a lower than 1 Ω resistance indicates a short circuit.
4. Check the electrical resistance of the thermocouple between the brown [+] and white
[-] wires. Use a multimeter at the 100 Ω range. Measure the thermocouple resistance
first with one polarity, then reverse the polarity. Take the average value. The typical
resistance of the copper wiring is 0.1 Ω/m, for the constantan wiring this is 2.5 Ω/m.
Typical resistance should be the nominal thermocouple resistance of 2.5 Ω plus 2.6 Ω for
the total resistance of the two wires of each metre (back and forth). Infinite resistance
indicates a broken circuit; zero or a lower than 1 Ω resistance indicates a short circuit.
5. Check if the sensor reacts to heat: put the multimeter at its most sensitive range of
DC voltage measurement, typically the 100 x 10-3 VDC range or lower. Expose the sensor
to heat. Exposing the back side (the side without sticker) to heat should generate a
positive signal between the red [+] and black [-] wires. Doing the same at the front side
(the side with sticker), reverses the sign of the output.

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2Instrument principle and theory
FHF03’s scientific name is heat flux sensor. A heat flux sensor measures the heat flux
density through the sensor itself. This quantity, expressed in W/m2, is usually called
“heat flux”.
FHF03 users typically assume that the measured heat flux is representative of the
undisturbed heat flux at the location of the sensor. Users may also apply corrections
based on scientific judgement.
The sensor in FHF03 is a thermopile. This thermopile measures the temperature
difference across the polyimide body of FHF03. Working completely passive, the
thermopile generates a small voltage that is a linear function of this temperature
difference. The heat flux is proportional to the same temperature difference divided by
the effective thermal conductivity of the heat flux sensor body.
Using FHF03 is easy. For readout the user only needs an accurate voltmeter that works
in the millivolt range. To convert the measured voltage, U, to a heat flux Φ, the voltage
must be divided by the sensitivity S, a constant that is supplied with each individual
sensor.
Figure 2.1 The general working principle of a heat flux sensor. The sensor inside FHF03
is a thermopile. A thermopile consists of a number of thermocouples, each consisting of
two metal alloys (marked 1 and 2), electrically connected in series. A single
thermocouple generates an output voltage that is proportional to the temperature
difference between its hot- and cold joints. Putting thermocouples in series amplifies the
signal. In a heat flux sensor, the hot- and cold joints are located at the opposite sensor
surfaces (4 and 5). In steady state, the heat flux (6) is a linear function of the
temperature difference across the sensor and the average thermal conductivity of the
sensor body (3). The thermopile generates a voltage output proportional to the heat flux
through the sensor. The exact sensitivity of the sensor is determined at the manufacturer
by calibration, and can be found on the product certificate that is supplied with each
sensor.
5
4
321
6

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Figure 2.2 Heat flux from the back side to the front side (side with sticker, logo
readable) generates a positive voltage output signal
FHF03 is designed such that heat flux from the back side to the front side (side with
sticker, logo readable) generates a positive voltage output signal.
Unique features of the economical FHF03 include flexibility (bending radius ≥ 25 x 10-3 m),
low thermal resistance, a wide temperature range, a fast response time, cable with strain
relief for robustness, IP67 protection class rating (essential for outdoor application), and
the inclusion of thermal spreaders to reduce thermal conductivity dependence.
The FHF03 is calibrated under the following reference conditions:
•conductive heat flux (as opposed to radiative or convective heat flux)
•homogeneous heat flux across the sensor and guard surface

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•room temperature
•heat flux in the order of 600 W/m2
•mounted on aluminium heat sink
FHF03 has been calibrated using a well-conducting metal heat sink, representing a
typical industrial application, at 20 °C and exposing it to a conductive heat flux. When
used under conditions that differ from the calibration reference conditions, for example at
extremely high or low temperatures, or exposed to radiative flux, the FHF03 sensitivity to
heat flux may be different than stated on the certificate. In such cases, the user may choose:
•not to use the sensitivity and only perform relative measurements / monitor changes
•reproduce the calibration conditions by mounting the sensor on or between metal foils
•design a dedicated calibration experiment, for example using a foil heater which
generates a known heat flux
•paint the sensor surface (black) to absorb radiation
The user should analyse his own experiment and make his own uncertainty evaluation.
The FHF03 operating temperature range is -40 to +150 °C. Prolonged exposure to
temperatures near +150 °C can accelerate the aging process.

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3Specifications of FHF03
3.1 Specifications of FHF03
FHF03 measures the heat flux density through the surface of the sensor. This quantity,
expressed in W/m2, is called heat flux. Working completely passive, using a thermopile
sensor, FHF03 generates a small output voltage proportional to this flux. It can only be
used in combination with a suitable measurement system.
Table 3.1 Specifications of FHF03 (continued on next page)
FHF03 SPECIFICATIONS
Sensor type
foil heat flux sensor
Sensor type according to ASTM
heat flow sensor or heat flux transducer
Measurand
heat flux
Measurand in SI units
heat flux density in W/m2
Measurement range
(-10 to +10) x 103W/m2 at heat sink temperature 20 °C
see appendix for detailed calculations
Sensitivity range
(1.5 to 2.5) x 10-6 V/(W/m2)
Sensitivity (nominal)
2 x 10-6 V/(W/m2)
Directional sensitivity
heat flux from the back side to the front side (side
with sticker, logo readable) generates a positive
voltage output signal
Increased sensitivity
multiple sensors may be put electrically in series. The
resulting sensitivity is the sum of the sensitivities of
the individual sensors
Expected voltage output
(-25 to +25) x 10-3 V
turning the sensor over from one side to the other will
lead to a reversal of the sensor voltage output
Measurement function / required
programming
Φ= U/S
Required readout
1 differential voltage channel or 1 single ended
voltage channel, input resistance > 106Ω
Optional readout
1 temperature channel
Rated load on cable
≤10 kg
Rated bending radius
≥25 x 10-3 m
Operating temperature range
-40 to +150 °C
Temperature dependence
< 0.3 %/°C
Non-linearity
< 5 % (0 to 10 x 10³ W/m²)
Solar absorption coefficient
0.75 (indication only)
Thermal conductivity dependence
negligible
Sensor length and width
(31 x 14.5) x 10-3 m
Sensing area
2.5 x 10-4 m2
Sensing area length and width
(25 x 10) x 10-3 m
Passive guard area
2 x 10-4 m2
Guard width to thickness ratio
2.8 m/m
Sensor thickness
0.8 x 10-3 m
Sensor thermal resistance
28 x 10-4 K/(W/m2)
Sensor thermal conductivity
0.29 W/(m·K)
Response time (95 %)
15 s
Sensor resistance range
20 to 30 Ω
Required sensor power
zero (passive sensor)
Temperature sensor
type T thermocouple incorporated
Thermal spreaders
incorporated

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Table 3.1 Specifications of FHF03 (started on previous page)
Standard cable length
2 m
Wiring
3 x copper and 1 x constantan wire, AWG 24, stranded
Cable diameter
3.6 x 10-3 m
Marking
1 x sticker, showing serial number and sensitivity
IP protection class
IP67
Rated operating relative humidity range
0 to 100 %
Use under water
FHF03 is not suitable for continuous use under water
Gross weight including 2 m cable
approx. 0.5 kg
Net weight including 2 m cable
approx. 0.5 kg
INSTALLATION AND USE
Typical conditions of use
in experiments, in measurements in laboratory and
industrial environments. Exposed to heat fluxes for
periods of several minutes to several years.
Connected to user-supplied data acquisition
equipment. Regular inspection of the sensor.
Continuous monitoring of sensor temperature. No
special requirements for immunity, emission, chemical
resistance.
Recommended number of sensors
2 per measurement location
Installation
see recommendations in this user manual
Bending
see chapter on installation on curved surfaces
Wire extension
see chapter on cable extension or order sensors with
longer cable
CALIBRATION
Calibration traceability
to SI units
Product certificate
included
(showing calibration result and traceability)
Calibration method
method HFPC, according to ASTM C1130 - 17
Calibration hierarchy
from SI through international standards and through
an internal mathematical procedure
Calibration uncertainty
< ± 5 % (k = 2)
Recommended recalibration interval
2 years
Calibration reference conditions
20 °C, heat flux of 600 W/m2, mounted on aluminium
heat sink, thermal conductivity of the surrounding
environment 0.0 W/(m·K)
Validity of calibration
based on experience the instrument sensitivity will not
change during storage. During use the instrument
“non-stability” specification is applicable. When used
under conditions that differ from the calibration
reference conditions, the FHF03 sensitivity to heat flux
may be different than stated on its certificate. See the
chapter on instrument principle and theory for
suggested solutions
Field calibration
is possible by comparison to a calibration reference
sensor. Usually mounted side by side, alternative on
top of the field sensor. Preferably reference and field
sensor of the same model and brand. Typical duration
of test > 24 h
MEASUREMENT ACCURACY
Uncertainty of the measurement
statements about the overall measurement
uncertainty can only be made on an individual basis.

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Table 3.1 Specifications of FHF03 (started on previous page)
VERSIONS / OPTIONS
With 5 metres of cable
option code = cable length in metres
With black sticker applied
BLK-3015 applied to the sensor at the factory to
absorb radiation
With gold sticker applied
GLD-3015 applied to the sensor at the factory to
reflect radiation
ACCESSORIES
Hand-held read-out unit
LI19 handheld read-out unit / datalogger
Separate black stickers
BLK-3015 to absorb radiation, to be applied by the user
Separate gold sticker
GLD-3015 to reflect radiation, to be applied by the user

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3.2 Dimensions of FHF03
Figure 3.2.1 FHF03 heat flux sensor; dimensions in x 10-3 m
(1) sensing area with thermal spreader
(2) passive guard
(3) type T thermocouple
(4) sticker showing serial number and sensitivity
(5) strain relief
(6) cable, standard length 2 m

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4Standards and recommended practices
for use
FHF03 should be used in accordance with recommended practices.
4.1 Heat flux measurement in industry
FHF03 sensors are often used to measure on industrial walls and metal surfaces,
estimating the installation’s energy balance and the thermal transmission of walls.
Typically the total measuring system consists of multiple heat flux- and temperature
sensors. In many cases heat flux sensors are used for trend-monitoring. In such cases
reproducibility is more important than absolute measurement accuracy.
Figure 4.1.1 Example of an FHF03 foil heat flux sensor being installed for measurement on
a computer processor. The sensor is mounted on a well-prepared flat surface.

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5Installation of FHF03
5.1 Site selection and installation
Table 5.1.1 Recommendations for installation of FHF03 heat flux sensors
Location
choose a location that is representative of the process that is analysed
if possible, avoid exposure to sun, rain, etc.
do not expose to drafts and lateral heat fluxes
do not mount in the vicinity of thermal bridges, cracks, heating or cooling
devices and fans
Performing a
representative
measurement /
recommended
number of sensors
we recommend using > 2 sensors per measurement location. This
redundancy also improves the assessment of the measurement accuracy
Mounting
when mounting an FHF03, keep the directional sensitivity in mind
heat flux from the back side to the front side (side with sticker, logo
readable) generates a positive voltage output signal
Surface cleaning and
levelling
create a clean and smooth surface of (31 x 14.5) x 10-3 m
Mechanical mounting:
avoiding strain on the
sensor to cable
transition
the sensor-to-cable transition is vulnerable
during installation as well as operation, the user should provide proper
strain relief of the cable so that transition is not exposed to significant
force
first install the cable including strain relief and after that install the sensor
Short term
installation
avoid any air gaps between sensor and surface. Air thermal conductivity
is in the 0.02 W/(m·K) range, while a common glue has a thermal
conductivity around 0.2 W/(m·K). A 0.1 x 10-3 m air gap increases the
effective thermal resistance of the sensor by 200 %
to avoid air gaps, we recommend thermal paste or glycerol for short term
installation
use tape to fixate the sensor on the surface. If possible, tape only over
the passive guard area (surrounding the sensing area). See Figure 3.2.1
use tape to fixate the strain relief of the sensor
usually the cables are provided with an additional strain relief, for
example using a cable tie mount as in Figure 5.1.1
Permanent
installation
for long-term installation fill up the space between sensor and object with
silicone construction sealant, silicone glue or silicone adhesive, that can
be bought in construction depots.
we discourage the use of thermal paste for permanent installation
because it tends to dry out. silicone glue is more stable and reliable
Signal amplification
see the paragraph on electrical connection

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Figure 5.1.1 Installation of FHF03 using tape to fixate the sensor and the strain relief.
Extra strain relief of the cable is provided using cable tie mounts equipped with double
sided tape as adhesive. As indicated in Table 5.1.1, tapes fixating the sensor are
preferably taped over the passive guard area and not on the sensing area (the latter
indicated by grey shading in Figure 5.1.1). Please note the Hukseflux logo is readable in
this image; this indicates that we are viewing the front side and that the other side, the
back side, is attached on the object on which the sensor is mounted, as explained in
Chapter 2.

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5.2 Installation on curved surfaces
The flexibility of the FHF03 makes it perfectly suitable to be installed on singly curved
surfaces. Bend the sensor in the direction indicated in Figure 5.2.1.
Figure 5.2.1 Bending of an FHF03 foil heat flux sensor, in this image on a pipe.
FHF03 is not suited for dynamic bending.
The recommendations of the previous chapter apply. For installation on curved surfaces,
it is usually not achievable to tape only over the passive guard area. Use sufficient tape
to make sure the sensor remains fixed and in good thermal contact with curved surface.
Avoid air gaps. Tape can be used over the sensing area when necessary.