Hukseflux FHF04 User manual

Copyright by Hukseflux | manual www.hukseflux.com | [e-mail]
USER MANUAL FHF04
Foil heat flux sensor with thermal spreaders, flexible,
50 x 50 mm, with temperature sensor
Hukseflux
Thermal Sensors
Copyright by Hukseflux | manual v2101 | www.hukseflux.com | [e-mail]

FHF04 manual v2101 2/39
Cautionary statements
Cautionary statements are subdivided into four categories: danger, warning, caution and
notice according to the severity of the risk.
DANGER
Failure to comply with a danger statement will lead to death or serious
physical injuries.
WARNING
Failure to comply with a warning statement may lead to risk of death or
serious physical injuries.
CAUTION
Failure to comply with a caution statement may lead to risk of minor or
moderate physical injuries.
NOTICE
Failure to comply with a notice may lead to damage to equipment or may
compromise reliable operation of the instrument.

FHF04 manual v2101 3/39
Contents
Cautionary statements 2
Contents 3
List of symbols 4
Introduction 5
1Ordering and checking at delivery 8
1.1 Ordering FHF04 8
1.2 Included items 8
1.3 Quick instrument check 9
2Instrument principle and theory 10
3Specifications of FHF04 13
3.1 Specifications of FHF04 13
3.2 Dimensions of FHF04 16
4Standards and recommended practices for use 17
4.1 Heat flux measurement in industry 17
5Installation of FHF04 18
5.1 Site selection and installation 18
5.2 Installation on curved surfaces 20
5.3 Electrical connection 21
5.4 Requirements for data acquisition / amplification 24
6Maintenance and trouble shooting 25
6.1 Recommended maintenance and quality assurance 25
6.2 Trouble shooting 26
6.3 Calibration and checks in the field 27
7Appendices 29
7.1 Appendix on wire extension 29
7.2 Appendix on installation of FHF04 sensor foil 30
7.3 Appendix on using FHF04 with BLK – GLD sticker series 33
7.4 Appendix on standards for calibration 34
7.5 Appendix on calibration hierarchy 34
7.6 Appendix on correction for temperature dependence 35
7.7 Appendix on measurement range for different temperatures 36
7.8 Appendix on temperature measurement accuracy 37
7.9 EU declaration of conformity 38

FHF04 manual v2101 4/39
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

FHF04 manual v2101 5/39
Introduction
FHF04 is the latest standard model for general-purpose heat flux measurement.
Significantly thinner and more flexible, FHF04 replaces earlier models FHF01 and FHF02.
FHF04 is very versatile: it has an integrated temperature sensor and thermal spreaders
to reduce thermal conductivity dependence. It is applicable over a temperature range
from –70 to +120 °C. FHF04 measures heat flux from conduction, radiation and
convection. Optionally, black BLK and gold GLD stickers are available to separately
determine heat transport by radiation and convection.
FHF04 measures heat flux through the object in which it is incorporated or on which it is
mounted, in W/m2. The sensor in FHF04 is a thermopile. This thermopile measures the
temperature difference across FHF04’s flexible body. A type T thermocouple is integrated
as well. The thermopile and thermocouple are passive sensors; they do not require
power.
Multiple small thermal spreaders, which form a conductive layer covering the sensor,
help reduce the thermal conductivity dependence of the measurement. With its
incorporated spreaders, the sensitivity of FHF04 is independent of its environment. Many
competing sensors do not have thermal spreaders. The passive guard area around the
sensor reduces measurement errors due to edge effects and is also used for mounting.
Figure 0.1 FHF04 foil heat flux sensor with thermal spreaders: thin, flexible and versatile
Using FHF04 is easy. It can be connected directly to commonly used data logging systems.
The heat flux in W/m2is calculated by dividing the FHF04 output, a small voltage, by the
sensitivity. The sensitivity is provided with FHF04 on its product certificate.

FHF04 manual v2101 6/39
FHF04 foil heat flux sensor has unique features and benefits:
•flexible (bending radius ≥ 7.5 x 10-3 m)
•low thermal resistance
•wide temperature range
•fast response time
•large guard area
•integrated type T thermocouple
•robustness, including wiring and metal connection block, may be used as strain
relief between sensor and wires
•IP protection class: IP67 (essential for outdoor application)
•integrated thermal spreaders for low thermal conductivity dependence
Equipped with a metal connection block, which may serve as strain relief, and with
potted protective covers on both sides so that moisture does not penetrate, FHF04 has
proven to be very robust and stable.
Figure 0.2 FHF04 foil heat flux sensor being installed to measure heat flux on a pipe
FHF04 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 FHF04 sensitivity to heat flux may be
different than stated on its certificate. See Chapter 2 in this manual for suggested
solutions.
Would you like to study energy transport / heat flux in detail? Hukseflux helps taking this
measurement to the next level: order FHF04 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

FHF04 manual v2101 7/39
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 FHF04 heat flux sensor: with BLK-5050 and GLD-5050 stickers
See also:
•model FHF04SC for a self-calibrating version of FHF04
•model FHF03, our most economical foil heat flux sensor
•model HFP01 for increased sensitivity (also consider putting two or more FHF04s 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
absorbs radiation
reflects radiation

FHF04 manual v2101 8/39
1Ordering and checking at delivery
1.1 Ordering FHF04
The standard configuration of FHF04 is with 2 metres of wire.
Common options are:
•with 5 metres wire length
•without wiring, without metal connection block
•with LI19 hand-held read-out unit / datalogger; NOTE: LI19 measures heat flux only,
not temperature
•BLK-5050 black sticker (to measure radiative as well as convective heat flux)
•GLD-5050 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 FHF04 with wires of the length as ordered
•product certificate matching the instrument serial number
Figure 1.2.1 FHF04’s serial number and sensitivity are visible on the metal connection
block. FHF04 is delivered with bundled wiring.

FHF04 manual v2101 9/39
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 on the metal connection
block 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 1k Ω range. Measure the sensor resistance first with one
polarity, then reverse the polarity. Take the average value. The typical resistance of the
wiring is 0.1 Ω/m. Typical resistance should be the nominal sensor resistance of 200 Ω
plus 0.2 Ω for the total resistance of two wires for each metre (back and forth). 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 Ωfor
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 the dot) to heat should generate a
positive signal between the red [+] and black [-] wires. Doing the same at the front side
(the side with the dot), reverses the sign of the output.

FHF04 manual v2101 10/39
2Instrument principle and theory
FHF04’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”.
FHF04 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 FHF04 is a thermopile. This thermopile measures the temperature
difference across the polyimide body of FHF04. 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 FHF04 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.
Φ= U/S (Formula 2.1.1)
Figure 2.1 The general working principle of a heat flux sensor. The sensor inside FHF04
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

FHF04 manual v2101 11/39
Figure 2.2 Heat flux from the back side to the front side generates a positive voltage
output signal. The dot on the foil indicates the front side.
FHF04 is designed such that heat flux from the back side to the front side generates a
positive voltage output signal. The dot on the foil indicates the front side.
Unique features of the FHF04 include flexibility (bending radius ≥ 7.5 x 10-3 m), low
thermal resistance, a wide temperature range, a fast response time, IP67 protection class
rating (essential for outdoor application), and the inclusion of thermal spreaders to reduce
thermal conductivity dependence.

FHF04 manual v2101 12/39
FHF04 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
•room temperature
•heat flux in the order of 600 W/m2
•mounted on aluminium heat sink
FHF04 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 FHF04 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
•apply our BLK-5050 sticker to the sensor surface to absorb radiation
•apply our GLD-5050 sticker to the sensor surface to reflect radiation
The user should analyse his own experiment and make his own uncertainty evaluation.
The FHF04 rated temperature range for continuous use is -70 to +120 °C, for short
intervals, a peak temperature of +150 °C is allowed. Prolonged exposure to
temperatures near +150 °C can accelerate the aging process.

FHF04 manual v2101 13/39
3Specifications of FHF04
3.1 Specifications of FHF04
FHF04 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, FHF04 generates a small output voltage proportional to this flux. It can only be
used in combination with a suitable measurement system.
Table 3.1.1 Specifications of FHF04 (continued on next pages)
FHF04 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
9 - 13 x 10-6 V/(W/m2)
Sensitivity (nominal)
11 x 10-6 V/(W/m2)
Directional sensitivity
heat flux from the back side to the front side (side with
the dot) 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
(-100 to +100) 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 > 10
6
Ω
Optional readout
1 temperature channel
Rated load on wires
≤1.6 kg
Rated bending radius
≥7.5 x 10-3 m
Rated temperature range,
continuous use
-70 to +120 °C
Rated temperature range,
short intervals
-160 to +150 °C
(contact Hukseflux when measuring at -160 °C)
Temperature dependence
< 0.2 %/°C
Non-linearity
< 5 % (0 to 10 x 10³ W/m²)
Solar absorption coefficient
0.75 (indication only)
Thermal conductivity dependence
negligible, < 3 %/(W/m·k) from 270 to 0.3 W/m·K
Sensor length and width
(50 x 50) x 10-3 m
Sensing area
9 x 10-4 m2
Sensing area length and width
(30 x 30) x 10-3 m
Passive guard area
16 x 10-4 m2
Guard width to thickness ratio
40
Sensor thickness
0.4 x 10-3 m
Sensor thermal resistance
11 x 10-4 K/(W/m2)
Sensor thermal conductivity
0.36 W/(m·K)
Response time (95 %)
3 s
Sensor resistance range
160 to 240 Ω
Required sensor power
zero (passive sensor)

FHF04 manual v2101 14/39
Table 3.1.1Specifications of FHF04 (started on previous page)
Temperature sensor
type T thermocouple
Temperature sensor accuracy
± 2 % (of temperature in ˚C), see appendix
Standard wire length
2 m
Wiring
3 x copper and 1 x constantan wire, AWG 24, stranded
Wire diameter
1 x 10-3 m
Marking
dot on foil indicating front side of the heat flux sensor;
1 x label on metal connection block, showing serial
number and sensitivity
IP protection class
IP67
Rated operating relative humidity range
0 to 100 %
Use under water
FHF04 is not suitable for continuous use under water
Gross weight including 2 m wires
approx. 0.5 kg
Net weight including 2 m wires
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 Chapter 5 on installation for recommendations
Bending
see Section 5.2 on installation on curved surfaces
Wire extension
see appendix on wire extension, or order sensors with
longer wire
Sensor foil installation
see appendix on installation of FHF04 without wiring,
without metal connection block
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 FHF04 sensitivity to heat flux
may be different than stated on its certificate. See the
chapter on instrument principle 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.

FHF04 manual v2101 15/39
Table 3.1.1 Specifications of FHF04 (started on previous pages)
VERSIONS / OPTIONS
With longer wire length
option code = wire length in metres
Without wires, without metal connection
block
calibrated FHF04 sensor foil
to be soldered / connected by the user
see appendix for more information
With black sticker applied
BLK-5050 applied to the sensor at the factory to
absorb radiation
With gold sticker applied
GLD-5050 applied to the sensor at the factory to
reflect radiation
ACCESSORIES
Hand-held read-out unit
LI19 handheld read-out unit / datalogger
NOTE: LI19 does not measure temperature, only heat flux
Separate black stickers
BLK-5050 to absorb radiation, to be applied by the user
Separate gold sticker
GLD-5050 to reflect radiation, to be applied by the user

FHF04 manual v2101 16/39
3.2 Dimensions of FHF04
Figure 3.2.1 FHF04 heat flux sensor;dimensions in x 10-3 m
(1) sensing area with thermal spreaders
(2) passive guard
(3) type T thermocouple
(4) dot indicating front side
(5) metal connection block, showing serial number and sensitivity
(6) wires, standard length 2 m
50
1
30 0.3
4.7
30 50
67
97
2
3
4
5
6

FHF04 manual v2101 17/39
4Standards and recommended practices
for use
FHF04 should be used in accordance with recommended practices.
4.1 Heat flux measurement in industry
FHF04 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 FHF04 foil heat flux sensor being installed for measurement on
an industrial pipe. The sensor is mounted on a well-prepared curved surface.

FHF04 manual v2101 18/39
5Installation of FHF04
5.1 Site selection and installation
Table 5.1.1 Recommendations for installation of FHF04 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 FHF04, keep the directional sensitivity in mind
heat flux from the back side to the front side (side with dot) generates a
positive voltage output signal
to achieve the highest accuracy temperature measurement, fix the metal
connection block to the object of interest, so that the temperature of the
connection block remains as close as possible to that of the heat flux
sensor (see appendix)
Surface cleaning and
levelling
create a clean and smooth surface of at least (50 x 50) x 10-3 m
Mechanical mounting:
avoiding strain on the
sensor to wire
transition
during installation as well as operation, the user should provide proper
strain relief on the wires so that the metal connection block is not
exposed to significant force
first install the sensor by providing strain relief on the metal connection
block and after that install the wires including additional strain relief
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 metal connection block of the sensor
usually the wires are fixated 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 at 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

FHF04 manual v2101 19/39
Figure 5.1.1 Installation of FHF04 using tape to fixate the sensor and the metal
connection block. Extra strain relief on the wires 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 dot is visible 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.
See also our application note on how to install a heat flux sensor.

FHF04 manual v2101 20/39
5.2 Installation on curved surfaces
The flexibility of the FHF04 makes it perfectly suitable to be installed on singly curved
surfaces. The sensor can be bent around any axis.
Figure 5.2.1 Bending of an FHF04 foil heat flux sensor, in this image on a pipe.
When measuring on curved surfaces, the same recommendations of the previous chapter
apply, except that the use of thermal paste is recommended over glycerol. 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.
Table 5.2.1 Extra recommendations for installation of FHF04 foil heat flux sensors on
curved surfaces
Bending Sensor can be bent in both directions
Rated bending radius ≥7.5 x 10-3 m
Effect on sensitivity No significant influence on sensitivity