Hukseflux

Thermal Sensors

USER MANUAL
FHF05 series
Foil heat flux sensors with thermal spreaders, flexible,
5 different dimensions and sensitivities, with temperature
sensorCopyright by Hukseflux | FHF05 series manual v2205 | www.hukseflux.com | info@hukseflux.com

Cautionary statements
Cautionary statements are subdivided into four categories: danger, warning, caution andnotice 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 orserious physical injuries.

CAUTION
Failure to comply with a caution statement may lead to risk of minor ormoderate physical injuries.

NOTICE
Failure to comply with a notice may lead to damage to equipment or may
compromise reliable operation of the instrument.

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Contents
Cautionary statements
Contents
List of symbols
Introduction
1
Ordering and checking at delivery
1.1
Ordering FHF05 series
1.2
Included items
1.3
Quick instrument check2
Instrument principle and theory
3
Specifications of FHF05 series
3.1
Specifications of FHF05 series
3.2
Dimensions of FHF05 series
4Standards and recommended practices for use
4.1
Heat flux measurement in industry
5
Installation of FHF05 series
5.1
Site selection and installation
5.2
Installation on curved surfaces
5.3
Electrical connection
5.4
Requirements for data acquisition / amplification
6
Maintenance and trouble shooting
6.1
Recommended maintenance and quality assurance
6.2
Trouble shooting
6.3
Calibration and checks in the field
7
Appendices
7.1
Appendix on cable and wire extension
7.2
Appendix on installation of FHF05 series sensor foil
7.3
Appendix on using FHF05 series with BLK – GLD sticker series
7.4
Appendix on standards for calibration
7.5
Appendix on calibration hierarchy
7.6
Appendix on correction for temperature dependence
7.7
Appendix on measurement range for different temperatures
7.8
Appendix on temperature measurement accuracy
7.9
EU declaration of conformity

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List of symbols
Quantities

SymbolUnit

Heat flux
Voltage output
Sensitivity
Temperature
Thermal resistance per unit areaΦU
S
T
Rthermal,A

W/m²
V
V/(W/m2)°C
K/(W/m²)

subscripts
property of heatsink
maximum value, specification limit

FHF 05 s e r i es m an u a l v 220 5

heatsink
maximum

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Introduction
FHF05 series are the latest standard model for general-purpose heat flux measurement.
Models are available in five dimensions and sensitivities. Significantly thinner and more
flexible, FHF05 series replaces earlier models FHF01 to FHF04. All FHF05 models are very
versatile: they have an integrated temperature sensor and thermal spreaders to reduce
thermal conductivity dependence. It is applicable over a temperature range from –70 to+120 °C. FHF05 series measures heat flux from conduction, radiation and convection.
Optionally, black BLK and gold GLD stickers are available for every sensor model toseparately determine heat transport by radiation and convection.
All FHF05’s measure heat flux through the object in which it is incorporated or on which it
is mounted, in W/m2. The sensor within is a thermopile. This thermopile measures thetemperature difference across FHF05’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 FHF05 series is independent of its environment.
Many competing sensors do not have thermal spreaders. The passive guard area aroundthe sensor reduces measurement errors due to edge effects and is also used for
mounting. Looking for heat flux and temperature measurement with a heater? See our
FHF05SC series heat flux sensors.

Figure 0.1 Model FHF05-50X50 foil heat flux sensor with thermal spreaders: thin, flexible
and versatile.
Using FHF05 series is easy. It can be connected directly to commonly used data logging
systems. The heat flux in W/m2 is calculated by dividing the sensor output, a small voltage,by the sensitivity. The sensitivity is provided with FHF05 series on its product certificate.

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All FHF05’s have unique features and benefits:
•
•
•
•
•
•
•
•
•

flexible (bending radius ≥ 7.5 x 10-3 m)
low thermal resistancewide temperature range
fast response timelarge guard area
integrated thermal spreaders for low thermal conductivity dependence
integrated type T thermocouple
robustness, including cable and connection block which may be used as strain relief
between sensor and cable
IP protection class: IP67 (essential for outdoor application)

Equipped with a protective potted connection block, which may serve as strain relief sothat moisture does not penetrate, FHF05 series has proven to be very robust and stable.

Figure 0.2 Model FHF05-15X85 foil heat flux sensor being installed to measure heat flux
on a pipe.
FHF05 series has calibration that is traceable to international standards. The factory
calibration method follows the recommended practice of ASTM C1130 - 21. When used
under conditions that differ from the calibration reference conditions, the sensitivity of
the FHF05 series to heat flux may be different than stated on its certificate. See Chapter
2 in this manual for best practices.
Would you like to study energy transport / heat flux in detail? Hukseflux helps taking thismeasurement to the next level: order FHF05 series with radiation-absorbing black BLK andradiation-reflecting gold GLD stickers. You can then measure convective + radiative flux
with one, and convective flux only with the other. Subtract the two measurements and you
have radiative flux. They can be applied to the sensor by the user or ordered pre-appliedat the factory; see the BLK – GLD sticker series user manual and installation video for
instructions. Stickers are available for every sensor model.

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Figure 0.3 Model FHF05-50X50 heat flux sensor: with BLK-50X50 and GLD-50X50
stickers.
See also:
•
•
•
•
•

FHF05SC series for a self-calibrating version of FHF05
HTR02 series heater, for calibration and verification of performance of FHF-type
sensors
model HFP01 for increased sensitivity (also consider putting two or more FHF05’s inseries)
BLK - GLD sticker series for every sensor dimension to separate radiative andconvective heat fluxes
Hukseflux offers a complete range of heat flux sensors with the highest quality forany budget

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1

Ordering and checking at delivery

1.1

Ordering FHF05 series

The standard configuration of FHF05 series is FHF05-50X05-02, model 50X50 with 2
metres of cable. Common options are:• model FHF05-10X10• model FHF05-15X30• model FHF05-15X85• model FHF05-85X85• change -02 to -05 or -10 metres for the respective cable length• without wiring, without connection block• with a separate cable in 2, 5, or 10 metres cable length• with LI19 hand-held read-out unit / datalogger; NOTE: LI19 measures heat flux only,
not temperature• with HTR02 series, a foil heater of calibration and verification of performance• BLK black sticker (to measure radiative as well as convective heat flux)• GLD gold sticker (to measure convective heat flux only)•
BLK - GLD sticker series can also be ordered pre-applied at the factory for every
sensor dimension

1.2

Included items

Arriving at the customer, the delivery should include:••

heat flux sensor FHF05 with wires of the length as ordered
product certificate matching the instrument serial number

Figure 1.2.1 Model FHF05-50X50 with serial number and sensitivity shown at the end of
the cable.

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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 label at the end of FHF05’s cableagainst 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 thewiring is 0.3 Ω/m. Typical resistance should be the nominal sensor resistance mentioned
in table 3.1.1 plus 0.6 Ω for the total resistance of two wires for each metre (back andforth). Infinite resistance indicates a broken circuit; zero or a lower than 1 Ω resistanceindicates 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 resistancefirst with one polarity, then reverse the polarity. Take the average value. The typicalresistance of the copper wiring is 0.3 Ω/m, for the constantan wiring this is 6.5 Ω/m.
Typical resistance should be the nominal thermocouple resistance of 2.5 Ω plus 6.8 Ω forthe total resistance of the two wires of each metre (back and forth). Infinite resistanceindicates 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 apositive 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.

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2

Instrument principle and theory

FHF05 series’ 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”.
FHF05 series users typically assume that the measured heat flux is representative of theundisturbed heat flux at the location of the sensor. Users may also apply corrections
based on scientific judgement.
The sensor in FHF05 series is a thermopile. This thermopile measures the temperature
difference across the polyimide body of the sensor. Working completely passive, thethermopile 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 FHF05 series is easy. For readout the user only needs an accurate voltmeter thatworks in the millivolt range. To convert the measured voltage, U, to a heat flux Φ, thevoltage must be divided by the sensitivity S, a constant that is supplied with eachindividual sensor.
Φ = U/S(Formula 2.1.1)

1

2

3

4

5
6

Figure 2.1 The general working principle of a heat flux sensor. The sensor inside FHF05
series is a thermopile. A thermopile consists of a number of thermocouples, eachconsisting 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 thesignal. 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 thetemperature difference across the sensor and the average thermal conductivity of thesensor 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 eachsensor.

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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.
All FHF05’s are 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 FHF05 series include flexibility (bending radius ≥ 7.5 x 10-3 m), lowthermal 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.

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All FHF05’s are 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 300 or 600 W/m2 generated with our HTR02 series
mounted on aluminium heat sink

FHF05 series has been calibrated using a well-conducting metal heat sink, representing atypical industrial application, at 20 °C and exposing it to a conductive heat flux. Whenused under conditions that differ from the calibration reference conditions, for example atextremely high or low temperatures, or exposed to radiative flux, the FHF05 series
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
• correct the sensitivity for the temperature dependence. See appendix 7.6 for more
information• apply our BLK black sticker to the sensor surface to absorb radiation• apply our GLD gold sticker to the sensor surface to reflect radiation
The user should analyse his own experiment and make his own uncertainty evaluation.
The FHF05 series rated temperature range for continuous use is -70 to +120 °C, forshort intervals, peak temperatures -160 to +150 °C are allowed. Please contact
Hukseflux when measuring at -160 °C. Prolonged exposure to temperatures near +150
°C will accelerate the ageing process.

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3

Specifications of FHF05 series

3.1

Specifications of FHF05 series

FHF05 series measures the heat flux density through the surface of the sensor. Thisquantity, expressed in W/m2, is called heat flux. Working completely passive, using athermopile sensor, FHF05 series generates a small output voltage proportional to thisflux. It can only be used in combination with a suitable measurement system.

Table 3.1.1 Specifications of FHF05 series (continued next pages).
FHF05 SERIES SPECIFICATIONS
Sensor type
Sensor type according to ASTM
Measurand
Measurand in SI units
Measurement range
Sensitivity per dimension (nominal)
FHF05-10X10
FHF05-15X30
FHF05-50X50
FHF05-15X85
FHF05-85X85
Directional sensitivity
Asymmetry
Increased sensitivity
Expected voltage output
Measurement function / required
programming
Required readout
Optional readout
Rated load on wires
Rated bending radius
Rated temperature range,
continuous use
Rated temperature range,
short intervals
Temperature dependence
Non-linearity
Solar absorption coefficient
Thermal conductivity dependence
Sensor length and width
FHF05-10X10
FHF05-15X30
FHF05-50X50
FHF05-15X85
FHF05-85X85

FHF 05 s e r i es m an u a l v 220 5

foil heat flux sensor
heat flow sensor or heat flux transducer
heat flux
heat flux density in W/m2(-10 to +10) x 103 W/m2 at heat sink temperature 20 °C
see appendix for detailed calculations
1 x 10-6 V/(W/m2)
3 x 10-6 V/(W/m2)
13 x 10-6 V/(W/m2)
7 x 10-6 V/(W/m2)
50 x 10-6 V/(W/m2)
heat flux from the back side to the front side (side withthe dot) generates a positive voltage output signal<2%
multiple sensors may be put electrically in series. The
resulting sensitivity is the sum of the sensitivities of
the individual sensors(-100 to +100) x 10-3 V
turning the sensor over from one side to the other willlead to a reversal of the sensor voltage output
Φ = U/S
1 differential voltage channel or 1 single ended
voltage channel, input resistance > 106 Ω
1 temperature channel≤ 1.6 kg
≥ 7.5 x 10-3 m-70 to +120 °C-160 to +150 °C(contact Hukseflux when measuring at -160 °C)< 0.2 %/°C
< 5 % (0 to 10 x 10³ W/m²)
0.75 (indication only)
negligible, < 3 %/(W/m·k) from 270 to 0.3 W/m·K(10 x 10) x 10-3 m(15 x 30) x 10-3 m(50 x 50) x 10-3 m(15 x 85) x 10-3 m(85 x 85) x 10-3 m

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Table 3.1.1 Specifications of FHF05 series (started on previous pages).
Sensing area

FHF05-10X10
FHF05-15X30
FHF05-50X50
FHF05-15X85
FHF05-85X85
Sensing area length and width
FHF05-10X10
FHF05-15X30
FHF05-50X50
FHF05-15X85
FHF05-85X85
Passive guard area
FHF05-10X10
FHF05-15X30
FHF05-50X50
FHF05-15X85
FHF05-85X85
Guard width to thickness ratio
FHF05-10X10
FHF05-15X30
FHF05-50X50
FHF05-15X85
FHF05-85X85
Sensor thickness
Sensor thermal resistanceSensor thermal conductivity
Response time (95 %)Sensor resistance range per dimension
FHF05-10X10
FHF05-15X30
FHF05-50X50
FHF05-15X85
FHF05-85X85
Required sensor power
Temperature sensor
Temperature sensor accuracyStandard wire length
Optional wire length
WiringCable diameter
Marking
IP protection class
Rated operating relative humidity range
Use under water
Gross weight including 2 m wires
Net weight including 2 m wires

FHF 05 s e r i es m an u a l v 220 50.64 x 10-4 m2
2.70 x 10-4 m2
12.96 x 10-4 m2
7.10 x 10-4 m2
47.70 x 10-4 m2(8 x 8) x 10-3 m(10 x 27) x 10-3 m(36 x 36) x 10-3 m(10 x 71) x 10-3 m(70 x 71) x 10-3 m
0.36 x 10-4 m2
2.25 x 10-4 m2
12.04 x 10-4 m2
5.65 x 10-4 m2
22.55 x 10-4 m2
2.5
6.25
17.5
6.25
18.75
0.4 x 10-3 m
11 x 10-4 K/(W/m2)
0.36 W/(m·K)
3s
5 – 30 Ω
50 – 90 Ω
200 – 300 Ω
100 – 180 Ω
800 – 1300 Ωzero (passive sensor)
type T thermocouple± 5 % (of temperature in ˚C), see appendix 7.8 fordirections how to reduce the uncertainty to ± 2 %which is the normal specification for Class 2
thermocouples
2m
0, 5 or 10 m
3 x copper and 1 x constantan wire, AWG 28, solidcore, bundled with a MFA sheath2 x 10-3 m
dot on foil indicating front side of the heat flux sensor;
1 x label at end of FHF05’s cable, showing serial
number and sensitivity
IP67
0 to 100 %FHF05 is not suitable for continuous use under water
approx. 0.5 kg
approx. 0.5 kg

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Table 3.1.1 Specifications of FHF05 series (started on previous pages).
INSTALLATION AND USE
Typical conditions of use

Recommended number of sensors
Installation
Bending
Cable extension
Sensor foil installation
CALIBRATION
Calibration traceability
Product certificate
Calibration method
Calibration hierarchy
Calibration uncertainty
Recommended recalibration interval
Calibration reference conditions

Validity of calibration

Field calibration

MEASUREMENT ACCURACY
Uncertainty of the measurement

in experiments, in measurements in laboratory andindustrial environments. Exposed to heat fluxes forperiods of several minutes to several years. Connected
to user-supplied data acquisition equipment. Regularinspection of the sensor. Continuous monitoring of
sensor temperature. No special requirements for
immunity, emission, chemical resistance.
2 per measurement location
see Chapter 5 on installation for recommendations
see Section 5.2 on installation on curved surfaces
see Appendix 7.1 on cable extension, or order sensors
with longer cable lengthsee Appendix 7.2 on installation of FHF05 without
wiring, without connection block
to SI units
included (showing calibration result and traceability)
method HFPC, according to ASTM C1130 - 21from SI through international standards and throughan internal mathematical procedure< ± 5 % (k = 2)
2 years
20 °C, heat flux of 300 (models -15X85 and -85X85)
or 600 (models -10X10, -15X30 and 50X50) W/m2,
mounted on aluminium heat sink, thermal conductivityof the surrounding environment 0.0 W/(m·K)
based on experience the instrument sensitivity will notchange during storage. During use the instrument“non-stability” specification is applicable. When used
under conditions that differ from the calibrationreference conditions, the FHF05 sensitivity to heat flux
may be different than stated on its certificate. See thechapter on instrument principle for suggested solutions
is possible by comparison to a calibration reference
sensor. Usually mounted side by side, alternative on
top of the field sensor. Preferably reference and fieldsensor of the same model and brand. Typical durationof test > 24 h
statements about the overall measurement
uncertainty can only be made on an individual basis.

VERSIONS / OPTIONSWith longer cable length
Without cable, without connection block
With black sticker appliedWith gold sticker applied
ACCESSORIES
Hand-held read-out unit

FHF 05 s e r i es m an u a l v 220 5

option code = cable length in metrescalibrated FHF05 sensor foil
to be soldered / connected by the user
see appendix for more information
BLK sticker applied to the sensor at the factory toabsorb radiation
GLD sticker applied to the sensor at the factory toreflect radiation

LI19 handheld read-out unit / datalogger
NOTE: LI19 does not measure temperature, only heat flux

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Separate foil heater
Separate cable
Separate black stickers
Separate gold sticker

FHF 05 s e r i es m an u a l v 220 5

HTR02 general-purpose heater, that can be used for test
and calibration purposes
cable with 3 x copper and 1 x constantan wire, AWG
28, solid core, bundled with an MFA sheathavailable in 2, 5 or 10 m length
BLK sticker to absorb radiation, to be applied by the user
GLD sticker to reflect radiation, to be applied by the user

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3.2

Dimensions of FHF05 series

Figure 3.2.1 Models FHF05 15X30 and 15X85; Y1 = 27 or 70, Y2 = 18 or 25 withdimensions in x 10-3 m(1)
sensing area with thermal spreaders(2)
passive guard(3)type T thermocouple(4)
dot indicating front side(5)
connection block for strain relief(6)cable, standard length C = 2 m

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Figure 3.2.2 Models FHF05 10X10, 50X50 and 85X85 heat flux sensor; Y = 8, 36 or 70,
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)
connection block for strain relief(6)cable, standard length C = 2 m

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4

Standards and recommended practices
for useFHF05 series should be used in accordance with recommended practices.

4.1

Heat flux measurement in industry

FHF05 series 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 model FHF05-85X85 foil heat flux sensor being installed formeasurement on an industrial pipe. The sensor is mounted on a well-prepared curved
surface.

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5

Installation of FHF05 series

5.1

Site selection and installation

Table 5.1.1 Recommendations for installation of FHF05 series heat flux sensors.
Locationchoose 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 coolingdevices 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 a FHF05, 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 theconnection block to the object of interest, so that the temperature of theconnection block remains as close as possible to that of the heat flux
sensor (see appendix)

Surface cleaning andlevellingcreate a clean and smooth surface of at least the outer dimensions of thesensor in use

Mechanical mounting:avoiding strain on thesensor to cabletransition

during installation as well as operation, the user should provide proper
strain relief on the cable so that the connection block is not exposed tosignificant force
first install the sensor by providing strain relief on the connection blockand after that install the cable 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 theeffective 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 overthe passive guard area (surrounding the sensing area). See Figure 3.2.1
use tape to fixate the connection block of the sensor
usually, the cables 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 withsilicone construction sealant, silicone glue or silicone adhesive, that canbe 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

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Figure 5.1.1 Installation of model FHF05-50X50 using tape to fixate the sensor and theconnection block. Extra strain relief on 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 dot is visible in thisimage; this indicates that we are viewing the front side and that the other side, the backside, 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.

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5.2

Installation on curved surfaces

The flexibility of the FHF05 series 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 model FHF05-50X50 foil heat flux sensor, in this image on apipe.
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 goodthermal 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 FHF05 series foil heat flux sensors
on curved surfaces.
Bendingsensor can be bent in both directions

Rated bending radius≥ 7.5 x 10-3 m

Effect on sensitivity

no significant influence on sensitivity

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5.3

Electrical connection

5.3.1 Normal connection
A heat flux sensor should be connected to a measurement system, typically a so-called
datalogger. All FHF05 models are passive sensors that do not need any power. Wires
may act as a source of distortion, by picking up capacitive noise. We recommend keeping
the distance between a datalogger or amplifier and the sensor as short as possible and tokeep the signal wires close to each other. For wire extension, see the appendix on thissubject.
Table 5.3.1.1 The electrical connection of FHF05 series.
WIRE

MEASUREMENT SYSTEM

Red

heat flux signal [+]voltage input [+]Blackheat flux signal [−]voltage input [−]Brown

thermocouple type T [+]thermocouple input [+]White

thermocouple type T [−]thermocouple input [−]The sensor serial number and sensitivity are shown on the FHF05 series productcertificate and on the end of FHF05’s cable.
NOTICE
Putting more than 24 Volt across the sensor wiring
can lead to permanent damage to the sensor.

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5.3.2 Increasing sensitivity, connecting multiple sensors in seriesMultiple sensors may be electrically connected in series. The resulting sensitivity is thesum of the sensitivity of the individual sensors. Below the equations in case two sensors
are used. If needed, more than two sensors may be put in series, again increasing thesensitivity.
Φ = U/(S1 + S2)(Formula 5.3.2.1)

and
U = U1 + U 2(Formula 5.3.2.2)

Table 5.3.2.1 The electrical connection of two FHF05 series models, 1 and 2, in series.
In such case the sensitivity is the sum of the two sensitivities of the individual sensors.
More sensors may be added in a similar manner.
SENSORWIRE

MEASUREMENT SYSTEM

1Red

signal 1 [+]voltage input [+]

1

Black

signal 1 [‒]connected to signal 2 [+]

1

Brown

thermocouple type T [+]

1

White

thermocouple type T [−]2

Redsignal 2 [+]connected to signal 1 [‒]

2

Black

signal 2 [‒]voltage input [‒] or ground

2

Brown

thermocouple type T [+]

2

White

thermocouple type T [‒]The serial number and sensitivity of the individual sensors are shown on the FHF05 series
product certificate and at the end of FHF05’s cable.

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5.3.3 Connection to read out half signals
See the figure on the left: FHF05 series can be connected to read out
only the heat flux through the left half of the sensing area or theheat flux though the right half of the sensing area. This feature may
be used for quality assurance purposes; if the sensor is correctly
installed, a constant percentage of the signal will be generated by theleft – and right.

Figure 5.3.3.1 model FHF05-50X50 with left half indicated by diagonal lines.
Table 5.3.3.1 The electrical connection of FHF05 series for 100 % signal.
WIRE

MEASUREMENT SYSTEM

Red

heat flux signal [+]voltage input [+]Blackheat flux signal [−]voltage input [−] or groundBrown

thermocouple type T [+]White

thermocouple type T [−]Table 5.3.3.2 The electrical connection of FHF05 series for left 50 % signal.
WIRE

MEASUREMENT SYSTEM

Red

heat flux signal [+]Blackheat flux signal [−]Brown

thermocouple type T [+]White

thermocouple type T [−]voltage input [+]voltage input [−] or ground

Table 5.3.3.3 The electrical connection of FHF05 series for right 50 % signal.
WIRE

MEASUREMENT SYSTEM

Red

heat flux signal [+]Blackheat flux signal [−]voltage input [−] or groundBrown

thermocouple type T [+]voltage input [+]White

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5.4

Requirements for data acquisition / amplification

The selection and programming of dataloggers is the responsibility of the user. Pleasecontact the supplier of the data acquisition and amplification equipment to see if
directions for use with the FHF05 series are available. In case a program for similar
instruments is available, this can be used. All FHF05’s can be treated in the same way asother heat flux sensors and (analogue) thermopile pyranometers.
NOTICEDo not use “open circuit detection” when measuring the sensor output.

Table 5.4.1 Requirements for data acquisition and amplification equipment for FHF05
series in the standard configuration.
Capability to measure small voltage
signals

preferably: < 5 x 10-6 V uncertainty
minimum requirement: 20 x 10-6 V uncertainty(valid for the entire expected temperature range of theacquisition / amplification equipment)

Capability for the data logger or thesoftwareto store data, and to perform division by the sensitivity to
calculate the heat flux.
Φ = U/S

Capability to measure thermocouple
type Tpreferably: < ± 3 °C uncertainty

Data acquisition input resistance> 1 x 106 ΩOpen circuit detection(WARNING)

open-circuit detection should not be used, unless this is done
separately from the normal measurement by more than 5times the sensor response time and with a small current
only. Thermopile sensors are sensitive to the current that isused during open circuit detection. The current will generate
heat, which is measured and will appear as a temporary
offset.

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6

Maintenance and trouble shooting

6.1

Recommended maintenance and quality assurance

FHF05 series measures reliably at a low level of maintenance. Unreliable measurement
results are detected by scientific judgement, for example by looking for unreasonably
large or small measured values. The preferred way to obtain a reliable measurement is aregular critical review of the measured data, preferably checking against othermeasurements.
Table 6.1.1 Recommended maintenance of FHF05 series. If possible, the data analysis isdone daily
MINIMUM RECOMMENDED HEAT FLUX SENSOR MAINTENANCE
INTERVALSUBJECTACTION1

1 weekdata analysis

compare measured data to the maximum possible or
maximum expected heat flux and to other measurements forexample from redundant instruments.
look for any patterns and events that deviate from what isnormal or expected. compare to acceptance intervals.

2

6 months

inspection

inspect sensor for wear, cable and wire quality, inspect
mounting, inspect location of installation

3

2 years

recalibration

recalibration by comparison to a calibration standard
instrument in the field, see Paragraph 6.3.
recalibration by the sensor manufacturer

4

2 years

lifetime
assessment

FHF 05 s e r i es m an u a l v 220 5

judge if the instrument will be reliable for another 2 years, or
if it should be replaced

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6.2

Trouble shooting

Table 6.2.1 Trouble shooting for FHF05 series.
GeneralInspect the sensor for any damage. Inspect the quality of mounting / installation.
Inspect if the wires are properly attached to the data logger.
Check the condition of the cable and wires.
Check the datalogger program, in particular if the right sensitivity is entered.
FHF05 series sensitivity and serial number are shown on the product certificate
and at the end of FHF05’s cable.
Check the electrical resistance of the sensor between the black [‒] and red [+]wires. See specifications table 3.1.1 for the nominal sensitivities per dimension.
Measure the sensor resistance first with one polarity, then reverse the polarity.
Take the average value. The typical resistance of the wiring is 0.3 Ω/m. Typicalresistance should be the nominal sensor resistances stated in table 3.1.1 plus 0.6Ω for the total resistance of two wires (back and forth) of each m. Infinite
resistance indicates a broken circuit; zero or a lower than 1 Ω resistance indicatesa short circuit.

The sensor
does not give
any signal

Check if the sensor reacts to heat: put the multimeter at its most sensitive rangeof 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 heatshould generate a positive signal between the red [+] and black [-] wires, doing
the same at the front side (the side with dot), the sign of the output reverses.
Check the data acquisition by replacing the sensor with a spare unit.

The sensor
signal isunrealisticallyhigh or low

Check the wire condition.
Check the data acquisition by applying a 1 x 10-6 V source to it in the1 x 10-6 V range. Look at the measurement result. Check if it is as expected.
Check the data acquisition by short circuiting the data acquisition input with a
10 Ω resistor. Look at the output. Check if the output is close to 0 W/m2.

The sensor
signal shows
unexpectedvariationsThe
temperature
measurementshows
unrealistic
valuesCheck the presence of strong sources of electromagnetic radiation (radar, radio).
Check the condition of the sensor wires.
Check if the wires are not moving during the measurement.
Check if the thermocouple type T is selected in the datalogger program.
Check if a correct reference temperature is selected in the program.
Check the electrical resistance of the thermocouple between the brown [+] andwhite [-] wires. Use a multimeter at the 100 Ω range. Measure the thermocouple
resistance first with one polarity, then reverse the polarity. Take the averagevalue. The typical resistance of the copper wiring is 0.3 Ω/m, for the constantan
wiring this is 6.5 Ω/m. Typical resistance should be the nominal thermocouple
resistance of 2.5 Ω plus 6.8 Ω for the total resistance of the two wires of eachmetre (back and forth). Infinite resistance indicates a broken circuit; zero or alower than 1 Ω resistance indicates a short circuit.
Make sure the temperature of the connection block remains as close as possible tothat of the heat flux sensor. See appendix on temperature measurement accuracy
for more information.

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6.3

Calibration and checks in the field

The recommended calibration interval of heat flux sensors is 2 years.
Recalibration of field heat flux sensors is ideally done by the sensor manufacturer.
On-site field calibration is possible by comparison to a calibration reference sensor.
Usually mounted side by side, alternatively mounted on top of the field sensor.
Hukseflux main recommendations for field calibrations are:1) to compare to a calibration reference of the same brand and type as the field sensor
2) to connect both to the same electronics, so that electronics errors (also offsets) areeliminated3) to mount all sensors on the same platform, so that they have the same bodytemperature4) typical duration of test: > 24 h5) typical heat fluxes used for comparison: > 200 W/m2
6) to correct deviations of more than ± 20 %. Lower deviations should be interpreted as
acceptable and should not lead to a revised sensitivity
Users may also design their own calibration experiment, for example using a wellcharacterised foil heater.

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7

Appendices

7.1

Appendix on cable and wire extension

FHF05 series is equipped with one cable containing four wires. Standard cable length is 2
m. It is possible to order FHF05 series with longer cable lengths or without any cable. A
separate cable is available in 2, 5 or 10 m length.
Wires may act as a source of distortion by picking up capacitive noise. Keep the distance
between data logger or amplifier and sensor as short as possible.
In an electrically “quiet” environment the FHF05 series cable may be extended without
problem. If done properly, the sensor signal, although small, will not significantly
degrade because the sensor resistance is very low (which results in good immunity toexternal sources) and because there is no current flowing (so no resistive losses).
Wire and connection specifications are summarised below.
Table 7.1.1 Preferred specifications for wire extension of FHF05 series.
Wire

3 x copper and 1 x constantan wire, AWG 28, solid core, bundled with an
PFA sheath

Separate cable

Available in 2, 5 or 10 m length

Extension sealing

make sure any connections are sealed against humidity ingress

Conductor resistance< 0.3 Ω/m (copper wire)

Outer cable diameter2 x 10-3 m

Lengthcable should be kept as short as possible, in any case the total cable
length should be less than 100 m

Connection

either use gold plated waterproof connectors, or solder the new wire
conductors and shield to those of the original sensor wire, and make awaterproof connection using heat-shrink tubing with hot-melt adhesive
when using connectors, use dedicated type T thermocouple connectors
for extending the thermocouple wires

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7.2

Appendix on installation of FHF05 series sensor foil

FHF05 series can optionally be ordered without cable and without connection block. It is
possible to order a separate cable. The user should ensure a good connection to thesensor by either soldering wires or alternatively, using a FFC / FPC ZIF connector. See
Table 7.2.1 and 7.2.2 for recommendations.

Figure 7.2.1 FHF05-50X50 sensor foil; detail A is the same for all FHF05’s; dimensions
in x 10-3 m.
(1)
(2)
(3)
(4)

heat flux signal [+], copper
thermocouple type T [+], copper
thermocouple type T [-], constantan (Cu55Ni45)
heat flux signal [-], copper

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Table 7.2.1 Recommendations for soldering FHF05 series sensor foils.
Wire

use insulated wires of preferably at least AWG28
it is possible to order a separate cable
see Figure 7.2.1 for which material to use on which contact

Preparation

clean soldering pad before soldering with isopropyl alcohol (IPA)

Solder material

preferably use lead free solder

Solderingtemperature

use a soldering temperature of max 350 °C

Contact time

as short as possible (± 2 seconds)

Surfaceplace sensor with soldering pad on a well-insulated surface

Strain relief

add additional strain relief on the solder connections, for example bypotting the connection with epoxy

NOTICE
Cross-connecting the wires while soldering will short circuit the sensor.
NOTICE
Avoid long contact while soldering as excess heat can damage the soldering
contacts.

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Figure 7.2.2 FHF05 series sensor foil with soldered wires.
Table 7.2.2 Recommendations for connectors for FHF05 series sensor foils.
Connector type

FFC / FPC (Flat Flex Cable / Flexible Printed Circuit)

Connector variant

ZIF (Zero Insertion Force)

Number of contacts

8 contacts, where only position 1, 3, 6 and 8 are used

Pitch

1 x 10-3 m

External environment

use FHF05 sensor foils with connector in a dry and stable environment

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7.3

Appendix on using FHF05 series with BLK – GLD sticker series

BLK black and GLD gold stickers are accessories to the FHF05 series and FHF05SC series
heat flux sensors. A sensor equipped with a BLK black sticker is sensitive to bothradiative and convective heat flux. A sensor equipped with a GLD gold sticker reflects
radiation and measures convective heat flux only. To calculate the radiative heat flux,
subtract the two measurements.
There are BLK – GLD stickers for every sensor in FHF05 series.
BLK - GLD stickers are designed to be applied by the user. Optionally, it is also possible
to order FHF05 with stickers pre-applied at the factory.
For more details, see the BLK – GLD sticker series user manual.

Figure 7.3.1 FHF05-50X50 heat flux sensor: with BLK-50X50 and GLD-50X50 stickers.

Table 7.3.1 Recommendations for use of FHF05 series heat flux sensors with BLK – GLD
stickers.
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Mountingwhen mounting an FHF05 with a BLK or GLD sticker, 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.

Mounting on curved
surfaces

apply BLK – GLD stickers before mounting the sensor

Location

avoid direct exposure to the sun

Effect on sensitivity

BLK-GLD stickers have no significant influence on sensitivity

7.4

Appendix on standards for calibration

The standard ASTM C1130 - 21 Standard Practice for Calibrating Thin Heat Flux
Transducers specifies in chapter 6 that a guarded hot plate, a heat flowmeter, a hot box
or a thin heater apparatus are all allowed. Hukseflux employs a thin heater apparatus,
uses a linear function according to X1.1 and uses a nominal temperature of 20 °C, in
accordance with X2.2.
The Hukseflux HFPC method relies on a thin heater apparatus according to principles as
described in paragraph 4 of ASTM C1114 - 06, used in the single sided mode of operation
described in paragraph 8.2 and in ASTM C1044 - 16.
ISO does not have a dedicated standard practice for heat flux sensor calibration.
We follow the recommended practice of ASTM C1130 - 21.
Table 7.4.1 Heat flux sensor calibration according to ISO and ASTM.
STANDARDS ON INSTRUMENT CLASSIFICATION AND CALIBRATIONISO STANDARD

EQUIVALENT ASTM STANDARD

no dedicated heat flux calibration standard
available.

ASTM C1130 - 21 Standard Practice for
Calibrating Thin Heat Flux Transducers
ASTM C 1114 - 06 Standard Test Method for
Steady-State Thermal Transmission Properties
by Means of the Thin-Heater Apparatus
ASTM C1044 - 16 Standard Practice for Using a
Guarded-Hot-Plate Apparatus or Thin-Heater
Apparatus in the Single-Sided Mode

7.5

Appendix on calibration hierarchy

FHF05 series factory calibration is traceable from SI through international standards andthrough an internal mathematical procedure that corrects for known errors. The formal

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traceability of the generated heat flux is through voltage and current to electrical powerand electric power and through length to surface area.
The Hukseflux HFPC method follows the recommended practice of ASTM C1130 - 21. It
relies on a thin heater apparatus according to principles as described in Paragraph 4 of
ASTM C1114 - 06, in the single sided mode of operation described in Paragraph 8.2 andin ASTM C1044 - 16. The method has been validated in a first-party conformity
assessment, by comparison to calibrations in a guarded hot plate.

7.6

Appendix on correction for temperature dependence

The sensitivity of a FHF05 series depends on the temperature of the sensor. The
temperature dependence of FHF05 series is specified as < 0.2 %/°C.
The calibration reference temperature is 20 °C.
Users that measure at temperatures that deviate much from 20 °C, or users thatmeasure over a wide range of temperatures, may wish to correct for this temperaturedependence.
To correct for the temperature dependence of the sensitivity, use the measurement
functionΦ = U/(S∙(1 + 0.002∙(T – 20)))(Formula 7.6.1)

with Φ the heat flux in W/m², U the FHF05 series voltage output in V, S the sensitivity in
V/(W/m²) at 20 °C and T the FHF05 temperature.
S is shown on the product certificate and at the end of FHF05’s cable .

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7.7

Appendix on measurement range for different temperatures

The measurement range of FHF05 series is specified as (-10 to +10) x 103 W/m2 at 20°C heat sink temperature. This is a very conservative specification.
In reality, the rated temperature for continuous use of +120 °C is the limiting
specification. The sensor temperature T in ˚C in a specific application depends on theheatsink temperature Theatsink in ˚C, the heat flux Φ in W/m² and the thermal resistance
per unit area Rthermal,A of the sensor in K/(W/m²).
T= Theatsink + Φ·Rthermal,A(Formula 7.7.1)

This means the measurement range is lower for higher heat sink temperatures.
Φmaximum = (120 – Theatsink)/Rthermal,A(Formula 7.7.2)

Table 7.5.1 shows measurement ranges for different heat sink temperatures. For
applications where the sensor is not mounted on a heatsink, use the ambient
temperature instead of heatsink temperature.
Table 7.7.1 Measurement range for different heat sink temperatures.
HEATSINK TEMPERATURE (In ˚C)

MEASUREMENT RANGE

20 °C
40 °C
60 °C
80 °C
100 °C84 x 103 W/m²
67 x 103 W/m²
51 x 103 W/m²
34 x 103 W/m²
17 x 103 W/m²

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7.8

Appendix on temperature measurement accuracy

All FHF05’s have an integrated thermocouple to measure temperature of the object under
test. This thermocouple is supplied as a secondary measurement, in addition to the mainheat flux measurement.
Expanded uncertainty
The total measurement uncertainty is the sum of the thermocouple measurement
uncertainty + the cold junction uncertainty. The cold junction uncertainty can be found in
the specifications of electronics. Typically this is ± 1 °C.
Thermocouple measurement uncertainty
The FHF05 series has a cable with thermocouple extension wires specified as a type T
thermocouple, IEC 60584-1:2013 class 2. They consist of a brown positive copper (Cu)wire and a negative white constantan (Cu55Ni45) wire. Accuracy is ± 2 % for temperature
differences between T2 and T3 (see figure 7.8.1).
In the FHF05 sensor itself, the thermocouple junction (T1) consists of copper andconstantan traces that are extended from the connection block to the edge of the heat
flux sensor sensitive area. These traces have slightly different Seebeck coefficientscompared to the wires, which results in a higher measurement uncertainty of ± 5 % fortemperature differences between T1 and T2 junctions.
Take home: make sure T1 = T2. If that is the case then:uc (T1) =cold junction + 2% (∆T1 + ∆T2)
The total expanded uncertainty in T1 ( Uc(T1) in °C, coverage factor of 2) based on the ofuncertainty in the measurement of the temperature differences between T1 and T2, ∆T1,
and between T2 and T3 or ∆T2 and the error in their Seebeck coefficients.
If T1 ≠ T2 the combined standard uncertainty can be calculated by the law ofpropagation of uncertainty:
uc (T1) = 5 % · ∆T1 + 2 % · ∆T2(Formula 7.8.1)

For simplicity, the worst-case scenario of ± 5 % accuracy of the absolute temperature
measurement in °C is given as specification for the measurement uncertainty.
It is clear from formula 7.8.1 that the accuracy is best, i.e., in the 2 % range, T1 is kept
close to the temperature T2. If the temperature measurement is critical, consider using aseparate temperature sensor.

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Figure 7.8.1 Model FHF05-50X50 with its thermocouple junctions. To minimise uncertainty, please
make sure that ∆T1 is close to zero.

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7.9

EU declaration of conformity

We,

Hukseflux Thermal Sensors B.V., Delftechpark 31, Delft,
The Netherlands

hereby declare under our sole responsibility that:
Product model
Product type

FHF05 series, all models
Heat flux sensors

conform with the following directive(s):2011/65/EU, EU 2015/863

The Restriction of Hazardous Substances Directive

This conformity is declared using the relevant sections and requirements of the following
standards:
Hazardous substances

RoHS 2 and EU 215/863 amendment known as RoHS 3

Eric HOEKSEMA
Director
Delft, 09 November, 2022

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