Hukseflux

Thermal Sensors

USER MANUAL FHF06
Foil heat flux sensor for use to 250 °C, with thermal
spreaders, flexible, with temperature sensor, 25 x 50 mm

Copyright by Hukseflux | FHF06 manual v2301 | 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 FHF06
1.2
Included items
1.3
Quick instrument check2
Instrument principle and theory
3
Specifications of FHF06
3.1
Specifications of FHF06
3.2
Dimensions of FHF06
4
Standards and recommended practices for use
4.1
Heat flux measurement in industry
5
Installation of FHF06
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 FHF06 sensor foil
7.3
Appendix on standards for calibration
7.4
Appendix on calibration hierarchy
7.5
Appendix on correction for temperature dependence
7.6
Appendix on measurement range for different temperatures
7.7
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

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heatsink
maximum

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Introduction
FHF06 is a foil heat flux sensor for use to 250 °C. The instrument is thin, flexible andversatile: it has an integrated temperature sensor and thermal spreaders to reduce
thermal conductivity dependence. It is applicable over a temperature range from –70 to+250 °C for continuous use. FHF06 measures heat flux from conduction, radiation andconvection.
This sensor measures heat flux through the object in which it is incorporated or on which
it is mounted, in W/m2. The sensor in FHF06 is a thermopile. This thermopile measures
the temperature difference across FHF06’s flexible body. A type T thermocouple isintegrated 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 FHF06 is independent of its environment. Many
competing sensors do not have thermal spreaders. The passive guard area around thesensor reduces measurement errors due to edge effects and is also used for mounting.

Figure 0.1 Model FHF06 foil heat flux sensor with thermal spreaders: thin, flexible andversatile.
Using FHF06 is easy. It can be connected directly to commonly used data logging systems.
The heat flux in W/m2 is calculated by dividing the FHF06 output, a small voltage, by thesensitivity. The sensitivity is provided with FHF06 on its product certificate.
FHF06 has unique features and benefits:
•
•
•
•
•
•

high temperature resistance up to T = 250 °C continuous use
flexible (bending radius ≥ 7.5 x 10-3 m)
low thermal resistance
fast response time
integrated type T thermocouple
robustness, including cable and connection block which may be used as strain relief
between sensor and cable

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•
•
•

IP protection class: IP67 (essential for outdoor application)
integrated thermal spreaders for low thermal conductivity dependence
sensor foil only: may be used in vacuum

Equipped with a protective potted connection block, which may serve as strain relief sothat moisture does not penetrate, FHF06 has proven to be very robust and stable.

Figure 0.2 Model FHF06 high temperature foil heat flux sensor being used to monitor the
performance of an oven. The sensor can be continuously used for temperatures up to T =250 °C.
FHF06 has calibration that is traceable to international standards. The factory calibration
method follows the recommended practice of ASTM C1130 - 21. When used underconditions that differ from the calibration reference conditions, the sensitivity of FHF06 toheat flux may be different than stated on its certificate. See Chapter 2 in this manual forbest practices.
See also:
•
•
•
•
•

FHF05 series, foil heat flux sensors in five different dimensions and sensitivities
FHF05SC series for a self-calibrating version of FHF05 series
HTR02 series heater, for calibration and verification of performance of FHF-type
sensors
model HFP01 for increased sensitivity (also consider putting two or more FHF06’s inseries)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 FHF06

The standard configuration of FHF06 is FHF06-25X50-02, model 25X50 with 2 metres ofcable. Common options are:
•
•
•
•
•

change -02 to -05 or -10 metres for the respective cable length
without wiring, without connection blockwith a separate cable in 2, 5, or 10 metres cable lengthwith 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

1.2

Included items

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

heat flux sensor FHF06 with wires of the length as ordered
product certificate matching the instrument serial number

Figure 1.2.1 Model FHF06 with serial number and sensitivity shown at the end of thecable. Please note that the label can handle temperatures up to 120 °C. See figure 5.1.2.

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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 on the label at the end of FHF06’s cable against thesensitivity on 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 of 200 Ω plus 0.6 Ω
for the total resistance of two wires for each metre (back and forth). Infinite resistanceindicates 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 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

FHF06’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”.
FHF06 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 FHF06 is a thermopile. This thermopile measures the temperature
difference across the polyimide body of FHF06. 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 FHF06 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)

1

2

3

4

5
6

Figure 2.1 The general working principle of a heat flux sensor. The sensor inside FHF06
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 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.
FHF06 is designed in such a way, 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 FHF06 include high temperature resistance up to 250 °C, flexibility(bending radius ≥ 7.5 x 10-3 m), low thermal resistance, a fast response time, IP67
protection class rating (essential for outdoor application), and the inclusion of thermal
spreaders to reduce thermal conductivity dependence.
FHF06 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 300 W/m2 generated with our HTR02 heater series
mounted on aluminium heat sink

FHF06 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 FHF06 sensitivity toheat 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

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• 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
The user should analyse his own experiment and make his own uncertainty evaluation.
The FHF06 rated temperature range for continuous use is -70 to +250 °C. Please contact
Hukseflux when measuring at -160 °C.

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3

Specifications of FHF06

3.1

Specifications of FHF06

FHF06 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, FHF06 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 FHF06 (continued next pages).
FHF06 SPECIFICATIONS
Sensor type
Sensor type according to ASTM
Measurand
Measurand in SI units
Measurement range
Sensitivity
Directional sensitivity
Asymmetry
Increased sensitivity
Expected voltage output
Measurement function / required
programming
Required readout
Optional readout
Rated load on cable
Rated bending radius
Rated temperature range,
continuous use
Rated temperature range cable
Rated temperature range connection
block
Rated temperature range,
short intervals
Rated pressure
Outgassing of sensor foil

Temperature dependence
Non-linearity
Solar absorption coefficient
Thermal conductivity dependence
Sensor length and width
Sensing area
Sensing area length and width
Passive guard area
Guard width to thickness ratio
Sensor thickness

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high temperature foil heat flux sensor
heat flow sensor or heat flux transducer
heat flux
heat flux density in W/m2(-20 to +20) x 103 W/m2 at heat sink temperature 20 °C
see appendix for detailed calculations
5 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 +250 °C-70 to +250 °C-70 to +250 °C-160 to +250 °C(contact Hukseflux when measuring at -160 °C)
up to 25 bar
low outgassing, 0.36 % of total mass loss, 0.01 &collected volatile condensable material as per NASAJSC< 0.2 %/°C
< 5 % (0 to 10 x 10³ W/m²)
0.85 (indication only)
negligible, < 3 %/(W/m·k) from 270 to 0.3 W/m·K(25 x 50) x 10-3 m
8.9 x 10-4 m2(20 x 44.5) x 10-3 m
3.6 x 10-4 m2
6.25
0.38 x 10-3 m

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Sensor thermal resistanceSensor thermal conductivity
Response time (95 %)Sensor resistance (nominal)
Required sensor power
Temperature sensor
Temperature sensor accuracy

12 x 10-4 K/(W/m2)
0.31 W/(m·K)
3s
200 Ωzero (passive sensor)
type T thermocouple± 2.5 or 0.0075 × T °C, as per IEC 60584-1:2013
class 2Standard wire length
Optional wire length
Wiring2m
0, 5 or 10 m
3 x copper and 1 x constantan wire, AWG 28, solidcore, bundled with an MFA sheath2 x 10-3 m
dot on foil indicating front side of the heat flux sensor;
1 x label at end of FHF06’s cable, showing serial
number and sensitivity
IP67
0 to 100 %FHF06 is not suitable for continuous use under water
approx. 0.5 kg
approx. 0.5 kg

Cable 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
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

FHF 06 ma n u al v 230 1

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. Sensor foil
can be used in vacuum.
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 FHF06 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 W/m2, mounted on aluminium
heat sink, thermal conductivity of 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 FHF06 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 field

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MEASUREMENT ACCURACY
Uncertainty of the measurement

sensor 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
ACCESSORIES
Hand-held read-out unit
Separate foil heater
Separate cable

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option code = cable length in metrescalibrated FHF06 sensor foil to be soldered /connected by the user see appendix for more
information

LI19 handheld read-out unit / datalogger
NOTE: LI19 does not measure temperature, only heat flux
HTR02-50X50 general-purpose heater, that can be used
for test and calibration purposes. See HTR02 heater
manual for more information
cable with 3 x copper and 1 x constantan wire, AWG
28, solid core, bundled with a PFA sheathavailable in 2, 5 or 10 m length

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3.2

Dimensions of FHF06

Figure 3.2.1 Model FHF06 with 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)
potted connection block for strain relief(6)cable, standard length C = 2 m

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4

Standards and recommended practices
for useFHF06 should be used in accordance with recommended practices.

4.1

Heat flux measurement in industry

FHF06 is 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 manycases heat flux sensors are used for trend monitoring. In such cases reproducibility ismore important than absolute measurement accuracy.

Figure 4.1.1 Example of model FHF06 high temperature foil heat flux sensor being installed
for measurement on a car exhaust. FHF06 is flexible and can be easily mounted on a curved
surface like a pipe or tube.

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5

Installation of FHF06

5.1

Site selection and installation

Table 5.1.1 Recommendations for installation of FHF06 heat flux sensor.
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 FHF06, 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
please note that the label at the end of the cable showing serial number
and sensitivity can handle only temperatures up to 125 ⁰C, see figure
5.1.2
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

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

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Signal amplification

see the paragraph on electrical connection

Figure 5.1.1 Installation of model FHF06 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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Figure 5.1.2 The label at the end of the cable showing serial number and sensitivity canhandle only temperatures up to 125 °C. The sensor, cable and connection block have acontinuous operating temperature up to 250 °C.

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5.2

Installation on curved surfaces

The flexibility of the FHF06’s 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 FHF06 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 goodthermal contact with the curved surface. Avoid air gaps. Tape can be used over thesensing area when necessary.
Table 5.2.1 Extra recommendations for installation of FHF06 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. FHF06 is a passive sensor that does not need any power. Wires may act as asource 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 to keep thesignal wires close to each other. For wire extension, see the appendix on this subject.
Table 5.3.1.1 The electrical connection of FHF06.
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 FHF06 product certificate and at
the end of FHF06’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 FHF06 models, 1 and 2, in series. In suchcase, the sensitivity is the sum of the two sensitivities of the individual sensors. Moresensors 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 sensor serial number and sensitivity are shown on FHF06 product certificate and at
the end of FHF06’s cable.

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5.3.3 Connection to read out half signals
See the figure on the left: FHF06 can be connected to read out
only the heat flux through the left half of the sensing area or theheat flux through the right half of the sensing area. This feature may be used
for quality assurance purposes; if the sensor is correctly installed, a constantpercentage of the signal will be generated by the left – and right.

Figure 5.3.3.1 Model FHF06 with left half indicated by diagonal lines.
Table 5.3.3.1 The electrical connection of FHF06 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 FHF06 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 FHF06 for right 50 % signal.
WIRE

MEASUREMENT SYSTEM

Red

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

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

thermocouple type T [−]FHF 06 ma n u al v 230 1

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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 FHF06 are available. In case a program for similar instruments
is available, this can be used. All FHF06’s can be treated in the same way as other 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 FHF06 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
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6

Maintenance and trouble shooting

6.1

Recommended maintenance and quality assurance

FHF06 measures reliably at a low level of maintenance. Unreliable measurement results
are detected by scientific judgement, for example by looking for unreasonably large orsmall measured values. The preferred way to obtain a reliable measurement is a regularcritical review of the measured data, preferably checking against other measurements.
Table 6.1.1 Recommended maintenance of FHF06. If possible, the data analysis is done
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 06 ma n u al v 230 1

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 FHF06.
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. The
sensor serial number and sensitivity are shown on FHF06 product certificate and at
the end of FHF06’s cable
Check the electrical resistance of the sensor between the black [‒] and red [+]wires. Measure the sensor resistance first with one polarity, then reverse thepolarity. Take the average value. The typical resistance of the wiring is 0.3 Ω/m.
Typical resistance should be the nominal sensor of 200 Ω plus 0.6 Ω 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 indicates a 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.

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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

FHF06 is equipped with one cable containing four wires. Standard cable length is 2 m. It
is possible to order FHF06 with longer cable lengths or without any cable. A separate
cable is available in 2, 5 or 10 m length.
Cable 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 FHF06 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 FHF06.
Wire

3 x copper and 1 x constantan wire, AWG 28, solid core, bundled with a
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 FHF06 sensor foil

FHF06 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 soldering wires. Please check the temperature range of the connectors. See
Table 7.2.1 for recommendations.

Figure 7.2.1 FHF06 sensor foil; 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 FHF06 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
check the solder material on its temperature spec, commercially available
solders with tin have a melting temperature around 220 °C
we recommend a solder with gold

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
check the epoxy on its temperature spec, not all epoxies are capable tohandle this temperature

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 FHF06 sensor foil with soldered wires.

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7.3

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 7.3.1 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

7.4

Appendix on calibration hierarchy

FHF06 factory calibration is traceable from SI through international standards andthrough an internal mathematical procedure that corrects for known errors. The formal
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.

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7.5

Appendix on correction for temperature dependence

The sensitivity of a FHF06 depends on the temperature of the sensor. The temperaturedependence of FHF06 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 7.5.1)

with Φ the heat flux in W/m², U the FHF06 voltage output in V, S the sensitivity in
V/(W/m²) at 20 °C and T the FHF06 temperature.
S is shown on the product certificate and at the end of FHF06’s cable.

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7.6

Appendix on measurement range for different temperatures

The measurement range of FHF06 is specified as (-20 to +20) x 103 W/m2 at 20 °C heatsink temperature. This is a very conservative specification.
In reality, the rated temperature for continuous use of +250 °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 7.6.1)

This means the measurement range is lower for higher heat sink temperatures.
Φmaximum = (250 – Theatsink)/Rthermal,A(Formula 7.6.2)

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 7.6.1 Measurement range for different heat sink temperatures.
HEATSINK TEMPERATURE

MEASUREMENT RANGE

20 °C
40 °C
60 °C
80 °C
100 °C
120 °C
140 °C
160 °C
180 °C
200 °C
220 °C186 x 103 W/m²
170 x 103 W/m²
154 x 103 W/m²
138 x 103 W/m²
121 x 103 W/m²
105 x 103 W/m²
89 x 103 W/m²
73 x 103 W/m²
57 x 103 W/m²
41 x 103 W/m²
25 x 103 W/m²

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7.7

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

FHF06
foil heat flux sensor

is in conformity with the following directives:
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, 15 April 2023

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© 2023, Hukseflux Thermal Sensors B.V.
www.hukseflux.com
Hukseflux Thermal Sensors B.V. reserves the right to change specifications without notice.