Hukseflux

Thermal Sensors

USER MANUAL
FHF05SC series
Self-calibrating foil heat flux sensor with thermal
spreaders and heater

Copyright by Hukseflux | FHF05SC series manual v2204 |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 FHF05SC series
1.2
Included items
1.3
Quick instrument check2
Instrument principle and theory
2.1
Theory of operation
2.2
The self-test
2.3
Calibration
2.4
Application example: stable performance check
2.5
Application example: non-invasive core temperature measurement
3
Specifications of FHF05SC series
3.1
Specifications of FHF05SC series
3.2
Dimensions of FHF05SC series
4Standards and recommended practices for use
4.1
Heat flux measurement in industry
5
Installation of FHF05SC 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 extension
7.2
Appendix on using FHF05SC series with BLK – GLD sticker series
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
Appendix on temperature measurement accuracy
7.8
EU declaration of conformity

FHF 05S C s e ri e s m an u al v2 204

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List of symbols
Quantities

SymbolUnit

Heat flux
Voltage output
Sensitivity
Temperature
Thermal resistance per unit area
Area
Electrical resistanceElectrical power

ΦUS
T
Rthermal,A
A
R
P

W/m²
V
V/(W/m2)°C
K/(W/m²)
m²Ω
W

subscripts
property of heatsink
property of heater
property of sensor
maximum value, specification limit

FHF 05S C s e ri e s m an u al v2 204

heatsink
heatersensormaximum

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Introduction
FHF05SC series is a combination of the standard model FHF05 heat flux sensor and aheater. The heater allows the user to perform self-tests, verifying sensor functionality
and stability during use, without having to remove the sensor. FHF05SC series are ideal
for high-accuracy and long-term heat flux measurement, construction of calorimeters,(zero heat flux) core temperature measurement and thermal conductivity test
equipment. Available in two models: size 50X50 mm and a larger size of 85X85 mm.
FHF05SC series measures heat flux through the object in which it is incorporated or onwhich it is mounted, in W/m2. The sensor within is a thermopile. This thermopile
measures the temperature difference across FHF05SC’s flexible body. A type T
thermocouple is integrated as well to provide a temperature measurement. 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 FHF05SC series is independent of its
environment. Many competing sensors do not have thermal spreaders. The passive guard
area around the sensor reduces edge effects and is also used for mounting. Looking for
only heat flux and temperature measurement without a heater? See our FHF05 series heat
flux sensors.

Figure 0.1 Model FHF05SC-50X50 self-calibrating foil heat flux sensor with thermal
spreaders and heater, showing its back and front side.

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Measuring heat flux, users may wish to regularly check their sensor performance. Duringuse, the film heater is activated to perform a self-test. The heat flux sensor response to
the self-test results in a verification of sensor performance. Implicitly also wire
connection, data acquisition, thermal connection of the sensor to its environment anddata processing are tested. Heat flux sensors are often kept installed for as long as
possible. Using self-testing, the user no longer needs to take sensors to the laboratory toverify their stable performance. In a laboratory environment, using a metal heat sink,you may even perform a formal calibration. The heater has a well characterised andtraceable surface area and electrical resistance.
The FHF05SC series self-calibrating foil heat flux sensor has unique features andbenefits:
•
•
•
•
•
•
•
•
•

heater for self-test
flexible (bending radius ≥ 15 x 10-3 m)
low thermal resistancewide temperature range
fast response time
integrated type T thermocouple
robustness, including cable and connection block which may be used as strain relief
IP protection class: IP67 (essential for outdoor application)
integrated thermal spreaders for low thermal conductivity dependence

FHF05SC series suggested use:
•
•
•
•
•

high-accuracy scientific measurement of heat flux, with a high level of data quality
assurance
study of convective heat transfer mechanisms
calorimeter prototyping(zero heat flux) non-invasive core temperature measurement
thermal conductivity test equipment\

Figure 0.2 Application example: FHF05SC-50X50 being installed to measure heat flux ona pipe. The sensor is mounted on a well-prepared curved surface.

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Using FHF05SC 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 FHF05SC series on its product certificate.
Equipped with a protective potted connection block, which may serve as strain relief sothat moisture does not penetrate, FHF05SC series has proven to be very robust andstable.
FHF05SC series calibration 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 FHF05SC series
sensitivity to heat flux may be different than stated on its certificate. See Chapter 2 inthis manual for suggested solutions.
Would you like to study energy transport / heat flux in detail? Hukseflux helps taking thismeasurement to the next level: order FHF05SC series with radiation-absorbing black andradiation-reflecting gold stickers. You can then measure convective + radiative flux with
one, and convective flux only with the other. Subtract the 2 measurements and you haveradiative flux. They can be applied to the sensor by the user or at the factory; see the BLK– GLD sticker series user manual and installation video for instructions.

Figure 0.3 FHF05SC-50X50 heat flux sensor: with BLK-5050 and GLD-5050 stickers.

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See also:
•
•
•
•
•

FHF05 series, our standard model for general-purpose heat flux measurement
model HFP01 for increased sensitivity (also consider putting two or more FHF05s inseries)
HTR02 series heater, for calibration and verification of performance of FHF-type sensors.
BLK - GLD sticker series to separate radiative and convective heat fluxes
Hukseflux offers a complete range of heat flux sensors with the highest quality for any
budgetFHF 05S C s e ri e s m an u al v2 204

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1

Ordering and checking at delivery

1.1

Ordering FHF05SC series

The standard configuration of FHF05SC series is FHF05SC-50X50-02, model 50X50 with
2 metres of cable. Common options are:
•
•
•
•
•
•
•

model FHF05SC-85X85
change -02 to -05 or -10 metres cable lengthwith a separate cable in 2, 5 or 10 metres cable lengthwith LI19 hand-held read-out unit / datalogger; NOTE: LI19 does not measuretemperature, only heat flux and does not support self-test functionality
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 FHF05SC with cable of the length as ordered
product certificate matching the instrument serial number

Figure 1.2.1 Model FHF05SC-50X50 with serial number and sensitivity shown at the endof 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 sticker on the potted connection
block against the product certificate provided with the sensor.
2. Inspect the instrument for any damage.
3. Check the electrical resistance of the sensor between the red [+] and black [-] wires.
Use a multimeter at the 1k Ω range. Measure the sensor resistance first with one
polarity, then reverse the polarity. Take the average value. The typical resistance of 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 with the heater) 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.
6. Check the electrical resistance of the heater between purple or yellow wire and pink or
green wire. Use a multimeter at the 1 kΩ range. Typical resistance should be around 120Ω for model -50X50 and around 40 Ω for model -85X85 . Infinite resistance indicates a
broken circuit; zero or a lower than 1 Ω resistance indicates a short circuit. 7. Check theelectrical resistance between the purple and yellow wires. . These resistances should bein the 0.1 Ω/m range, so 0.2 Ω in case of the standard 2 m wire length. Higher
resistances indicate a broken circuit. Repeat this measurement for the pink and green
wire.

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2

Instrument principle and theory

FHF05SC series’ scientific name is heat flux sensor. A heat flux sensor measures theheat flux density through the sensor itself. This quantity, expressed in W/m2, is usuallycalled “heat flux”.
FHF05SC series users typically assume that the measured heat flux is representative of
the undisturbed heat flux at the location of the sensor. Users may also apply corrections
based on scientific judgement. FHF05SC series has an integrated film heater. At a regularinterval the film heater can be activated to perform a self-test. The self-test results in averification of sensor performance. See the next chapters for examples how the self-test
may be used. Implicitly also wire connection, data acquisition and data processing aretested.

2.1

Theory of operation

The sensor in FHF05SC 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.

1

2

3

4

5
6

Figure 2.1.1 The general working principle of a heat flux sensor. The sensor inside
FHF05SC series 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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Using FHF05SC series is easy. For readout the user only needs an accurate voltmeterthat 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 eachindividual sensor.
Φ = U/S(Formula 2.1.1)

FHF05SC series 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 FHF05SC series include flexibility (bending radius ≥ 15 x 10-3 m), highsensitivity, low thermal resistance, a wide temperature range, a fast response time, IP67
protection class rating (essential for outdoor application), and the inclusion of thermal
spreaders to reduce thermal conductivity dependence.
FHF05SC’ 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 to 600 W/m2
mounted on aluminium heat sink

FHF05SC series has been calibrated using a well-conducting metal heat sink,
representing a typical industrial application, at 20 °C and exposing it to a conductive heat
flux. When used under conditions that differ from the calibration reference conditions, forexample at extremely high or low temperatures, or exposed to radiative flux, theFHF05SC 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, using the integrated heater
apply our BLK black sticker to the sensor surface to absorb radiation
apply our GLD black sticker to the sensor surface to reflect radiation

The user should analyse his own experiment and make his own uncertainty evaluation.
The FHF05SC series rated operating temperature range for continuous use is -70 to +120
°C, for short intervals a peak temperature of +150 °C is allowed. Prolonged exposure totemperatures near +150 °C can accelerate the aging process.

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Figure 2.1.2 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. The backside of the FHF05SC
has a heater.

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2.2

The self-test

A self-test is started by switching on FHF05SC’s heater, while recording the sensor
output signal and the heater power and is finalised by switching the heater off. During
the heating interval a current is fed through the film heater, which generates a known
heat flux. To calculate this heat flux, the heater power Pheater must be measured
accurately. This power can be measured in several different ways;•

heater voltage and current, Pheater = Uheater∙Iheater(Formula 2.2.1)•

heater voltage and known heater resistance, Pheater = Uheater2/Rheater(Formula 2.2.2)•

heater current and known heater resistance, Pheater = Iheater2∙Rheater(Formula 2.2.3)

The user must interrupt the normal measurement of the heat flux during the self-test.
Analysis of the heat flux sensor response to the heating, the self-test, serves several
purposes:•••

first, the amplitude and response time under comparable conditions are indicators of
the sensor stability. See Section 2.4 and 2.5 for application examples.
second, the functionality of the complete measuring system is verified. For example:
a broken cable is immediately detected.
third, under the right conditions, after taking the sensor out of its normal environment,
the self-test may be used as calibration. See Section 2.3 for more details.

2.3

Calibration

FHF05SC series calibration 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 FHF05SC series
sensitivity to heat flux may be different than stated on its certificate.
In a typical calibration setup as shown in the next figure, the FHF05SC series ispositioned between an insulating material and a heatsink with the FHF05SC series heateron the side of the insulating material. In such a setup, the heat losses through theinsulation may be ignored. In this case all heat generated by the heater flows throughthe heat flux sensor to the heat sink. Measuring the heater power Pheater, and dividing by
the surface area Aheater, gives the applied heat flux:Φ = Pheater/Aheater(Formula 2.3.1)

The heat flux sensor sensitivity S is the voltage output Usensor divided by the applied heat
flux Φ:S = Usensor/Φ

FHF 05S C s e ri e s m an u al v2 204(Formula 2.3.2)

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The reproducibility of this test is much improved when using contact material (such as
glycerol or a thermal paste) between sensor and heat sink.

Figure 2.3.1 Calibration of FHF05SC series; a typical stack used for calibration consistsof a block of metal (mass > 1 kg), for example aluminium (5), the heat flux sensor (3),with heater (2) and an insulation foam (1). Under these conditions, heat losses throughthe insulation are negligible. Heat flux (4) flows from hot to cold.

2.4

Application example: stable performance check

The FHF05SC series heater can be used to check for stable performance of the sensor at
regular intervals without the need to uninstall the sensor from its application.
A typical stability check is performed based on the step response of the measured heat
flux and sensor temperature to a heat flux applied by the heater. Upon installing thesensor, a reference measurement should be made. A time trace of the heater power, themeasured heat flux and the measured sensor temperature should be stored as reference
data. Stable operation of the sensor can then be confirmed at any time by comparing to
the reference measurement. The test protocol consists of the following steps:
1. Make sure that the absolute temperature is similar to that during the reference
measurement.
2. Check the heater resistance stability. This can be done accurately by using the four
heater wires to conduct a four-point resistance measurement.
3. Record a time trace of the heater power, the measured heat flux and the sensor
temperature; the same parameters as in the reference data. Normalise the data by
the heater power. Under normal circumstances (if the heater is stable) this processscales with Uheater2.
4. Compare patterns of heat flux and temperature rise and fall. In both cases relative tothe values just before heating.
• When the signal patterns match, amplitude differences, after correction forheater power, point towards sensor instability. In this case recalibration of thesensor may be required (Figure 2.4.1).

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•

Non-matching patterns point towards changes in sensor environment. This canfor example be the result of a loss of thermal contact between sensor andobject (Figure 2.4.2) or the presence of convective heat losses (Figure 2.4.3).

Figure 2.4.1 In-situ sensor stability check. Comparison of responses to stepwise heating
relative to reference curves. Normalised to heater power (P) and relative to the heat flux
and the temperature just before heating. Solid graphs show heat flux, dotted graphs showtemperature. The black HF and T signals are the reference curves at installation. The
sensor shows non-stability, loses sensitivity over time, which results in the red responses:
equal response times, lower heat flux and equal temperature rise.

Figure 2.4.2 In situ sensor stability check. Comparison of responses to stepwise heating
relative to reference curves. Normalised to heater power (P) and relative to the heat flux
and the temperature just before heating. Solid graphs show heat flux, dotted graphs showtemperature. The black HF and T signals are the reference curves at good thermal contact.
The sensor loses thermal contact, which results in the blue responses: slower response
times, lower heat flux and higher temperature rise.

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Figure 2.4.3 In-situ sensor stability check. Comparison of responses to stepwise heating
relative to reference curves. Normalised to heater power (P) and relative to the heat flux
and the temperature just before heating. Solid graphs show heat flux, dotted graphs
show temperature. The black HF and T signals are the reference curves at zero windspeed. The sensor is exposed to convection, which results in the grey responses: faster
response times at lower heat flux and lower temperature rise.

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2.5

Application example: non-invasive core temperature
measurement

FHF05SC series may be used for non-invasively measuring the core temperature ofobjects, for example of human beings.
The measurement is done by securely fixate the sensor on the object under test. The
side of the heater should be surrounded with insulation material. All the heat is forced
through the sensor. To determine the core temperature, the heater power should be
adjusted such that the heat flux equals zero. When zero heat flux is attained, thetemperature gradient equals zero and the measured temperature equals the coretemperature.
To perform such a measurement a PID controller can be used to regulate the heating
power. The setpoint of the PID controller should be set to zero heat flux. The PID
controller can regulate the heater power either through a 0 – 12 V programmable power
supply or via a solid-state relay controlled with a pulse-width-modulated signal.

Figure 2.5.1 FHF05SC series in a non-invasive core-temperature measurement. For
measurement of the core temperature (1), the heater (5) is controlled to a setpoint of zeroheat flux (2) measured by the heat flux sensor (3). At zero heat flux, the temperature of
the core (1) and the temperature sensor (6) are equal. Insulation material (4) is attached
to work at stable boundary conditions.

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3

Specifications of FHF05SC series

3.1

Specifications of FHF05SC series

FHF05SC 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, FHF05SC 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 FHF05SC series (continued next pages).
FHF05SC SERIES SPECIFICATIONS
Sensor type
Sensor type according to ASTM
Measurand
Measurand in SI units
Measurement range
Sensitivity range (nominal)
FHF05SC-50X50
FHF05SC-85X85
Directional sensitivity
Increased sensitivity
Expected voltage output
Required readout
Optional readout
Rated load on cable
Rated bending radius
Rated operating temperature range,
continuous use
Rated operating temperature range,
short interval
Temperature dependence
Non-linearity
Solar absorption coefficient
Thermal conductivity dependence
Sensor length and width
FHF05SC-50X50
FHF05SC-85X85
Sensor sensing area
FHF05SC-50X50
FHF05SC-85X85
Sensing area length and width
FHF05SC-50X50
FHF05SC-85X85
Sensor passive guard area
FHF05SC-50X50
FHF05SC-85X85
Guard width to thickness ratio

FHF 05S C s e ri e s m an u al v2 204

self-calibrating 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
13 x 10-6 V/(W/m2)
50 x 10-6 V/(W/m2)
heat flux from the back side (side with the heater) to
the front side (side with the dot) generates a positive
voltage output signal
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
1 differential voltage channel or 1 single ended
voltage channel, input resistance > 106 Ω
1 temperature channel≤ 1.6 kg≥ 15 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)(50 x 50) x 10-3 m(85 x 85) x 10-3 m
12.96 x 10-4 m2
47.70 x 10-3 m²(36 x 36) x 10-3 m(70 x 71) x 10-3 m
12.04 x 10-4 m2
22.55 x 10-4 m2

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FHF05SC-50X50
FHF05SC-85X85

Sensor thickness
Sensor thermal resistanceSensor thermal conductivity
Response time (95 %)Sensor resistance range per dimension
FHF05SC-50X50
FHF05SC-85X85
Required sensor power
Temperature sensor
Temperature sensor accuracyStandard cable length
Optional cable 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
HEATER

17.5
18.25
0.7 x 10-3 m
24 x 10-4 K/(W/m2)
0.29 W/(m·K)
6s
200 – 300 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
7 x copper and 1 x constantan wire, AWG 28, solidcore, bundled with a PFA sheath
2.7 x 10-3 m
dot on foil indicating front side of the heat flux sensor;
1 x label at the end of FHF05SC’s cable,
showing serial number and sensitivity
IP67
0 to 100 %FHF05SC is not suitable for continuous use underwater
approx. 0.5 kg
approx. 0.5 kg

Heater length and width per dimension
FHF05SC-50X50 (48 x 47.6) x 10-3 m
FHF05SC-85X85 (83 x 82.6) x 10-3 m
Heater area
FHF05SC-50X50 2381 x 10-6 m2
FHF05SC-85X85 7022 x 10-6 m2
Passive guard area
FHF05SC-50X50 2152 x 10-4 m2
FHF05SC-85X85 3692 x 10-4 m2
Heater resistance (nominal) per dimension (measured value supplied with each sensor in the
production report)
FHF05SC-50X50 120 Ω
FHF05SC-85X85 40 Ω
Heater rated power supply
24 VDC
Heater power supply
12 VDC (nominal)
Suggested current sensing resistor
10 Ω ± 0.1 %, 0.25 W, < 15 ppm/°C
SELF-TEST
Power consumption during heating interval (nominal)
FHF05SC-50X50 1.20 W (@ 12 VDC)
FHF05SC-85X85 3.60 W (@ 12 VDC)
Nominal heat flux at 12 VDC per dimension
FHF05SC-50X50 500 W/m²
FHF05SC-85X85 500 W/m²
INSTALLATION AND USE
Typical conditions of use

FHF 05S C s e ri e s m an u al v2 204

in experiments, in measurements in laboratory andindustrial environments. Exposed to heat fluxes forperiods of several minutes to several years.

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Recommended number of sensors
Installation
Bending
Wire extension
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
VERSIONS / OPTIONSWith longer cable lengthWith black sticker appliedWith gold sticker applied
ACCESSORIES
Hand-held read-out unit
Separate black stickers
Separate gold sticker

FHF 05S C s e ri e s m an u al v2 204

Connected to user-supplied data acquisition
equipment. Regular inspection of the sensor.
Continuous monitoring of sensor temperature. No
special requirements for immunity, emission, chemical
resistance.
2 per measurement location
see recommendations in this user manual
see chapter on installation on curved surfaces
see chapter on cable extension or order sensors withlonger cable length
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 (model -50X50) or 600 (model-85X85) 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 FHF05SC sensitivity to heat
flux may be different than stated on its certificate. See
the chapter on instrument principle and theory forsuggested 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.
option code = cable length in metres
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 fluxand does not support self-test functionality
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 FHF05SC series

5

Figure 3.2.1 FHF05SC series models -50X50 and -85X85 heat flux sensor; Y = 36 or 70,
H1 = 47.6 or 82.6 and H2 = 48 or 83. All dimensions in x 10-3 m(1)
sensing area with thermal spreaders(2)
passive guard(3)
contour of the heater area for self-test(4)
type T thermocouple(5)
dot indicating front side
(6)
connection block for strain relief(7)wires, standard length C = 2 m

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4

Standards and recommended practices
for useFHF05SC series should be used in accordance with recommended practices.

4.1

Heat flux measurement in industry

FHF05SC series sensors are often used to measure on industrial walls and metal
surfaces, estimating the installation’s energy balance and the thermal transmission ofwalls. Typically, the total measuring system consists of multiple heat flux- andtemperature 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 FHF05SC-85X85 foil heat flux sensor being installed formeasurement on an object. The sensor is mounted on a well-prepared flat surface.

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5

Installation of FHF05SC series

5.1

Site selection and installation

Table 5.1.1 Recommendations for installation of FHF05SC 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

we recommend using > 2 sensors per measurement location. This
redundancy also improves the assessment of the measurement accuracy

Mounting

when mounting a FHF05SC model, keep the directional sensitivity in mindorient the heater away from the object on which it is mounted
heat flux from the back side (side with heater) to the front side (side withdot) generates a positive voltage output signal
fix the connection block to the object of interest so that the temperatureor the connection block remains as close as possible to that of the heat
flux sensor. This is a way to achieve the highest accuracy temperature
measurement (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 wiretransition

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 wires including additional strain relief

Short term
installation

avoid any air gaps between sensor and surface. Air thermal conductivity
is in the 0.02 W/(m·K) range, while a common glue has a thermal
conductivity around 0.2 W/(m·K). A 0.1 x 10-3 m air gap increases 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 cable is 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 FHF05SC-50X50 using tape to fixate the sensor andthe connection block. Extra strain relief on the wires is provided using cable tie mounts
equipped with double sided tape as adhesive. As indicated in Table 5.1.1, tapes fixating
the sensor are preferably taped over the passive guard area and not on the sensing area(the latter indicated by grey shading in Figure 5.1.1). Please note we are viewing theback side (heater side) and that the other side, the front side, is attached on the object
on which the sensor is mounted, as explained in Chapter 2.
See also our application note on how to install a heat flux sensor.

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5.2

Installation on curved surfaces

The flexibility of the FHF05SC 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 FHF05SC-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 FHF05SC series foil heat flux
sensors on curved surfaces.
Bending

Sensor can be bent in both directions

Rated bending radius≥ 15 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
FHF05SC series has one bundled cable. It contains two sets of wires, one set for the heat
flux signal, and one set for the heater. The latter are yellow, purple, pink and green.
To read out the heat flux sensor, FHF05SC series should be connected to a measurement
system, such as a voltmeter, an amplifier, a datalogger or a data-acquisition (DAQ)
system. The FHF05SC series electrical connections are explained in Table 5.4.1.1.
FHF05SC series heat flux sensor is a passive sensor that does not require any power.
FHF05SC’s heater does require power.
Wires may pick up capacitive noise, which can lead to signal distortion. Therefore, we
recommend keeping the electrical connections between the measurement system andthe FHF05SC series as short as possible and to keep the signal wires close to each other.
For instructions on wire extension please refer to Appendix 7.1.
Table 5.3.1.1 The electrical connection of FHF05SC series.
WIRE

FHF05SCRed

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

thermocouple type T [−]Brown

thermocouple type T [+]Yellowheater power [+]Purpleheater measure [+]Pinkheater power [−]Green

heater measure [−]The sensor serial number and sensitivity of the individual sensors are shown on the
FHF05SC series product certificate and at the end of FHF05SC’s cable.
NOTICE
Putting more than 24 Volt across the sensor wiring
can lead to permanent damage to the sensor.
To apply power to the FHF05SC series heater, it should be connected to a 12 V power
supply.
To measure the power Pheater, the heater can be connected in several different ways,
measuring:•heater voltage and current, Pheater = Uheater∙Iheater•

heater voltage and known heater resistance, Pheater = Uheater2/Rheater (Formula 5.3.1.2)

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•

heater current and known heater resistance, Pheater = Iheater2∙Rheater (Formula 5.3.1.3)

To apply a variable controlled heat flux, the heater can either be connected via a solidstate relay controlled by a pulse-width modulated (PWM) signal or to a 0 – 12 V
programable power supply. The power generated by the heater can be accurately
measured by making a four-point measurement. To this end the heater has a four-wireconnection.

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 FHF05SC series models, 1 and 2, inseries. 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 FHF05SC
series product certificate and on the sticker.

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5.3.3 Connection to read out half signals
See the figure on the left: FHF05SC 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 iscorrectly installed, a constant percentage of the signal will be
generated by the left – and right.

Figure 5.3.3.1 FHF05SC-50X50 with left half indicated by diagonal lines.
Table 5.3.3.1 The electrical connection of FHF05SC 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 FHF05SC 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 FHF05SC 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

thermocouple type T [−]FHF 05S C s e ri e s m an u al v2 204

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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 FHF05SC series are available. In case a program for similar
instruments is available, this can be used. FHF05SC series 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 FHF05SC
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

FHF05SC 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 FHF05SC series. 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 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

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 FHF05SC 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 wires.
Check the datalogger program in particular if the right sensitivity is entered.
FHF05SC series sensitivity and serial number are shown on the product certificate
and on the sticker.
Check if the correct thermocouple type is chosen in the datalogger program.
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 theaverage value. The typical resistance of the wiring is 0.3 Ω/m. Typical resistanceshould be the nominal sensor resistances stated in table 3.1.1 plus 0.6 Ω for the
total resistance of two wires (back and forth) of each m. Infinite resistanceindicates a broken circuit; zero or a lower than 1 Ω resistance indicates a shortcircuit.

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 with the heater) to heatshould generate a positive signal between the red [+] and black [-] wires, doing
the same at the front side (the side with the 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 heater

Check if the correct thermocouple type (Type T) is chosen in the datalogger
programCheck if a correct reference temperature is chosen 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.
Check the electrical resistance between the yellow and purple wires. These
resistances should be in the 0.1 Ω/m range, so 0.2 Ω in case of the standard 2 mwire length. Higher resistances indicate a broken circuit. Repeat this measurement
for the pink and green wire.
Check the resistance between the yellow or purple wires and the green or pinkwires.
Use a multimeter at the 1 kΩ range. Typical resistance should be around 100 Ω.
Infinite resistance indicates a broken circuit; zero or a lower than 1 Ω resistanceindicates a short circuit.

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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, using the integrated heater

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7

Appendices

7.1

Appendix on cable extension

FHF05SC series is equipped with a cable with eight wires. Standard cable length is 2 m.
It is possible to order FHF05SC series with longer cable lengths.
Cables 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 FHF05SC series wires 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 cable extension of FHF05SC series.
Wire

7x copper and 1 x constantan wire, AWG 28, stranded, 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 diameter

typically 2.7 x 10-3 m

Lengthwires should be kept as short as possible, in any case the total wire
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 using FHF05SC series with BLK – GLD sticker
series

BLK black and GLD gold stickers are accessories to 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 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 and FHF05SC series.
BLK - GLD stickers are designed to be applied by the user. Optionally, it is also possible
to order FHF05(SC) with stickers pre-applied at the factory. For more details, see the
BLK – GLD sticker series user manual.

Figure 7.2.1 Models FHF05SC-50X50 heat flux sensor: with BLK-5050 and GLD-5050
stickers.
Table 7.2.1 Recommendations for use of FHF05SC heat flux sensors with BLK – GLD stickers.
Mounting

when mounting an FHF05SC with a BLK or GLD sticker, keep thedirectional sensitivity in mind
heat flux from the back side to the front side (side with dot, side without
the heater) 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

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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

FHF05SC series factory calibration is traceable from SI through international standards
and through an internal mathematical procedure that corrects for known errors. The
formal traceability of the generated heat flux is through voltage and current to electrical
power and 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 FHF05SC series depends on the temperature of the sensor. The
temperature dependence of the FHF05SC 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 7.5.1)

with Φ the heat flux in W/m², U the FHF05SC series voltage output in V, S the sensitivity
in V/(W/m²) at 20 °C and T the FHF05SC temperature.
S is shown on the product certificate and at the end of FHF05SC’s cable.

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7.6

Appendix on measurement range for different temperatures

The measurement range of FHF05SC 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 operating temperature continuous range 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 7.6.1)

This means the measurement range is lower for higher heat sink temperatures.
Φmaximum = (120 – 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 °C43 x 103 W/m²
35 x 103 W/m²
26 x 103 W/m²
18 x 103 W/m²
9 x 103 W/m²

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7.7

Appendix on temperature measurement accuracy

All FHF05SC’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 main heat 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 FHF05SC 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 FHF05SC 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 7.7.1 Model FHF05SC-50X50 with its thermocouple junctions. To minimise
uncertainty, please make sure that ∆T1 is close to zero.

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7.8

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

FHF05SC series, all models
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 215/863 amendment known as RoHS 3

Eric HOEKSEMA
Director
Delft, 15 November 2022

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FHF 05S C s e ri e s m an u al v2 204

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