Hukseflux

Thermal Sensors

USER MANUAL
HFP01 & HFP03
Heat flux plate / heat flux sensor

Copyright by Hukseflux | manual v2124 | www.hukseflux.com | info@hukseflux.com

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

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Contents
Warning statements
Contents
List of symbols
Introduction
1
Ordering and checking at delivery
1.1
Ordering HFP01
1.2
Included items
1.3
Quick instrument check2
Instrument principle and theory
3
Specifications of HFP01
3.1
Specifications of HFP01
3.2
Dimensions of HFP01
4
Standards and recommended practices for use
4.1
Heat flux measurement in building physics
4.2
Heat flux measurement in meteorology
5Installation of HFP01
5.1
Site selection and installation in building physics
5.2
Site selection and installation in meteorology / the soil
5.3
Electrical connection
5.4
Requirements for data acquisition / amplification
6
Making a dependable measurement
6.1
Uncertainty evaluation
6.2
Typical measurement uncertainty budget
6.3
Contributions to the uncertainty budget
7
Maintenance and trouble shooting
7.1
Recommended maintenance and quality assurance
7.2
Trouble shooting
7.3
Calibration and checks in the field
8
HFP03
8.1
Introduction HFP03
9
Appendices
9.1
Appendix on cable extension / replacement
9.2
Appendix on standards for calibration
9.3
Appendix on calibration hierarchy
9.4
Appendix on using HFP01 with BLK – GLD sticker series
9.5
EU declaration of conformity

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List of symbols
Quantities

SymbolUnit

Heat flux
Voltage output
Sensitivity
Temperature
Temperature differenceTime constant
Thermal resistance per unit area
Λ-Value, thermal conductance
Thermal resistance per unit area,
including ambient air boundary layer resistances
U-Value, thermal transmittanceTime
Thermal conductivity
Thermal resistivity
Ambient air / wind speed
Volumic heat capacity
ResistanceHeat transfer coefficient
Convection heat transfer coefficient
Radiation heat transfer coefficient
Storage term
Thermal conductivity dependence
Depth of installation
Water content (on mass basis)

Φ
U
S
T
ΔT
τ
Rthermal,A
Λ

W/m²
V
V/(W/m2)°C°C, Ks
K/(W/m²)
W/(m²·K)

Rthermal,A,B
U
t
λ
r
V
cvolumic
R
h
hc
hr
S
Dλ
x
Q

K/(W/m²)
W/(m²·K)s
W/(m∙K)m∙K/W
m/s
J/(m³∙K)Ω
W/(m2·K)
W/(m2·K)
W/(m2·K)
W/m²%/(W/(m∙K))
mkg/kg

Subscripts
property of thermopile sensor
property of ambient air
calibration reference condition
property of the object on which HFP01 is mounted
property at the (wall or soil) surface
property at indoor location
property at outdoor location
property of the surrounding environment

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sensorambient
reference
objectsurfaceindooroutdoorenvironment

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Introduction
HFP01 is the world’s most popular sensor for heat flux measurement in the soil as well as
through walls and building envelopes. The total thermal resistance is kept small by using
a ceramics-plastic composite body. The sensor is very robust and stable. It is suitable forlong term use on one location as well as repeated installation when a measuring system
is used at multiple locations.
HFP03 is the high-sensitivity version of HFP01. It differs in sensor technology and has alarger size. The sensor working principle and considerations for use are the same. Thismanual is written for model HFP01, but most content is applicable to HFP03 as
well. HFP03 has a dedicated chapter in this manual focusing on the differences
between HFP03 and HFP01.
HFP01 measures heat flux through the object in which it is incorporated or on which it ismounted, in W/m2. The sensor in HFP01 is a thermopile. This thermopile measures thetemperature difference across the ceramics-plastic composite body of HFP01. A
thermopile is a passive sensor; it does not require power.
Using HFP01 is easy. It can be connected directly to commonly used data logging
systems. The heat flux, Φ, in W/m2, is calculated by dividing the HFP01 output, a small
voltage U, by the sensitivity S.
The measurement function of HFP01 is:Φ = U/S(Formula 0.1)

The sensitivity is provided with HFP01 on its product certificate.

Figure 0.1 HFP01 heat flux plate / sensor. The opposite side has a blue coloured cover.

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A typical measurement location is equipped with 2 heat flux sensors for good spatial
averaging. If the sensitivity of a single sensor is too low, two or more sensors canelectrically be put in series, creating an amplified single output signal.
HFP01 can be used for on-site measurement of building envelope thermal resistance perunit area (R-value) and thermal transmittance (U-value) according to the standardised
practices of ISO 9869, ASTM C1046 and ASTM 1155.
Equipped with heavy duty cabling, protective covers at both sides and potted so thatmoisture does not penetrate the sensor, HFP01 has proven to be very robust and stable.
It survives long-term installation in soils, as well as repeated installation when ameasuring system such as TRSYS01 is used at multiple locations.
Suggested use of HFP01:• building heat flux
• U-value and R-value measurements• soil heat flux

Figure 0.2 HFP01. The opposite side has a red coloured cover. Standard cable length is 5 m.
The uncertainty of a measurement with HFP01 is a function of:• calibration uncertainty• differences between reference conditions during calibration and measurement
conditions, for example uncertainty caused by temperature dependence of thesensitivity• the duration of sensor employment (involving the non-stability)• application errors: the measurement conditions and environment in relation to thesensor properties, the influence of the sensor on the measurand, therepresentativeness of the measurement location
The user should make his own uncertainty evaluation. Detailed suggestions forexperimental design and uncertainty evaluation can be found in the following chapters.

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HFP01 calibration is traceable to international standards. The factory calibration method
follows the recommended practice of ASTM C1130. The recommended calibration intervalof heat flux sensors is 2 years.
Would you like to study energy transport / heat flux in detail? Hukseflux helps taking thismeasurement to the next level: order HFP01 with radiation-absorbing black and radiationreflecting gold stickers. You can then measure convective + radiative flux with one, andconvective flux only with the other. Subtract the 2 measurements and you have radiativeflux. They can be applied to the sensor by the user or ordered pre-applied at the factory;
see the BLK – GLD sticker series user manual and installation video for instructions.

Figure 0.3 HFP01 heat flux plate: with BLK-80 and GLD-80 stickers to measure radiativeand convective heat fluxes separately
See also:
•
•

•
•
•

if measuring in soil, in case a high level quality assurance and accuracy of themeasurement is needed, consider use of model HFP01SC.
model HFP03 for increased sensitivity; an alternative is putting two or more HFP01’s
electrically in series. HFP03 specifications can be found in a dedicated chapter of thismanual.
BLK - GLD sticker series to separate radiative and convective heat fluxes
view our complete product range of heat flux sensors.
view the TRSYS01 building thermal resistance measuring system which includes
2 x HFP01 and 4 x matched thermocouples type K.

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1

Ordering and checking at delivery

1.1

Ordering HFP01

The standard configuration of HFP01 is with 5 metres cable.
Common options are:••••longer cable in multiples of 5 m, cable lengths above 20 m in multiples of 10 m.
specify total cable length.
BLK-80 black sticker (to measure radiative as well as conventive heat flux)
GLD-80 gold sticker (to measure convective heat flux only)
BLK - GLD sticker series can also be ordered pre-applied at the factory

1.2

Included items

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

heat flux sensor HFP01
cable of the length as ordered
product certificate matching the instrument serial number

1.3

Quick instrument check

A quick test of the instrument can be done by connecting it to a multimeter.
1 Check the electrical resistance of the sensor between the green [-] and white [+]wires. Use a multimeter at the 100 Ω range. Measure the sensor resistance first with one
polarity, then reverse the polarity. Take the average value. The typical resistance of thewiring is 0.1 Ω/m. Typical resistance should be the nominal sensor resistance of 2 Ω forplus 1.5 Ω for the total resistance of two wires (back and forth) of each 5 m. Infinite
resistance indicates a broken circuit; zero or a lower than 1 Ω resistance indicates a shortcircuit.
2. 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
heat, for instance touching it with your hand. The signal should read > 2 x 10-3 V now.
Touching or exposing the red side should generate a positive signal, doing the same at
the opposite side the sign of the output voltage reverses.
3. Inspect the instrument for any damage.
4. Check the sensor serial number and sensitivity on the 2 x cable labels (one at sensor
end, one at cable end) against the certificate provided with the sensor.

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2

Instrument principle and theory

HFP01’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”.
HFP01 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 HFP01 is a thermopile. This thermopile measures the temperature
difference across the ceramics-plastic composite body of HFP01. Working completely
passive, the thermopile generates a small voltage that is a linear function of thistemperature difference. The heat flux is proportional to the same temperature difference
divided by the effective thermal conductivity of the heat flux sensor body.
Using HFP01 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.
1

2

3

4

5
6

Figure 2.1 The general working principle of a heat flux sensor. The sensor inside HFP01
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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Heat flux sensors such as HFP01, for use in the soil and on building envelopes, aretypically 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 350 W/m2

Unique features of HFP01 are:
•
•
•
•
•
•

low thermal resistance (essential for use on walls and windows)
large guard area (required by the ISO 9869 standard)
low electrical resistance (low pickup of electrical noise)high sensitivity (good signal to noise ratio in low-flux enviroments such asbuildings)
robustness, including a strong cable
IP protection class: IP67 (essential for outdoor application)

Measuring with heat flux sensors, errors may be caused by differences between
calibration reference conditions and the conditions during use. The user should analyse
his own experiment and make his own uncertainty evaluation. Comments on the mostcommon error sources can be found in the chapter about uncertainty evaluation.

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3

Specifications of HFP01

3.1 Specifications of HFP01
HFP01 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, HFP01 generates a small output voltage proportional to this flux. It can only be
used in combination with a suitable measurement system. The sensor should be used inaccordance with the recommended practices of ISO and ASTM.
Table 3.1 Specifications of HFP01 (continued on next page)
HFP01 SPECIFICATIONS
Sensor type
Sensor type according to ISO 9869
Sensor type according to ASTM
Measurand
Measurand in SI units
Measurement range
Sensitivity range
Sensitivity (nominal)
Directional sensitivity
Increased sensitivity
Expected voltage output

Measurement function / required
programming
Required readout
Rated operating temperature range
Temperature dependence
Non-stability
Thermal conductivity dependence
Sensor diameter including guard
Sensing area
Sensing area diameter
Passive guard area
Guard width to thickness ratio
Sensor thickness
Sensor thermal resistanceSensor thermal conductivity
Response time (95 %)Sensor resistance range
Required sensor power

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heat flux plate / heat flux sensor
heat flow meterheat flow sensor or heat flux transducer
heat flux
heat flux density in W/m2
-2000 to 2000 W/m2
40 to 80 x 10-6 V/(W/m2)
60 x 10-6 V/(W/m2)
heat flux from the red to the blue side 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. Also see model HFP03
application in meteorology: -10 to +20 x 10-3 V
application in building physics: -10 to +75 x 10-3 V
turning the sensor from one side to the other will leadto a reversal of the sensor voltage output
Φ = U/S
1 x differential voltage channel or 1 single ended
voltage channel, input resistance > 106 Ω-30 to +70 °C< 0.1 %/°C
< 1 %/yr(for typical use in meteorology and building physics)
7 %/(W/(m·K)) (order of magnitude only)
80 x 10-3 m
8 x 10-4 m2
32 x 10-3 m
42 x 10-4 m2(a passive guard is required by ISO 9869)
5 m/m(as required by ISO 9869 D.3.1)
5.4 x 10-3 m
71 x 10-4 K/(W/m2)
0.76 W/(m·K)
180 s
1 to 4 Ωzero (passive sensor)

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Table 3.1 Specifications of HFP01 (started on previous page, continued on the next page)
Standards governing use of the
instrument

Standard cable length (see options)
WiringCable diameter
Cable markers
IP protection class
Rated operating relative humidity range
Gross weight including 5 m cable
Net weight including 5 m cable

INSTALLATION AND USE

Recommended number of sensors
Orientation
Installation
Cable extension
Mounting with double sided tape

CALIBRATION
Calibration traceability
Production report
Calibration method
Calibration hierarchy
Calibration uncertainty
Recommended recalibration interval
Calibration reference conditions
Validity of calibration
Field calibration

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ISO 9869 Thermal insulation – Building elements – Insitu measurement of thermal resistance and thermaltransmittance.
ASTM C 1155-95 Standard Practice for Determining
Thermal Resistance of Building Envelope Components
from the In Situ Data.
ASTM 1046-95 Standard Practice for In Situ
Measurement of Heat Flux and Temperature on
Building Envelope Components
5m
0.15 m wires and shield at cable end
4 x 10-3 m
2 x sticker, 1 x at sensor and 1 x cable end, wrappedaround the heat flux sensor cable. Both stickers show
sensitivity and serial number.
IP67
0 to 100 %approx. 0.5 kg
approx. 0.5 kg

2 per measurement locationno preferred orientation
see recommendations in this user manual
see chapter on cable extension or order sensors withlonger cable
we recommend use of double-sided “removable”
carpet laying tape such as TESA 4939, which has freeremovability up to 14 days from the most common
surfaces (needs to be tested individually before
usage)to SI units
included(showing calibration result and traceability)
method HFPC01, according to ASTM C1130
from SI through international standards and throughan internal mathematical procedure< 3 % (k = 2)
compliant with ISO 9869 requirement < 2 % (k = 1)
2 years
20 °C, heat flux of 350 W/m2, thermal conductivity ofthe 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
is possible by comparison to a calibration reference
sensor. usually mounted side by side, alternatively
mounted on top of the field sensor. Preferablyreference and field sensor of the same model andbrand. Typical duration of test > 24 h

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Table 3.1 Specifications of HFP01 (started on previous 2 pages)
MEASUREMENT ACCURACY
Uncertainty of the measurement
VERSIONS / OPTIONSLonger cableWith black sticker appliedWith gold sticker applied
ACCESSORIES
Handheld read-out unit

Separate black stickers
Separate gold sticker

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statements about the overall measurement
uncertainty can only be made on an individual basis.
see the chapter on uncertainty evaluationin multiples of 5 m, cable lengths above 20 m in
multiples of 10 m
option code = total cable length
BLK-80 applied to the sensor at the factory to absorb
radiation
GLD-80 applied to the sensor at the factory to reflect
radiation

LI19
programmed LI19 handheld read-out unit /datalogger, two spare batteries, one USB cable,
software and a transport case
BLK-5050 to absorb radiation, to be applied by the user
GLD-5050 to reflect radiation, to be applied by the user

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3.2

Dimensions of HFP01

1
3

2

5m

5.4
80

Figure 3.2.1 HFP01 heat flux sensor dimensions in x 10-3 m
(1)
(2)
(3)

sensing areapassive guard of ceramics-plastic composite
cable (standard length 5 m, optionally longer cable in multiples of 5 m, cable
lengths above 20 m in multiples of 10 m.)

Total sensor thickness including covers is 5.4 x 10-3 m.

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4

Standards and recommended practices
for use

HFP01 should be used in accordance with the recommended practices of ISO and ASTM.

4.1

Heat flux measurement in building physics

Many HFP01 sensors measure heat flux in buildings, estimating the building’s energy
budget and thermal transmission of walls. Typically the total measurement system
consist of multiple heat flux- and temperature sensors, sometimes combined withmeasurements of solar radiation, wind speed and wind direction.

Figure 4.1.1 HFP01 heat flux sensors in use on a wall
Hukseflux offers a complete measurement system for analysis of building envelopes:
TRSYS01.
Table 4.1.1.1 contains a listing of applicable standards. We recommend users topurchase the latest version of the standard.

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4.1.1 Applicable standards
Table 4.1.1.1 Standards with recommendations for instrument use in building physics
STANDARDS FOR INSTRUMENT USE FOR BUILDING ENVELOPE THERMAL RESISTANCEMEASUREMENTISO STANDARD
EQUIVALENTASTM STANDARD
ISO 9869 Thermal insulation – Building
elements – In-situ measurement of thermal
resistance and thermal transmittance

ASTM C 1155-95 Standard Practice for
Determining Thermal Resistance of Building
Envelope Components from the In Situ Data
ASTM 1046-95 Standard Practice for In Situ
Measurement of Heat Flux and Temperature on
Building Envelope Components

4.1.2 ISO 9869 thermal conductance (Λ-value) and transmittance (U-value)
ISO 9869 may be applied both in “hot-box” steady state laboratory methods and in longterm averaging in field measurements. In this standard the heat flux sensor name is heatflow meter (HFM).
ISO 9869 makes a distinction between:••

thermal resistance R from surface to surface by conduction, calculated from heat
flux and surface temperature difference (or the inverse value: Λ-Value or thermal
conductance)
thermal resistance RT from environment to environment by convection plus
conduction, calculated from heat flux and ambient air temperature difference (orthe inverse value: U-value, or thermal transmittance)

At Hukseflux we typically measure the wall thermal conductance using surface
temperatures on the wall:Λ-value = 1/Rthermal,A= Φ/(Tsurface,indoor – Tsurface,outdoor)(Formula 4.1.2.1)

The thermal resistance Rthermal,A of an old insulated wall is of the order of 2.5 K/(W/m2), a
modern insulated wall may attain 6.7 K/(W/m2).
When measuring the thermal transmittance:
U-value = 1/Rthermal A,B(Formula 4.1.2.2)

The U-value includes Rambient,indoor and Rambient,outdoor thermal boundary layer plus radiativetransport resistance.

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Rthermal A,B = Rthermal,A + Rambient,indoor + Rambient,outdoor(Formula 4.1.2.3)

A typical assumption for non-ventilated walls is, for 2 surfaces, using the figures ofequation 4.1.2.6 below:
Rambient,indoor + Rambient,outdoor = 0.25 K/(W/m2)(Formula 4.1.2.4)

The convective transport of heat from the wall to the ambient air, Φ, is a function of theconvection heat transfer coefficient, hc, and the temperature difference between ambient
air and sensor.
Φ = hc·(Tambient – Tobject ) = 1/Rambient(Formula 4.1.2.5)

In buildings under indoor conditions we expect wind speeds of < 1 m/s. Working
environments will typically have wind speeds < 0.5 m/s. Outdoors, wind speeds may
reach 15 m/s under normal conditions, and up to 60 m/s in case of heavy storm.
An approximation of the heat transfer coefficient at a single surface at moderate ambient
air speeds, V, and taking 5 W/(m2·K) for the radiative transfer coefficient, is given by:
h = hr + hc = 5 + 4·V(Formula 4.1.2.6)

According to ISO 9869, A.3.1, a common value for the heat transfer coefficient by
convection, hc, for a single surface is 3.0 W/(m2·K); in the equation above this would
represent a wind speed of 0.75 m/s. The total heat transfer coefficient h for one surface
then is 8 W/(m2·K). For two surfaces it the Rambient,indoor + Rambient,outdoor then becomes 0.25
of equation 4.1.2.4. The radiative heat transfer coefficient of 5 W/(m2·K) follows from the
Stefan–Boltzmann law, linearised around 20 °C.
Measuring the thermal resistance of a building element, the duration of the test
according to ISO 9869 should at least be 96 h (ISO 9869 paragraph 7.2.3). The user
should verify the representativeness of the area with a thermal camera. The installation
should not be in the vicinity of potential sources of error such as thermal bridges, cracks,
heating or cooling devices and fans. Sensors should not be exposed to rain, snow, anddirect solar radiation.
Installation is described in ISO 9869 paragraph 6.1.2. The standard recommends use ofthermal paste and a passive guard ring with a width to thickness ratio of >5. Hukseflux
discourages the use of thermal paste because it tends to dry out. Silicone glue anddouble sided tape are more reliable. HFP01 is equipped with a guard ring.
In some cases only night time data may be included in the analysis. At the end of a test
the obtained R–value should not deviate by more than ± 5 % from the value obtained 24
h before. Chapters 7 and 8 of the ISO standard describe corrections for storage effects(changes in average wall temperature), added thermal resistance by the heat flux
sensor, which we call the resistance error, and errors caused by the finite dimensions of
the sensor. We use the term deflection error, while ISO uses the term operational error.
ISO 9869 chapter 9 shows examples of uncertainty evaluation, arriving at typical

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uncertainties of the order of ± 20 % of on-site measurements of thermal resistances(between 14 and 28 %).
Sensors for measurement of temperature difference should be calibrated to an accuracyof ± 0.1 °C, reference paragraph 5.2.
Annex D.3.2 states that “the width of the guard ring should be at least 5 times the
thickness of the heat flux meter”.
ASTM C 1155 and ASTM 1046 focus on the measurement of thermal resistance R (fromsurface to surface) only. This is the Rthermal,A of equation 4.1.2.1.
ASTM 1155 defines a Heat Flow Sensor or Heat Flux Transducer (HFT). Paragraph 5.8
specifies that during the test the indoor temperature changes less than 3 °C, andspecifies that the density of the construction material is < 440 kg/m3. Areas with a highlateral heat flux should be avoided. Time constants should be estimated according to
ASTM 1046. The duration of the test is at least 24 h, and a convergence test may be
used to determine total required timespan.
ASTM 1046 offers good practices for installation and site selection.

4.1.3 Measurements on glass windows
HFP01 may be mounted on glass windows; please note the following:••

we recommend using night-time data only. During daytime, the window material
typically transmits solar radiation, while the HFP01 absorbs this radiation. During
daytime the measurement is not representative of the heat flux through thewindow
the user may correct for the resistance error

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4.2

Heat flux measurement in meteorology

Many HFP01 sensors are used to measure heat flux in soils, as part of meteorologicalsurface flux measurement systems. Typically the total measuring system consists ofmultiple heat flux- and temperature sensors, combined with measurements of air
temperature, humidity, solar- or net radiation and wind speed.
In meteorological applications a heat flux sensor measures the energy that flows throughthe soil, typically at around 0.05 m depth. Usually this measurement is combined withmeasurements of the soil temperature to be able to estimate the heat flux at the soil
surface. Knowing the heat flux at the soil surface, it is possible to “close the energybalance" and estimate the uncertainty of the measurement of the other (convective andevaporative) fluxes.
In most meteorological experiments, the main source of energy during daytime isdownward solar radiation. The maximum power of the sun is about 1500 W/m2, aroundnoon at low latitudes under clear sky conditions. The solar radiation is either reflected orabsorbed by the soil. The absorbed heat is divided between evaporation of water, heatingof the ambient air and heating of the soil. At night, the sun is no longer present, the net
irradiance is upward. The soil then looses energy through far infra-red radiation to thesky. The maximum upward net irradiance is about 150 W/m2, under clear sky conditions.
The heat flux in the soil at 0.05 m depth is usually between -100 and +300 W/m2.
Measuring soil heat flux with HFP01, the main sources of uncertainty are:
1. representativeness: measurement at one location has an uncertain validity for thelarger area under observation
2. deflection errors that cannot be corrected because soil thermal properties are
unknown and variable over time and the contact to the soil may be unreliable
3. temperature dependence
4. non-stability
When estimating the surface heat flux at the soil surface, as opposed to the flux at 0.05
m depth, there is a fifth source of uncertainty:
5. uncertainty of the storage term (not formally part of the HFP01 measurement)
In case a more accurate measurement is required: see model HFP01SC self calibratingheat flux plate / sensor.
Ad 1: In field experiments it is difficult to find a single location that is representative of
the whole region. On a limited timescale effects of shading of the soil surface can give an
unrepresentative measurement. To be less sensitive to such effects, we recommend
using two sensors for each station or measurement location, usually at a distance of > 5m.

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Ad 2: A second significant source of measurement uncertainty in soil heat flux
measurement is the deflection error. This error may be modelled as a change of thesensitivity of the heat flux sensor as a function of the thermal conductivity of thesurrounding environment. This deflection error is described in detail in chapter 6. The
properties of the surrounding environment, soil, are unknown and also and change withtime as a function of the soil moisture content.
A typical HFP01 has a thermal conductivity of 0.8 W/(m·K), while thermal conductivity ofsoils can vary between 0.2 and 2.5 W/(m·K). Sand in relatively dry condition has athermal conductivity in the order of 0.2 W/(m·K), saturated with water it may reach 2.5
W/(m·K). A heat flux sensor model HFP01 performing a correct measurement in dry sandwill make a significant error (> 10 %) in saturated sand. The calibration reference
condition actually is 0 W/(m·K), so in absolute terms the heat flux will always be
underestimated.
Ad 3: The third important source of uncertainty is temperature dependence. Over theentire temperature range from -30 to +70 °C, the uncertainty is ± 5 %.
Ad 4: Soil heat flux sensors are preferably left as long as possible in the soil, so that thesoil properties and the soil surface become representative of the typical conditions of thearea under observation. The sensor sensitivity however potentially changes with time.
Under normal conditions this change is < 1 %/yr. The user must excavate HFP01 sensors
to verify their stable performance by laboratory calibration.

Figure 4.2.1 typical meteorological surface energy balance measurement system with
HFP01 installed under the soil.

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Ad 5: For various reasons, practical as well as scientific, the heat flux plate must be
installed under the soil, and not directly at the soil surface. First, mounting at the surface
would distort the flow of moisture, and the measured flux would no longer be
representative for the flux in the surrounding soil. Second, the absorption of solar
radiation would not be representative. Third, the sensor would be more vulnerable. The
mechanical stability of the installation then becomes an uncertain factor. Heat flux
sensors in meteorological applications are typically buried at a depth of about 0.05 m
below the soil surface. Installation at a depth of less than 0.05 m is not recommended.
In most cases a 0.05 m soil layer on top of the sensor offers just sufficient mechanicalstability to guarantee stable measurement conditions. Installation at a depth of morethan 0.08 m is not recommended, because at larger depths of installation the time delay
and amplitude of the measured heat flux becomes less accurately traceable tomomentous flux at the soil surface.
For the above reasons the flux at the soil surface Φsurface is usually estimated from theflux measured by the heat flux sensor plus the change of the energy stored in the layer
above the sensor during the measuring interval t1 to t2.
Φsurface = Φ0.05 m + S(Formula 4.2.1)

The quantity S is called the storage term.
The storage term is calculated from a space-averaged soil temperature measurement,using multiple soil temperature sensors, and an estimate of the volumic heat capacity
cvolumic of the soil above the sensor.
S = (T(t1) - T(t2)).cvolumic·x/(t1 - t2)(Formula 4.2.2)

Where T(t1) - T(t2) is the temperature difference in the measurement interval, x thedepth of installation of the soil heat flux sensors.
A correct estimate of Φsurface with a high time resolution requires a low installation depth
and a correct estimate of the storage term.
At an installation depth of 0.05 m, the storage term typically represents up to 50 % of
the total Φsurface. When the temperature T is measured closely below the surface, theresponse time of the storage term to a changing Φsurface is in the order of magnitude of
20 min, while the heat flux sensor buried at twice the depth is a factor 5 slower (squareof the depth). The volumic heat capacity is estimated from the specific heat capacity ofdry soil, csoil, dry, the bulk density of the dry soil ρ, the water content (on mass basis), Q,
and cwater, the volumic heat capacity of water.
cvolumic = ρsoil,dry·csoil,dry + Q·cvolumic,water(Formula 4.2.3)

The heat capacity of water is known, but the other quantities of the equation are difficult
to determine and vary with location and time. The storage term may be the main sourceof uncertainty in the soil energy balance measurement.
A typical value for dry soil heat capacity is 840 J/(kg·K).

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5

Installation of HFP01

5.1

Site selection and installation in building physics

Table 5.1.1 Recommendations for installation of heat flux sensors in building physics(continued on the next page)
Location

preferably mount heat flux sensors indoors and not outdoors
preferably use a large wall section which is relatively homogeneousin the northern hemisphere, north-facing walls are preferred
do not expose 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 fansclose window blinds and curtains
switch off artificial light sources
for detailed analysis of a single building element users may consider toinstall one heat flux sensor indoors, and the other outdoors. measuring
on two sides permits a detailed analysis of the thermal response time.
for mounting sensors on glass windows: see paragraph 4.1.3.

Orientation

the two sensor sides are equivalent.
recommended orientation is with the red face of the sensor facing
indoors, and the blue face connected to the indoor wall. This generates a
positive output signal when the heat flux direction is from indoors to
outdoors.
reversing the sensor orientation will result in a change of sign of thevoltage output. If necessary, this may be compensated by reversing thewiring at the datalogger connection, or in the post processing for example
by giving the sensitivity a negative sign.

Performing a
representative
measurement

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

Creating atemperature
difference

for the measurement of thermal resistance of walls it is best to have aconstant high level heat flux; strongly cooled or strongly heated rooms
are ideal. We recommend activating heaters or air conditioning to create
optimal conditions.

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Table 5.1.1 Recommendations for installation of heat flux sensors in building physics(started on the previous page)Mechanical mounting

avoid any air gaps between sensor and wall. Air thermal conductivity is in
the 0.02 W/(m·K) range, while a common glue has a thermal conductivity
around 0.2 W/(m·K). A 0.1 x 10-3 m air gap increases the effective
thermal resistance of the sensor by 60 %.
we recommend use of double-sided “removable” carpet laying tape suchas TESA 4939, which has free removability up to 14 days from the mostcommon surfaces (needs to be tested individually before usage).
for thermocouple mounting the same tape may be used as for the heat
flux sensors.
for long-term installation and for filling up large gaps, use silicone
construction sealant, glue or adhesive, that can be bought in construction
depots. During curing of the silicone, typically 24 h, the sensor must be
temporarily held in place by other means.
we discourage the use of thermal paste because it tends to dry out.
silicone glue and double sided tape are more stable and reliable.
usually the cables are provided with an additional strain relief, forexample using a cable tie mount as in figure 5.1.1.

Added temperature
sensorsfor Λ-value: temperature sensors should measure wall surface
temperature. They are typically located close to the sensor attached toboth sides of the wall.
for R-value: temperature sensors should measure ambient air
temperature. They are typically located close to the sensor at both sidesof the wall, however not attached to the wall. Ambient air temperature
sensors should be shielded from solar radiation.

Avoiding spectral
errorsin case of exposure solar radiation or to artificial light sources, thespectral properties of the sensor surface must match those of the wall.
This is attained by covering the sensor with paint or sheet material of thesame colour as the wall.

Signal amplification

see the paragraph on electrical connection. In case of low heat fluxes andsignals that are too small, consider use of multiple sensors electrically inseries or consider using model HFP03.

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Figure 5.1.1 Installation of HFP01 on a wall using sided “removable” carpet laying tapesuch as TESA 4939, and a strain relief of the cable using a cable tie mount equipped withthe same carpet laying tape as adhesive.

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5.2

Site selection and installation in meteorology / the soil

Table 5.2.1 Recommendations for installation of heat flux sensors in building physics
Location

preferably install in a large field which is relatively homogeneous andrepresentative of the area under observation.

Orientation

the two sensor sides are equivalent. Recommended orientation is with thered side of the sensor facing up. This generates a positive output signal
when the net heat flux is downward.
reversing the sensor orientation will result in a change of sign of the voltage
output. If necessary, this may be compensated by reversing the wiring at
the datalogger connection, or in the post processing for example by giving
the sensitivity a negative sign.

Performing a
representative
measurement
Installation in thesoil

we recommend using > 2 sensors per location at a distance of > 5 m. This
redundancy also improves the assessment of the measurement accuracy.

Fixation / strain
relieffor mechanical stability provide sensor cables with an additional strain relief,for example connecting the cable with a tie wrap to a metal pin that isinserted firmly into the soil.

Armoured cablein some cases cables are equipped with additional armour to avoid damage
by rodents. Make sure the armour does not act as a conductor of heat or atransport conduit or container of water.

Added
temperature
sensors

temperature sensors are typically located close to the heat flux sensor at 2
depths above it: when the sensor is buried at 0.05 m, temperature sensors
may be buried at 0.02 and 0.04 m below the soil surface.

Improvingmeasurement
accuracy

when measuring in soils, we recommend using model HFP01SC to get ahigher level of quality assurance and accuracy of the measurement. The
HFP01SC self-calibration compensates for the temperature dependence, thenon-stability and the deflection error.

Signalamplification

see the paragraph on electrical connection. In case of low heat fluxes andsignals that are too small to measure, consider use of multiple sensors inseries and consider using model HFP03.

depth of installation typically is 0.05 m.
if possible, install the sensor from the side of a small hole. There should beno air gaps between sensor and soil. A 0.1 x 10-3 m air gap increases theeffective thermal resistance of the sensor by 60 %.
Use a shovel to make a vertical slice in the soil. Make a hole in the soil at
one side of the slice. Keep the excavated soil intact so that after installingthe sensor the original soil structure can be restored. The sensor is installedin the undisturbed face of the excavated hole. Measure the depth from thesoil surface at the top of the hole. With a knife, make a horizontal cut at therequired depth of installation, for example at 0.05 m below the surface, into
the undisturbed face of the hole. Insert the heat flux sensor into thehorizontal cut.
Never run the sensor cable directly to the surface. Bury the sensor cablehorizontally over a distance of at least 1 m, to minimise thermal conduction
through the lead wire. Put the excavated soil back into its original position
after the sensor and cable are installed.

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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. HFP01 is a passive sensor that does not need any power. Cables may act asa source of distortion, by picking up capacitive noise. We recommend keeping the
distance between a datalogger or amplifier and the sensor as short as possible. For cable
extension, see the appendix on this subject.
Table 5.3.1.1 The electrical connection of HFP01. The cable internally also has a brown
wire, which is not used and which is not visible when supplied from the factory. The wires
extend 0.15 m from the cable end.
WIRE

MEASUREMENT SYSTEM

White

signal [+]voltage input [+]Greensignal [‒]voltage input [‒] or groundBlack

groundanalogue groundThe sensor serial number and sensitivity are shown on the HFP01 product certificate andon 2 cable labels (one at sensor end, one at cable end).

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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 2 sensors may be put in series, again increasing thesensitivity.
E = 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 HFP01’s, 1 and 2, in series. In such casethe sensitivity is the sum of the two sensitivities of the individual sensors. More sensors
may be added in a similar manner.
SENSORWIRE

MEASUREMENT SYSTEM

1

White

signal 1 [+]voltage input [+]

1

Green

signal 1 [‒]connected to signal 2 [+]

1

Black

groundanalogue ground

2

White

signal 2 [+]connected to signal 1 [-]

2

Green

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

2

Black

groundanalogue groundThe serial number and sensitivity of the individual sensor are shown on the HFP01
product certificate and on 2 cable labels (one at sensor end, one at cable end).

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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 HFP01 are available. In case a program for similar instruments
is available, this can be used. HFP01 can be treated in the same way as other heat flux
sensors and thermopile pyranometers.
Table 5.4.1 Requirements for data acquisition and amplification equipment for HFP01 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.
E = U/S (Formula 0.1)

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

Making a dependable measurement

6.1

Uncertainty evaluation

A measurement with a heat flux sensor is called “dependable” if it is reliable, i.e.
measuring within required uncertainty limits, for most of the time and if problems, once
they occur, can be solved quickly.
In case of heat flux sensors, the measurement uncertainty is a function of:••••calibration uncertainty
differences between reference conditions during calibration and measurement
conditions, for example uncertainty caused temperature dependence of the sensitivity
the duration of sensor employment (involving the non-stability)
application errors: the measurement conditions and environment in relation to thesensor properties, the influence of the sensor on the measurand, therepresentativeness of the measurement location

It is not possible to give one figure for heat flux sensor measurement uncertainty.
Statements about the overall measurement uncertainty can only be made on an
individual basis, taking all these factors into account.
Guidelines for uncertainty evaluation:
1) The formal evaluation of uncertainty should be performed in accordance with ISO 98-3
Guide to the Expression of Uncertainty in Measurement, GUM.
2) Uncertainties are entered in measurement equation (equation is usually Formula 0.1:
E = U/S), either as an uncertainty in E (non-representativeness, resistance error anddeflection error) in U (voltage readout errors) or in S (non-stability, temperaturedependence, calibration uncertainty).
3) In case of special measurement conditions, typical specification values are chosen.
These should for instance account for environmental conditions (working temperature
range).
4) Among the various sources of uncertainty, some are “correlated”; i.e. present during
the entire measurement process, and not cancelling or converging to zero when
averaged over time; the off-diagonal elements of the covariance matrix are not zero.
Paragraph 5.2 of GUM.
5) Among the various sources of uncertainty, some are “uncorrelated”; cancelling or
converging to zero when averaged over time; the off-diagonal elements of the covariancematrix are zero. Paragraph 5.1 of GUM.

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6.2

Typical measurement uncertainty budget

Table 6.2.1 typical measurement uncertainties (k = 2) when measuring heat flux with
HFP01 heat flux sensors.
APPLICATIONTYPICAL MEASUREMENT UNCERTAINTY BUDGET (k = 2)

Building physics

Under ideal conditions, measurements of heat flux in building physics
may attain uncertainties in the ± 6 % range.
Contributions to the uncertainty budget: calibration, temperaturedependence from -10 to + 30 °C, thermal conductivity of the surrounding
environment from 0.5 to 1.5 W/(m·K), representativeness of themeasurement location.
ISO 9869 chapter 9 shows examples of uncertainty evaluation, forthermal resistance measurement. This uncertainty budget also includes
contributions from the temperature measurements and dynamic effects.
It arrives at typical uncertainties of the order of ± 20 % of on-sitemeasurements of thermal resistances.
Corrections may be applied according to chapter 8 of ISO 9869. These
corrections include corrections for the thermal resistance and corrections
for the finite dimension of the sensor. ISO also calls the latter theoperational error, we use the term deflection error.

Meteorology

Measurements of heat flux in the soil may attain uncertainties in the ±20% range
Contributions to the uncertainty budget: calibration, temperaturedependence from -20 to +50 °C, thermal conductivity of the surrounding
soil from 0.15 to 2.5 W/(m·K), representativeness of the measurement
location.
Not included: estimates of the storage term, which are not part of the
HFP01 measurement.

6.3

Contributions to the uncertainty budget

6.3.1 Calibration uncertainty
HFP01’s factory calibration uncertainty under reference conditions is ± 3 % with acoverage factor k = 2.

6.3.2 Uncertainty caused by non-stability
HFP01’s non-stability specification is < 1 %/yr.
This means that for every year of operation, 1 % uncertainty should be added in theuncertainty budget.

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6.3.3 Uncertainty caused by - and correction of the resistance error
When mounting the sensor in a certain environment or on an object, its thermal
resistance may significantly change the total thermal resistance of the object underobservation (usually a wall). The resulting measurement error is called resistance error.

1
3
2

4
3

Figure 6.3.3.1 The resistance error: a heat flux sensor (2) increases or decreases thetotal thermal resistance of the object on which it is mounted (1) or in which it isincorporated. This can either lead to an increase of or decrease of the measured heat flux
(3).
1

2

3

Figure 6.3.3.2 The resistance error: a heat flux sensor (2) increases or decreases thetotal thermal resistance of the object on which it is mounted or in which it isincorporated. An otherwise uniform flux (1) is locally disturbed. Lines (3) represent
isotherms. In this case the measured heat flux is smaller than the actual undisturbed
flux, (1).

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Taking a sensor mounted on a wall as an example, a first order correction of themeasurement is:Φ = ((Rwall + Rsensor)/Rwall)·(U/S)(Formula 6.3.3.1)

This correction (Rwall + Rsensor)/ Rwall is often applied to measurements on thin or badly
insulated walls and measurements on windows. This correction can only be determinedfor objects with limited (finite) dimensions. For this reason this correction is notapplicable in soils.
Also contact resistances between sensor and the surrounding environment may
contribute to the resistance error. Air gaps add to the sensor thermal resistance. The
contact between sensor and surrounding environment should be as small and as stable
as possible. The thermal conductivity of air is approximately 0.02 W/(m·K), a factor 30
smaller than that of the heat flux sensor. A 0.1 x 10-3 m air gap increases the effective
thermal resistance of the sensor by 60 %. Air gaps form major contact resistances.
Avoiding the presence of air gaps should be a priority whenever heat flux sensors are
installed.

6.3.4 Uncertainty caused by - and correction of the deflection error
The thermal conductivity of the surrounding environment may differ from the sensor
thermal conductivity. The heat flux will then deflect. The resulting measurement error iscalled the deflection error. The deflection error may be estimated by experiments or bynumerical simulation.
Corrections may be applied according to chapter 8 of ISO 9869. These corrections
include corrections for the finite dimension of the sensor. ISO also calls this theoperational error, we use the term deflection error.
As figure 6.3.4.1 illustrates, the deflection error is largest at the sensor edges, andsmaller at the centre. For this reason sensors are equipped with a passive guard aroundthe sensitive sensor area.
For sensors that are fully surrounded by a uniform homogeneous material which have aperfect thermal connection, the deflection error may be expressed as thermal
conductivity dependence Dλ of the sensitivity S. The order of magnitude of Dλ is constantfor one sensor model. The order of magnitude of Dλ is 7 %/(W/(m·K)). The value of
λreference is 0.0 W/(m·K).
S = Sreference·(1+ Dλ·(λenvironment - λreference))(Formula 6.3.4.1)

A correction may be applied when there is a substantial amount (at least 40 x 10-3 m) of
the same material at both sides of the sensor. In soils λenvironment usually is not known.
We discourage correction for this error because it relies on the assumption that theproperties of the surrounding material as well as the contact resistance are known.

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When measuring in soils, we recommend using model HFP01SC to get a higher level ofquality assurance and accuracy of the measurement. The HFP01SC self-calibrationcompensates for the temperature dependence, the non-stability and the deflection error.
1

2

Figure 6.3.4.1 The deflection error. The heat flux (1) is deflected in particular at theedges of the sensor. The measurement will contain an error; the so-called deflection
error. The magnitude of this error depends on the thermal conductivity of theenvironment, sensor thermal conductivity as well as sensor design and contact
resistance.

6.3.5 Uncertainty caused by temperature dependence
HFP01’s temperature dependence specification is < 0.1 %/°C.
This means that for every °C deviation from the 20 °C reference temperature, 0.1 %uncertainty should be added in the uncertainty budget.
When measuring in soils, we recommend using model HFP01SC to get a higher level ofquality assurance and accuracy of the measurement. The HFP01SC self-calibrationcompensates for the temperature dependence.

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7

Maintenance and trouble shooting

7.1

Recommended maintenance and quality assurance

HFP01 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 7.1.1 Recommended maintenance of HFP01. If possible the data analysis is doneon a daily basis.
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 nearby stations or redundant instruments.
Historical seasonal records can be used as a source for
expected values. Look for any patterns and events thatdeviate from what is normal or expected. Compare toacceptance intervals. Plot heat flux data against othermeteorological measurands, in particular net radiation and soil
temperature.

2

6 months

inspection

inspect cable quality, inspect mounting, inspect location of
installation
look for seasonal patterns in measurement data

3

2 years

recalibration

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

4lifetime
assessment

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if it should be replaced

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7.2

Trouble shooting

Table 7.2.1 Trouble shooting for HFP01
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.
Inspect the connection of the shield (typically connected at the datalogger side).
Check the datalogger program in particular if the right sensitivity is entered.
HFP01 sensitivity and serial number are marked on its cable.
Check the electrical resistance of the sensor between the green [‒] and white [+]wires. Use a multimeter at the 100 Ω range. Measure the sensor resistance firstwith one polarity, then reverse the polarity. Take the average value. The typicalresistance of the wiring is 0.1 Ω/m. Typical resistance should be the nominal
sensor resistance of 1 to 4 Ω plus 1.5 Ω for the total resistance of two wires (backand forth) of each 5 m. Infinite resistance indicates a broken circuit; zero or a low
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 heat, for instance touching it with your hand. The signal should read >2 x 10-3 V now. Touching or exposing the red side should generate a positive
signal, doing the same at the opposite side the sign of the output voltage
reverses.
Check the data acquisition by replacing the sensor with a spare unit.

The sensor
signal isunrealisticallyhigh or low

Check the cable condition looking for cable breaks.
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
unexpectedvariations

Check the presence of strong sources of electromagnetic radiation (radar, radio).
Check the condition and connection of the shield.
Check the condition of the sensor cable.
Check if the cable is not moving during the measurement.

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7.3

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) areeliminated.
3) to mount all sensors on the same platform, so that they have the same bodytemperature.
4) typical duration of test: > 24 h5) typical heat fluxes used for comparison: > 20 W/m2
6) to correct deviations of more than ± 10 %. Lower deviations should be interpreted as
acceptable and should not lead to a revised sensitivity.

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8

HFP03

8.1

Introduction HFP03

Sensor model HFP03 is a high-sensitivity version of HFP01. The plate’s diameter is larger.
HFP03 is specifically suitable for measurement of small flux levels, in the order of lessthan 1 W/m2, for instance in geothermal applications. By using a ceramics-plastic
composite body the total thermal resistance is kept small. See also model HFP01: putting
several HFP01 sensors electrically in series is an alternative for using HFP03.

Figure 8.1.1 HFP03 ultra sensitive heat flux sensor. The opposite side has a blue cover.
Table 8.1.1 Specifications of HFP03, listing only those specifications which deviate from HFP01
HFP03 SPECIFICATIONS
Sensor type
Sensitivity (nominal)
Directional sensitivity
Sensor resistance range
Weight including 5 m cable
Sensing area
Sensing area diameter
Passive guard area
Sensor diameter including guard
Uncertainty of calibration
Gross weight including 5 m cable
Net weight including 5 m cable

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ultra sensitive heat flux plate / heat flux sensor
500 x 10-6 V/(W/m2)
heat flux from the white to the blue side generates a
positive voltage output signal
10 to 32 Ω (nominal 18 Ω)
approx. 1 kg
64 x 10-4 m2
8 area’s of 32 x 10-3 m
168 x 10-4 m2(a passive guard is required by ISO 9869)
172 x 10-3 m± 3 % (k= 2)
approx. 0.5 kg
approx. 0.5 kg

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1
3

5.4
172

Figure 8.1.2 HFP03 heat flux sensor dimensions in x 10-3 m
(1)
(2)
(3)

8 x sensing areapassive guard of ceramics-plastic composite
cable (standard length 5 m, optionally longer cable in multiples of 5 m, cable
lengths above 20 m in multiples of 10 m.)

Total sensor thickness including covers is 5.4 x 10-3 m

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.

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9

Appendices

9.1

Appendix on cable extension / replacement

HFP01 is equipped with one cable. Keep the distance between data logger or amplifierand sensor as short as possible. Cables may act as a source of distortion by picking up
capacitive noise. In an electrically “quiet” environment the HFP01 cable may be extended
without problem to 100 metres. If done properly, the sensor signal, although small, will
not significantly degrade because the sensor resistance is very low (which results in good
immunity to external sources) and because there is no current flowing (so no resistive
losses). Cable and connection specifications are summarised below.
Table 9.1.1 Preferred specifications for cable extension of HFP01
Cable

2-wire, shielded, with copper conductor (at Hukseflux 3-wire shieldedcable is used, of which only 2 wires are used)

Extension sealing

make sure any connections are sealed against humidity ingress

Conductor resistance< 0.1 Ω/mOuter diameter4 x 10-3 m

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

Outer mantle

with specifications for outdoor use(for good stability in outdoor applications)

Connection

either solder the new cable conductors and shield to those of the original
sensor cable, and make a waterproof connection using heat-shrink tubing
with hot-melt adhesive, or use gold plated waterproof connectors. Always
connect the shield.

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9.2

Appendix on standards for calibration

The standard ASTM C1130 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 thinheater 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 accordancewith X2.2.
The Hukseflux HFPC01 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 ofoperation described in paragraph 8.2 and in ASTM C1044.
ISO does not have a dedicated standard practice for heat flux sensor calibration. ISO
9869 paragraph 5.1 recommends calibration according to ISO 8302 (guarded hot plate)or ISO 8301 (heat flow meter apparatus) with a ± 2 % accuracy. We assume that thisstatement is with a coverage factor k = 1.
For a known model, paragraph 5.1.2 allows one heat flow, a typical temperature during
use and on a typical building material. ISO 9869 does not describe the calibration
apparatus of method. We follow the recommended practice of ASTM C1130.
Table 9.2.1 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. ISO 9869 allows the user to choose
his calibration method.

ASTM C1130 Standard Practice for CalibratingThin 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 - 12 Standard Practice for Using a
Guarded-Hot-Plate Apparatus or Thin-Heater
Apparatus in the Single-Sided Mode

9.3

Appendix on calibration hierarchy

HFP01 factory calibration is traceable from SI through international standards andthrough an internal mathematical procedure which 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 HFPC01 method follows the recommended practice of ASTM C1130. 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 and in
ASTM C1044. In accordance with ISO 9869, the method has been validated in a firstparty conformity assessment, by comparison to calibrations in a guarded hot plate.

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9.4

Appendix on using HFP01 with BLK – GLD sticker series

BLK-80 and GLD-80 stickers are accessories to HFP01 heat flux sensors. A sensor
equipped with a BLK black sticker is sensitive to both radiative 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, substract the two measurements.
BLK - GLD stickers are designed to be applied by the user. Optionally, it is also possible
to order HFP01 with stickers pre-applied at the factory.
For more details, see the BLK – GLD sticker series user manual.

Figure 9.4.1 HFP01 heat flux sensor: with BLK-80 and GLD-80 stickers
Table 9.4.1 Recommendations for use of HFP01 heat flux sensors with BLK – GLD
sticker series
Mounting

when mounting HFP01 with a BLK or GLD sticker, keep the directional
sensitivity in mind.
heat flux from the back side (coloured blue) to the front side (colouredred) 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

no significant influence on sensitivity

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9.5

EU declaration of conformity

We,

Hukseflux Thermal Sensors B.V.
Delftechpark 31
2628 XJ Delft
The Netherlands

in accordance with the requirements of the following directive:
2014/30/EU

The Electromagnetic Compatibility Directive

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

HFP01
Heat flux sensor / heat flux plate

has been designed to comply and is in conformity with the relevant sections andapplicable requirements of the following standards:
Emission:
Immunity:
Emission:
Emission:
Report:EN 61326-1 (2006)
EN 61326-1 (2006)
EN 61000-3-2 (2006)
EN 61000-3-3 (1995) + A1 (2001) + A2 (2005)
08C01340RPT01, 06 January 2009

Eric HOEKSEMA
Director
Delft
September 08, 2015

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© 2022, Hukseflux Thermal Sensors B.V.
www.hukseflux.com
Hukseflux Thermal Sensors B.V. reserves the right to change specifications without notice.