Hukseflux

Thermal Sensors

USER MANUAL HFP01SC
Self-calibrating heat flux sensorTM

Copyright by Hukseflux | manual v2026 | 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.
Putting more than 20 Volt across the heater wiring
can lead to permanent damage to the heater.
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 HFP01SC
1.2
Included items
1.3
Quick instrument check2
Instrument principle and theory
2.1
General heat flux sensor theory
2.2
The self-test and self-calibration
2.3
Programming the self-test and self-calibration
3
Specifications of HFP01SC
3.1
Dimensions of HFP01SC
4Standards and recommended practices for use
5
Installation of HFP01SC
5.1
Site selection and installation
5.2
Electrical connection
5.3
Requirements for data acquisition / amplification
6
Making a dependable measurement
6.1
Uncertainty evaluation
6.2
Typical measurement uncertainties
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
Appendices8.1
Appendix on cable extension / replacement
8.2
Appendix on standards for calibration
8.3
Appendix on calibration hierarchy
8.4
Electrical connection of HFP01SC supplied by Campbell Scientific USA
8.5
EU declaration of conformity

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List of symbols
Quantities

SymbolUnit

Heat flux
Voltage output
Sensitivity
Temperature
Temperature differenceTime constantTime
Thermal conductivity
Thermal resistivity
Volumic heat capacity
Resistance
Storage term
Depth of installation
Water content (on mass basis)
Water content (on volume basis)

Φ
U
S
T
ΔT
τ
t
λ
r
cvolumic
R
S
x
Q
QV

W/m²
V
V/(W/m2)
°C
°C, K
s
s
W/(m∙K)m∙K/W
J/(m³∙K)
Ω
W/m²
m
kg/kg
m³/m³

Subscripts
property of thermopile sensor
property obtained under calibration reference
conditions
property at the (soil) surface
property of the surrounding soil
property of the heater
property obtained by self-test
property obtained by self-calibration
property of the current-sensing resistor

HFP01SC manual v2026

sensorreference
surfacesoil
heaterselftest
selfcalibration
current

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Introduction
HFP01SC self-calibrating heat flux sensorTM is a heat flux sensor for use in the soil. It
measures soil heat flux in W/m2 and offers the best available accuracy and quality
assurance of the measurement. The on-line self-test verifies the stable performance andgood thermal contact of sensors that are buried and cannot be visually inspected andtaken to the laboratory for recalibration. The self-test also includes self-calibration whichcorrects for measurement errors caused by the thermal conductivity of the surrounding
soil (which varies with soil moisture content), for sensor non-stability and fortemperature dependence.
The total thermal resistance is kept small by using a ceramics-plastic composite body.
Equipped with heavy duty cabling, protective covers at both sides and potted so thatmoisture does not penetrate the sensor, HFP01SC has proven to be very robust andstable. It survives long-term installation in soils.
In essence, HFP01SC is a combination of a heat flux sensor and a film heater. The heat
flux sensor output is a voltage signal that is proportional heat flux through the sensor. At
a regular interval the film heater is activated to perform a self-test. The self-test results
in a verification of sensor contact to the soil and in a new sensitivity that is valid for thecircumstances at that moment. The latter is called self-calibration. Implicitly also cableconnection, data acquisition and data processing are tested. The result is a much
improved accuracy & quality assurance of the measurement relative to measurementswith conventional sensors such as model HFP01. Soil heat flux sensors are preferably left
in the soil for as long as possible, so that the soil properties become representative of thelocal conditions. Using self-testing, the user no longer needs to take sensors to thelaboratory to verify their stable performance.
The sensor in HFP01SC is a thermopile. This thermopile measures the temperature
difference across the ceramics-plastic composite body of HFP01SC. A thermopile is apassive sensor; it does not require power. HFP01SC can be connected directly tocommonly used data logging systems. The heat flux, Φ, in W/m2, is calculated by
dividing the HFP01SC output, a small voltage U, by the sensitivity Sreference.
The measurement function for HFP01SC is:Φ = U/Sreference(Formula 0.1)

The factory-determined sensitivity Sreference , as obtained under calibration reference
conditions, is provided with HFP01SC on its product certificate.
HFP01SC calibration is traceable to international standards. The factory calibration
method follows the recommended practice of ASTM C1130. The recommended calibrationinterval of common heat flux sensors is 2 years. With HFP01SC, using the self-test, thismay be extended to 5 years.

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Every 6 h, the HFP01SC film heater is switched on to perform a self-test. During the selftest the normal heat flux measurement is interrupted.
Analysis of the heat flux sensor response to heating, the self-test, serves two purposes:• the amplitude and response time are indicators of the quality of contact of the sensor
to the soil.
• the signal level during self-testing is used for self-calibration, which results in a newsensitivity Sselfcalibration.
If response time and signal level are within user-determined acceptance limits, from themoment a new sensitivity has been determined the user works with:
Φ = U/Sselfcalibration(Formula 0.2)

The new sensitivity Sselfcalibration compensates for:• the deflection error caused by non-perfect matching of the thermal conductivity of
sensor and soil, including changes of the thermal conductivity of the soil caused bychanging moisture content• temperature dependence of the sensitivity of the heat flux sensor• non-stability of the heat flux sensor
Additional quality assurance is offered by:• monitoring of seasonal and yearly patterns of the sensitivity Sselfcalibration, whichquantifies the sensor stability
A typical measurement location is equipped with 2 heat flux sensors for good spatial
averaging.

Figure 0.1 HFP01SC self-calibrating heat flux sensor. The opposite side has a blue cover.

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Figure 0.2 HFP01SC. The opposite side has a red cover. Standard cable length is 5 m (2 cables).
The uncertainty of a measurement with HFP01SC is a function of:• calibration uncertainty, use of self-calibration• 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
The user should make his own uncertainty evaluation. Detailed suggestions forexperimental design and uncertainty evaluation can be found in the following chapters.
See also:• in case a less accurate measurement is sufficient, consider model HFP01• view our complete product range of heat flux sensors

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1

Ordering and checking at delivery

1.1

Ordering HFP01SC

The standard configuration of HFP01SC is with 2 x 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.

1.2

Included items

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

heat flux sensor HFP01SC
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 [+] wiresof cable [1]. 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 2 Ω for plus 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 Ωresistance indicates a short circuit.
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, or activating the HFP01SC heater by
putting 9 to 12 VDC across the green and brown wires of cable [2]. 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 reverses.
3. Check the electrical resistance of the film heater between the wires of cable [2]. Use amultimeter at the 1000 Ω range. Typical resistance should be the typical heater
resistance of 100 Ω ± 15 %. Infinite resistance indicates a broken circuit; zero or a lower
than 1 Ω resistance indicates a short circuit.
4. Inspect the instrument for any damage.
5. Check the sensor serial number, and sensitivity on the cable labels of cable [1] (one at
sensor end, one at cable end) against the product certificate provided with the sensor.
6. Check the heater resistance value in Ω on the product certificate.

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2

Instrument principle and theory

HFP01SC’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”. HFP01SC users typically assume that the measured heat flux isrepresentative of the undisturbed heat flux at the location of the sensor. Users may also
apply corrections based on scientific judgement.
HFP01SC has an integrated film heater. At a regular interval the film heater is activatedto perform a self-test. The self-test results in a verification of sensor contact to the soil
and in a new sensitivity that is valid for the circumstances at that moment. The latter iscalled self-calibration. Implicitly also cable connection, data acquisition and dataprocessing are tested.
Unique features of HFP01SC are:
•
•
•
•
•
•
•

low thermal resistancelarge 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 environments)
robustness, including a strong cable (essential for permanently installed sensors)
IP protection class: IP67 (essential for outdoor application)
incorporated film heater for self-testing

2.1

General heat flux sensor theory

The sensor in HFP01SC is a thermopile. This thermopile measures the temperature
difference across the ceramics-plastic composite body of HFP01SC. 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 the heat flux sensor of HFP01SC is easy. For readout the user only needs anaccurate 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 Sreference, a constant that is
supplied with each individual sensor.

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1

2

3

4

5
6

Figure 2.1.1 The general working principle of a heat flux sensor. The sensor inside
HFP01SC is a thermopile. A thermopile consists of a number of thermocouples, eachconsisting of two metal alloys marked 1 and 2, electrically connected in series. A single
thermocouple will generate 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 the temperature
difference across the sensor and the average thermal conductivity of the sensor body, 3.
The thermopile generates a voltage output proportional to the heat flux through thesensor. The exact sensitivity of the sensor is determined at the manufacturer by
calibration, and is found on the calibration certificate that is supplied with each sensor.

Heat flux sensors such as HFP01SC, for use in the soil, are typically calibrated under thefollowing reference conditions:••••conductive heat flux (as opposed to radiative or convective)
homogeneous heat flux across the sensor and guard surface
room temperature
heat flux in the order of 350 W/m2

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.
One of the purposes of the self-test, described in the following chapter, is to reducethese errors.

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2.2

The self-test and self-calibration

A self-test is started by switching on HFP01SC’s heater, while recording the sensor
output signal and the heater power, and finalised by switching the heater off. During theheating interval a current is fed through the film heater, which generates a known heat
flux. To calculate this heat flux the heater current Iheater must accurately be measured.
For the highest accuracy measurements with the best level of quality assurance, thevalues Sreference and Rheater must be entered individually for every sensor.
The recommended interval between tests is 6 hr. The recommended duration of the test
is 360 s. It is divided in a heating interval of 180 s and a settling interval of 180 s.
Optionally the interval between tests may be chosen differently, for example 3 or 12 hr.
The user must interrupt the normal measurement of the soil heat flux during the selftest. We recommend that the soil heat flux value of just before the heating interval iscopied for at least 360 s. In case of very small soil heat fluxes, this interruption may be
600 s.
Analysis of the heat flux sensor response to the heating, the self-test, serves three
purposes:• first, the amplitude and response time are indicators of the quality of contact of thesensor to the soil. See 2.3.1 for more details.
• second, the signal level during self-testing is used for self-calibration, which results in
a new sensitivity Sselfcalibration, see figure 4 and paragraph 2.3.2 for an explanation.
• third, the functionality of the complete measuring system is verified. For example: abroken cable is immediately detected.
If response time and signal level are within acceptance limits, from the moment a newsensitivity has been determined the user works with:
Φ = U/Sselfcalibration(Formula 2.2.1)

The new sensitivity Sselfcalibration compensates for:•••the deflection error caused by non-perfect matching of the thermal conductivity of
sensor and soil, including changes of the thermal conductivity of the soil caused bychanging moisture content
temperature dependence of the sensitivity of the heat flux sensor
non-stability of the heat flux sensor

Additional quality assurance is offered by:•monitoring of seasonal and yearly patterns of the sensitivity Sselfcalibration , whichquantifies the sensor stability

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0.5

(1+X)

0.5

(1-X)

2
(1-X)

Figure 2.2.1 Explanation of the self-calibration: on the left, the heat flux sensor (2)measures a soil heat flux Φ. This flux is subject to a measurement error - X, thedeflection error which depends on the thermal conductivity of the soil compared to thatof the sensor and its thermal contact to the soil. On the right, during the self-test thefilm heater (1) is switched on to generate a known electrically generated heat flux. As afirst approximation, the division of the total heat flux between downward flux through thesensor and upward flux contains the same (1-X) term that also characterises thedeflection error. The signal level during the self-test, multiplied by 2, is used for selfcalibration. The newly measured sensitivity compensates for the deflection error, andalso for temperature dependence of the sensitivity and non-stability of the sensor.

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2.3

Programming the self-test and self-calibration

2.3.1 The self-test
Bad contact to the soil and also malfunctions cable and data acquisition will result in:• long response times and• high or low measured sensitivities Sselfcalibration during self-calibration.
We recommend defining acceptance intervals for these parameters. For exact definitionof the parameters and calculation of Sselfcalibration, see the next paragraph.
We suggest to generate an error message if Sselfcalibration is larger than the factory
determined Sreference outside a +5 to –20 % acceptance interval around the original
Sreference.
0.8 Sreference < Sselfcalibration < 1.05 Sreference(Formula 2.3.1.1)

We suggest generating an error message if response times are too long.
|U(360) – U(0)| > 0.1 Uselfcalibration(Formula 2.3.1.2)|Uselfcalibration (170) – Uselfcalibration (180)| > 0.1 Uselfcalibration(Formula 2.3.1.3)

2.3.2 Self-calibration
The difference in voltage output, Uselfcalibration, of the sensor without heating and after
heating for 180 s, multiplied by 2 (because only half of the flux passes the sensor) is
divided by the heat flux generated by the heater, Φselfcalibration, to calculate the newsensitivity, Sselfcalibration.
Typically measurements are taken at 0, 180 and 360 s. The heat flux sensor voltage
output at time t is U(t), with t = 0 just before switching on the heater.
Φselfcalibration = (U2current·Rheater)/(R2current·Aheater)
Uselfcalibration= |U(180) - 0.5·(U(0) + U(360))|

Sselfcalibration = 2·Uselfcalibration/Φselfcalibration(Formula 2.3.2.1)(Formula 2.3.2.2)(Formula 2.3.2.3)

Concluding:Sselfcalibration = 2·Uselfcalibration·R2current·Aheater/(U2current·Rheater)(Formula 2.3.2.4)

NOTE: Rheater is a property of the individual sensor. Its nominal value is 100 Ω. Its exact
value can be found on the calibration certificate. For accurate measurements the exact
value of Rheater must be entered into the equation. The value of Aheater is its nominal
value. The value of Rcurrent is its nominal value, assuming use of a 0.1 % resistor.

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Example:
Your HFP01SC product certificate might state: Aheater = 38.85 x 10-4 m² and Rheater = 100
Ohm. With a typical value for Rcurrent of 10 Ohm, you get
Sselfcalibration = 2·Uselfcalibration·10²·38.85 x 10-4/(Ucurrent²·100) = 7.77 x 10-3·Uselfcalibr /Ucurrent²
With a 12 VDC power supply, you get a value of Φselfcalibration of about 306 W/m².
Power during heating is 1.3 W, and time averaged power consumption is 0.02 W when
the interval between tests is 6 h.
With typical voltage readings of
Uselfcalibration = 9 x 10-3 V
Ucurrent = 1.09 V
The final result is
Sselfcalibration = 7.77 x 10-3·9 x 10-3/1.09²= 58.86 x 10-6 V/(W/m²)
In soils, corrections of up to +5 to -20 % relative to the factory supplied sensitivity canbe expected (of which +5 % due to temperature dependence).

2.3.3 Program summary
In case the user writes his own software program for controlling the HFP01SC selft-test,
the program flow in table 2.3.3.1 may be used.
Table 2.3.3.1 a summary of a program for control of the self-test
initialisation:enter sensor and system
information

every 6 h:stop measurement of Φ,
copy last measured value
during 360 s
360 s self-test

serial number, Rheater, Rcurrent, Aheater, Sreference

measure U
180 s heating interval
180 setting interval

self-test: self-calibration
self-test: quality checks

start measurement of Φ
using new Sselfcalibration

HFP01SC manual v2026

calculate Sselfcalibration
store Sselfcalibration
accept/reject Ssc
accept/reject response timeof U
if accepted, then: use selfcalibrationelse

heater on
measure U
measure Iheater
heater off
measure U

S = Sselfcalibration
S = Sreference
generate warning

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3

Specifications of HFP01SCHFP01SC measures the heat flux density through the surface of the sensor. Thisquantity, expressed in W/m2, is called heat flux. It is exclusively rated for use in the soil.
Working completely passive, using a thermopile sensor, HFP01SC generates a small
output voltage proportional to this flux. HFP01SC is equipped with a film heater. The
heater may be used to perform an on-line self-test. Analysis of the self-test results inimproved quality assurance and accuracy of the measurement. Part of the self-test is
self-calibration, which results in a new sensitivity that is valid under the circumstances ofthat moment. HFP01SC can only be used in combination with a suitable measurement
and control system.
Table 3.1 Specifications of HFP01SC (continued on next page)
HFP01SC SPECIFICATIONS
Sensor type
Sensor type according to ISO 9869
Sensor type according to ASTM
Measurand
Measurand in SI units
On-line functionality testing
Measurement range
Sensitivity range
Sensitivity (nominal)
Directional sensitivity
Rated operating environment
Expected voltage output
Measurement function / required
programming
Required programming
Required readout and control

Rated operating temperature range
Temperature dependence
Thermal conductivity dependence
Non-stability

HFP01SC manual v2026

self-calibrating heat flux sensor
heat flow meterheat flow sensor or heat flux transducer
heat flux
heat flux density in W/m2
self-test including self-calibration-2000 to 2000 W/m2
40 to 80 x 10-6 V/(W/m2)
60 x 10-6 V/(W/m2)(adapted using self-calibration)
heat flux from the red to the blue side generates a
positive voltage output signal
surrounded by soil
application in meteorology: -10 to +20 x 10-3 V
turning the sensor from one side to the other will leadto a reversal of the sensor voltage output.
without self-calibration: Φ = U/Swith self-calibration: Φ = U/Sselfcalibration
self-test, including self-calibration
heat flux sensor: 1 x differential voltage channel or 1
single ended voltage channel,
input resistance > 106 Ω
heater: 1 x current channel or alternatively 1 voltage
channel which acts as a current measurement channel
using a current sensing resistor
heater: 1 x switchable 12 VDC-30 to +70 °C< 0.1 %/°C
(compensated using self-calibration)
7 %/(W/(m·K))(order of magnitude only)(compensated using self-calibration)< 1 %/yr(compensated using self-calibration)

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Table 3.1 Specifications of HFP01SC (started on previous page, continued on next page)
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
Standard 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
FILM HEATER
Film heater resistance (nominal)
Film heater rated power supplyFilm heater power supplyFilm heater area
Suggested current sensing resistor
SELF-TEST
Power consumption during heatinginterval (nominal)
Power consumption daily average
Interval between self-tests
Self-test duration
Heating interval duration
Settling interval duration
INSTALLATION AND USE
Recommended number of sensors
Orientation
Installation
Cable extension

HFP01SC manual v2026

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)
5.6 x 10-3 m (6 x 10-3 m at cable exit from sensor)
81 x 10-4 K/(W/m2)
0.69 W/(m·K)
180 s
1 to 4 Ωzero (passive sensor)(provided that self-calibration is not used)
Not applicable
2x5m
0.15 m wires and shield at cable ends
4 x 10-3 m
2 x sticker, 1 x at sensor and 1 x cable end, wrappedaround the heat flux sensor cable (cable 1). Bothstickers show sensitivity and serial number.
IP67
0 to 100 %approx. 0.5 kg
approx. 0.5 kg
100 Ω ± 10 %(measured value supplied with each sensor in the
production report)
9 to 15 VDC
12 VDC (nominal)
0.003885 m2
10 Ω ± 0.1 %, 0.25 W, < 15 ppm/°C
1.5 W
0.02 W at 6 hr interval between tests and 180 s
heating interval
6 hr, optionally 3 or 12 hr
360 s
180 s
180 s
2 per measurement location
red side up
see recommendations in this user manual
see chapter on cable extension or order sensors withlonger cable16/37

Table 3.1 Specifications of HFP01SC (started on previous pages)
CALIBRATION
Calibration traceability
Product certificate
Factory calibration method
On-line calibration method
Calibration hierarchy
Factory calibration uncertainty
Recommended recalibration interval
Factory calibration reference conditions
Validity of factory calibration
Field calibration

MEASUREMENT ACCURACY
Uncertainty of the measurement
VERSIONS / OPTIONSLonger cable
ACCESSORIES

to SI units
included(showing calibration result and traceability, as well as
film heater resistance and surface area)
method HFPC01, according to ASTM C1130
self-calibration as part of the self-test
From SI through international standards and throughan internal mathematical procedure< 3 % (k = 2)
compliant with ISO 9869 requirement < 2 % (k = 1)
5 years, provided that on-line calibration is used
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. Preferablyreference and field sensor of the same model andbrand. Typical duration of test > 24 h.
statements about the overall measurement
uncertainty can only be made on an individual basis.
see the chapter on uncertainty evaluation.
in multiples of 5 m, cable lengths above 20 m in
multiples of 10 m
option code = total cable length

No accessories

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3.1

Dimensions of HFP01SC

80

5m

5.6
2

Figure 3.1.1 HFP01SC heat flux sensor dimensions in x 10-3 m
(1)
(2)
(3)

film heater
heat flux sensor plus passive guard
2 x cable (standard length 5 m, optionally longer cable in multiples of 5 m, cable
above 20 m in multiples of 10 m)

Total sensor thickness including heater and covers is 5.6 x 10-3 m (6 x 10-3 m at cable
exit from sensor)

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4

Standards and recommended practices
for use

HFP01SC sensors are used to measure heat flux in soils, as part of meteorologicalsurface flux measuring systems. Typically the total measuring system consists of multiple
heat flux- and temperature sensors, often 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 estimate the heat flux at the soil surface.
Knowing the heat flux at the soil surface, it is possible to “close the balance" andestimate the uncertainty of the measurement of the other (convective and evaporative)
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 the sky. The maximum upward net irradiance isabout 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.
The measurement of the soil heat flux with HFP01SC using the self-test is more reliable
and accurate than without the self-test. However, there still is a large source of
uncertainty:
1. representativeness: measurement at one location has an uncertain validity for thelarger area under observation
When estimating the surface heat flux, there is a second source of uncertainty:
2. uncertainty of the storage term (not formally part of the HFP01SC measurement)
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 > 5 m.

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Figure 4.2.1 typical meteorological surface energy balance measurement system with
HFP01SC installed under the soil.
Ad 2: 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, the self-calibration only
works when the sensor is surrounded by soil. Second, mounting at the surface would
distort the flow of moisture, and the measured flux would no longer be representative forthe flux in the surrounding soil. Third, the absorption of solar radiation would not be
representative. Fourth, the sensor would be more vulnerable. The mechanical stability of
the installation then becomes an uncertain factor. Heat flux sensors in meteorologicalapplications 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 mechanical stability to guarantee
stable measurement conditions. Installation at a depth of more than 0.08 m is notrecommended, because at larger depths of installation the time delay and amplitude of
the measured heat flux becomes less accurately traceable to momentous 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.

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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 depth of
installation 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, on
a volume basis Qv, and cwater, the specific heat capacity of water.
cvolumic = ρsoil·(csoil, dry + Q·dwater) = ρsoil·csoil, dry + ρwater·Qv·cwater(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 HFP01SC

5.1

Site selection and installation

Table 5.1.1 Recommendations for installation of HFP01SC
Location

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

Orientation

recommended orientation is with the red side of the sensor facing
upwards. This generates a positive output signal when the heat flux is
downward and also of the heat flux generated by the heater.
reversing the sensor orientation will result in a change of sign of thevoltage output of the ambient heat flux (not of the flux generated by
the heater). If necessary, this may be compensated by reversing thewiring at the datalogger connection, or in the post processing forexample by giving the sensitivity a negative sign.
the self-test will work independent of orientation.

Performing a
representative
measurement

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

Installation

depth of installation typically is 0.05 m.
If possible, install the sensor from the side of a small hole. There should
be no air gaps between sensor and soil. A 0.1 x 10-3 m air gap increases
the effective 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
installing the sensor the original soil structure can be restored. The
sensor is installed in the undisturbed face of the excavated hole.
Measure the depth from the soil surface at the top of the hole. With aknife, make a horizontal cut at the required depth of installation, forexample at 0.05 m below the surface, into the undisturbed face of thehole. Insert the heat flux sensor into the horizontal 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.

Fixation / strain relief

for mechanical stability provide sensor cables with an additional strain
relief, for example connecting the cable with a tie wrap to a metal pinthat is inserted 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 conductorof heat or a transport 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.

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5.2

Electrical connection

HFP01SC has two separate cables, one for the signal, cable 1, and one for the heater,
cable 2.
An HFP01SC should be connected to a measurement and control system, typically a socalled datalogger. HFP01SC’s heat flux sensor is a passive sensor that does not need any
power. The heater is powered from 12 VDC, using a relay to switch it on and off.
Cables may act as a source of distortion, by picking up capacitive noise. We recommend
keeping the distance between a datalogger or amplifier and the sensor as short as
possible. For cable extension, see the appendix on this subject.
The heat flux sensor output is connected to a differential or single-ended voltage input.
The voltage across the current sensing resistor is measured by a differential voltage
channel.
Table 5.2.1 connections of cable 1 heat flux sensor signal. Cable 1 internally also has abrown wire, which is not used and not visible when supplied from the factory. The wires
extend 0.15 m from the cable end.
CABLE

WIRE

MEASUREMENT SYSTEM

1

White

signal [+]voltage input [+]

1

Green

signal [‒]voltage input [‒] or ground

111

Black

groundanalogue ground

Table 5.2.2 connections of cable 2 heater connection. Cable 2 internally also has a white
wire, which is not used and not visible when supplied from the factory. The wires extend
0.15 m from the cable end.
CABLE

WIRE

2

Green

2

Brown

21

Black

HFP01SC manual v2026

MEASUREMENT SYSTEM
heater (brown and green wires
are equivalent)
heater (brown and green wires
are equivalent)
ground12 VDC power supply, possibly to a
current sensing resistor
12 VDC power supply, possibly to a
current sensing resistor
analogue ground

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4
CABLE 2

6

C1+

5

C1-

4

G

3

1

3 12VDCCABLE 1

2 C2+

2

1 C2-

Figure 5.2.1 electrical connection HFP01SC. Dashed line (4) separates between the
HFP01SC on the left and the cable and the datalogger on the right.
Heat flux sensor (2) wires are connected to datalogger inputs 1 and 2. Film heater (1)heater wires are connected to the power supply at 3 and 4. The heater current is usually
measured using a 10 Ω current sensing resistor (not included with HFP01SC) (3) in series
with the heater, and by measuring the voltage across the resistor, at datalogger inputs 5
and 6.
NOTE: the current sensing resistor is not part of the delivery.

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5.3

Requirements for data acquisition / amplification

The selection and programming of dataloggers is the responsibility of the user. To see if
directions for use with HFP01SC are available: contact the supplier of the data acquisition
equipment.
Table 5.3.1 Requirements for data acquisition, amplification and control equipment for
HFP01SC 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 to measure the heater
current

the heater is powered from 12 VDC, at 0.12 A. The current
should be measured with an uncertainty of < 1 %a 10 Ω ± 0.1% current sensor resistor is often used.

Capability to switch the heater onand off

a relay must be used, capable of switching the required 12
VDC at 0.12 A (nominal values).

Capability for the data logger or thesoftwareto store data, and to perform division by the sensitivity to
calculate the heat flux.
Φ = U/S (Equation 0.1)
to time and control the self-test
to perform comparison of test results against the acceptancelimitsto reset the sensitivity S to Ssc

Data acquisition input resistance forheat flux sensor> 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 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
corrections applied for example using self-calibration

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

Table 6.2.1 typical measurement uncertainties when measuring heat flux with HFP01SC
heat flux sensors.
APPLICATIONTYPICAL MEASUREMENT UNCERTAINTY BUDGET (K=2)

Meteorology

measurements of heat flux in the soil, using the factory sensitivity
Sreference only, may attain uncertainties in the ± 20% range.
The self-calibration partially compensates for the deflection error, nonstability and temperature dependence, ideally attaining uncertainties in
the ± 10% range. The uncertainty evaluation then assumes that the
self-check including the self-calibration reduces usual uncertainties
related to non-stability, temperature dependence and deflection by 50%.
In case the heater resistance is not entered as an individual value persensor, the electrical power measurement contains a resistance with anuncertainty of ± 10 %. The total uncertainty then is of the orderof ± 15 %.
Further contributions to the uncertainty budget: representativeness ofthe measurement location.
Not included: estimates of the storage term, which is not part of the
HFP01SC measurement.

6.3

Contributions to the uncertainty budget

When measuring in soils, we recommend using model HFP01SC to get a higher level ofquality assurance and accuracy of the measurement. HFP01SC’s self-calibrationcompensates for the temperature dependence, non-stability and the deflection error. We
usually assume that the self-check including the self-calibration reduces usual
uncertainties related to non-stability, temperature dependence and deflection by 50 %.

6.3.1 Calibration uncertainty
HFP01SC’s factory calibration uncertainty under reference conditions is ± 3 % with acoverage factor k = 2.

6.3.2 Uncertainty caused by non-stability
HFP01SC’s non-stability specification is < 1 %/yr. This means that for every year ofoperation, 1 % uncertainty should be added in the uncertainty budget, unless the selfcalibration is used. We usually assume that the self-check including the self-calibration
reduces usual uncertainties related to non-stability, by 50 %.

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6.3.3 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. Dλ used to be part of the HFP01SC product specifications. The
order of magnitude of Dλ is 7 %/(W/(m·K)). The value of λref 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.
When measuring in soils, we recommend using model HFP01SC to get a higher level ofquality assurance and accuracy of the measurement. We usually assume that the selfcheck including the self-calibration reduces usual uncertainties related to deflection
by 50 %.

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1

2

Figure 6.3.3.1 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.

6.3.4 uncertainty caused by temperature dependence
HFP01SC’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, unless the self-calibration isused. We usually assume that the self-check including the self-calibration reduces usual
uncertainties related temperature dependence by 50 %.

6.3.5 Uncertainty caused by the heater resistance
Rheater is a property of the individual sensor. Its nominal value is 100 Ω. Its exact value
can be found on the calibration certificate. For accurate measurements the exact value of
Rheater must be entered into the equation. In case the nominal value is used, the 10 %uncertainty should be used. In case the individual value is used, we may work with 1 %.

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7

Maintenance and trouble shooting

7.1

Recommended maintenance and quality assurance

HFP01SC 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 of heat flux, response time during the self-test andsensitivity measured during the self-test. The preferred way to obtain a reliablemeasurement is a regular critical review of the measured data, preferably checking
against other measurements.
Table 7.1.1 Recommended maintenance of HFP01SC. If possible the data analysis
should be done on a daily basis.
MINIMUM RECOMMENDED HEAT FLUX SENSOR MAINTENANCE
INTERVALSUBJECTACTION1

1 day

self-testat least one self-test per day

2

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. Analyse the self-test
measurement results. Compare to acceptance intervals.
Plot heat flux data against other meteorological measurands,
in particular net-radiation and soil temperature.

3

6 months

inspection

inspect cable quality, inspect mounting, inspect location of
installation
look for seasonal patterns in measurement data and results ofthe self-test

4

2 years

lifetime
assessment

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

5

5 years

recalibration

recalibration by the sensor manufacturer

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7.2

Trouble shooting

Table 7.2.1 Trouble shooting for HFP01SC
GeneralInspect the quality of mounting / installationl.
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.
HFP01SC sensitivity and serial number are marked on its cable.
Check the electrical resistance of the sensor between the green [-] and white [+]wires of cable [1]. Use a multimeter at the 100 Ω range. Measure the sensor
resistance first with one polarity, then reverse the polarity. Take the averagevalue. The typical resistance of the wiring is 0.1 Ω/m. Typical resistance should be
the nominal sensor resistance of 2 Ω for plus 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 short circuit.
Check the electrical resistance of the film heater between the wires of cable [2].
Use a multimeter at the 1000 Ω range. Typical resistance should be the typicalheater resistance of 100 Ω ± 15 %. Infinite resistance indicates a broken circuit;zero or a lower than 1 Ω resistance indicates a short circuit.
Check the sensor serial number, and sensitivity on the cable labels of cable [1](one at sensor end, one at cable end) against the product certificate provided withthe sensor.
Check the heater resistance value in Ω on the product certificate.

The sensor
does not give
any signal

Check if the sensor reacts to heat flux: expose the sensor to a strong heat source;
for example a high power lamp. The signal should read > 100 W/m2 now. Checkthe data acquisition by replacing the sensor with a spare sensor.
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, or activating the
HFP01SC heater by putting 9 to 12 VDC across the green and brown wires of cable[2]. The signal should read > 2 x 10-3 V now. Touching or exposing the red sideshould generate a positive signal, doing the same at the opposite side the sign ofthe output reverses.

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

Recalibration of field heat flux sensors is ideally done by the sensor manufacturer.
The recommended calibration interval of heat flux sensors is 2 years. Using the selfcalibration this may be reduced to 5 years.
Besides using the self-test which includes self-calibration, field calibration is also possible
by comparison to a calibration reference sensor. Usually mounted side by side.
Hukseflux main recommendations for field calibrations relative to a reference sensor 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

Appendices8.1

Appendix on cable extension / replacement

HFP01SC is equipped with two cables. 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 HFP01SC cables may be
extended without problem to 100 metres. If done properly, the sensor signal, althoughsmall, 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 (sono resistive losses). Cable and connection specifications are summarised below.
Table 8.1.1 Preferred specifications for cable extension of HFP01SC. Please note that thesensor has two separate cables.
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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8.2

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 8.2.1 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 recommends use of aguarded hot plate for characterisation

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

8.3

Appendix on calibration hierarchy

HFP01SC factory calibration is traceable from SI through international standards andthrough an internal mathematical procedure which corrects for know 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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8.4

Electrical connection of HFP01SC supplied by Campbell
Scientific USA

Sensors supplied by Campbell Scientific USA have a different wiring diagram. The 10 Ω
current sensing resistor is included in the sensor cabling using heat-shrink tubing with
hot-melt adhesive.

Table 8.4.1 FOR SENSORS SUPPLIED BY CAMPBELL USA ONLY: connections of cable 1heat flux sensor signal. Cable 1 internally also has a brown wire, which is not used andnot visible when supplied from the factory. The wires extend 0.15 m from the cable end.
CABLE

WIRE

FUNCTION

MEASUREMENT SYSTEM

1

Black

shield / groundanalogue ground

1

White

sensor signal [+]voltage input [+]

111

Green

sensor signal [−]voltage input [‒] or ground

Table 8.4.2 FOR SENSORS SUPPLIED BY CAMPBELL USA ONLY connections of cable 2
heater connection. Cable 2 internally also has a white wire, which is not used and not
visible when supplied from the factory. The 10 Ω current sensing resistor is included in
the sensor cabling using heat-shrink tubing with hot-melt adhesive. The wires extend
0.15 m from the cable end.
CABLE

WIRE

FUNCTION

MEASUREMENT SYSTEM

2

Clear

shield / groundanalogue ground2

Purpleheater low signal

2

Yellowheater high signal

2

Red

heater power [+]+ 12 VDC power supply

121

Black

heater power [−]system groundvoltage input [−](single ended analogue ground)voltage input [+](single ended [+] or [−])

The 10 Ω current sensing resistor is included in the sensor cabling using heat-shrink tubing with
hot-melt adhesive.

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8.5

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:

HFP01SC
Self-calibrating heat flux sensor

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.