Hukseflux HFP01SC User manual

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USER MANUALHFP01SC
Self-calibrating heat flux sensorTM
Hukseflux
Thermal Sensors

HFP01SC manual v1624 2/39
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.

HFP01SC manual v1624 3/39
Contents
Warning statements 2
Contents 3
List of symbols 4
Introduction 5
1Ordering and checking at delivery 8
1.1 Ordering HFP01SC 8
1.2 Included items 8
1.3 Quick instrument check 8
2Instrument principle and theory 9
2.1 General heat flux sensor theory 9
2.2 The self-test and self-calibration 11
2.3 Programming the self-test and self-calibration 13
3Specifications of HFP01SC 15
3.1 Dimensions of HFP01SC 18
4Standards and recommended practices for use 19
5Installation of HFP01SC 22
5.1 Site selection and installation 22
5.2 Electrical connection 23
5.3 Requirements for data acquisition / amplification 25
6Making a dependable measurement 26
6.1 Uncertainty evaluation 26
6.2 Typical measurement uncertainties 27
6.3 Contributions to the uncertainty budget 27
7Maintenance and trouble shooting 30
7.1 Recommended maintenance and quality assurance 30
7.2 Trouble shooting 31
7.3 Calibration and checks in the field 32
8Appendices 34
8.1 Appendix on cable extension / replacement 34
8.2 Appendix on standards for calibration 35
8.3 Appendix on calibration hierarchy 35
8.4 Electrical connection of HFP01SC supplied by Campbell Scientific USA 36
8.5 EU declaration of conformity 37

HFP01SC manual v1624 4/39
List of symbols
Quantities Symbol Unit
Heat flux ΦW/m²
Voltage output U V
Sensitivity S V/(W/m2)
Temperature T °C
Temperature difference ΔT °C, K
Time constant τs
Time t s
Thermal conductivity λW/(m∙K)
Thermal resistivity r m∙K/W
Volumic heat capacity cvolumic J/(m³∙K)
Resistance R Ω
Storage term S W/m²
Depth of installation x m
Water content (on mass basis) Q kg/kg
Water content (on volume basis) QVm³/m³
Subscripts
property of thermopile sensor sensor
property obtained under calibration reference
conditions reference
property at the (soil) surface surface
property of the surrounding soil soil
property of the heater heater
property obtained by self-test selftest
property obtained by self-calibration selfcalibration
property of the current-sensing resistor current

HFP01SC manual v1624 5/39
Introduction
HFP01SC self-calibrating heat flux sensorTM is a heat flux sensor for use in the soil. It
measures soil heat flux in W/m2and offers the best available accuracy and quality
assurance of the measurement. The on-line self-test verifies the stable performance and
good thermal contact of sensors that are buried and cannot be visually inspected and
taken to the laboratory for recalibration. The self-test also includes self-calibration which
corrects for measurement errors caused by the thermal conductivity of the surrounding
soil (which varies with soil moisture content), for sensor non-stability and for
temperature 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 that
moisture does not penetrate the sensor, HFP01SC has proven to be very robust and
stable. 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 the
circumstances at that moment. The latter is called self-calibration. Implicitly also cable
connection, data acquisition and data processing are tested. The result is a much
improved accuracy & quality assurance of the measurement relative to measurements
with 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 the
local conditions. Using self-testing, the user no longer needs to take sensors to the
laboratory 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 a
passive sensor; it does not require power. HFP01SC can be connected directly to
commonly 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 calibration
interval of common heat flux sensors is 2 years. With HFP01SC, using the self-test, this
may be extended to 5 years.

HFP01SC manual v1624 6/39
Every 6 h, the HFP01SC film heater is switched on to perform a self-test. During the self-
test 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 new
sensitivity Sselfcalibration.
If response time and signal level are within user-determined acceptance limits, from the
moment 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 by
changing 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, which
quantifies 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.

HFP01SC manual v1624 7/39
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 the
sensor properties, the influence of the sensor on the measurand, the
representativeness of the measurement location
The user should make his own uncertainty evaluation. Detailed suggestions for
experimental 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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1Ordering 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 [+] 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 average value. 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.
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 a
multimeter 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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2Instrument 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 is
representative 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 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 the circumstances at that moment. The latter is
called self-calibration. Implicitly also cable connection, data acquisition and data
processing are tested.
Unique features of HFP01SC are:
•low thermal resistance
•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 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 this
temperature difference. The heat flux is proportional to the same temperature difference
divided by the effective thermal conductivity of the heat flux sensor body.
Using the heat flux sensor of HFP01SC 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 Sreference, a constant that is
supplied with each individual sensor.

HFP01SC manual v1624 10/39
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, each
consisting 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 the
signal. 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 the
sensor. 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 the
following 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 most
common 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 reduce
these errors.
5
4
321
6

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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 the
heating 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, the
values 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 self-
test. We recommend that the soil heat flux value of just before the heating interval is
copied 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 the
sensor 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: a
broken cable is immediately detected.
If response time and signal level are within acceptance limits, from the moment a new
sensitivity 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 by
changing 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 , which
quantifies the sensor stability

HFP01SC manual v1624 12/39
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, the
deflection error which depends on the thermal conductivity of the soil compared to that
of the sensor and its thermal contact to the soil. On the right, during the self-test the
film heater (1) is switched on to generate a known electrically generated heat flux. As a
first approximation, the division of the total heat flux between downward flux through the
sensor and upward flux contains the same (1-X) term that also characterises the
deflection error. The signal level during the self-test, multiplied by 2, is used for self-
calibration. The newly measured sensitivity compensates for the deflection error, and
also for temperature dependence of the sensitivity and non-stability of the sensor.
2
(1-X) (1-X)0.5
(1+X)0.5

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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 definition
of 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 new
sensitivity, 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) (Formula 2.3.2.1)
Uselfcalibration = |U(180) - 0.5·(U(0) + U(360))| (Formula 2.3.2.2)
Sselfcalibration = 2·Uselfcalibration/Φselfcalibration (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.

HFP01SC manual v1624 14/39
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 can
be 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
serial number, Rheater, Rcurrent, Aheater, Sreference
every 6 h: stop measurement of Φ,
copy last measured value
during 360 s
360 s self-test
measure U
180 s heating interval
heater on
measure U
measure Iheater
180 setting interval
heater off
measure U
self-test: self-calibration
calculate Sselfcalibration
store Sselfcalibration
self-test: quality checks
accept/reject Ssc
accept/reject response time
of U
if accepted, then: use self-
calibration
S = S
selfcalibration
else
S = Sreference
generate warning
start measurement of Φ
using new Sselfcalibration

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3Specifications of HFP01SC
HFP01SC measures the heat flux density through the surface of the sensor. This
quantity, 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 in
improved 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 of
that 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
self-calibrating heat flux sensor
Sensor type according to ISO 9869
heat flow meter
Sensor type according to ASTM
heat flow sensor or heat flux transducer
Measurand
heat flux
Measurand in SI units
heat flux density in W/m2
On-line functionality testing
self-test including self-calibration
Measurement range
-2000 to 2000 W/m2
Sensitivity range
50 to 70 x 10-6 V/(W/m2)
Sensitivity (nominal)
60 x 10-6 V/(W/m2)
(adapted using self-calibration)
Directional sensitivity heat flux from the red to the blue side generates a
positive voltage output signal
Rated operating environment
surrounded by soil
Expected voltage output
application in meteorology: -10 to +20 x 10-3 V
180 ° rotation will lead to a reversal of the sensor
voltage output.
Measurement function / required
programming
without self-calibration: Φ= U/S
with self-calibration: Φ= U/Sselfcalibration
Required programming
self-test, including self-calibration
Required readout and control heat flux sensor: 1 x differential voltage channel or 1
single ended voltage channel,
input resistance > 106Ω
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
Rated operating temperature range
-30 to +70 °C
Temperature dependence < 0.1 %/°C
(compensated using self-calibration)
Thermal conductivity dependence 7 %/(W/(m·K))(order of magnitude only)
(compensated using self-calibration)
Non-stability
< 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
80 x 10-3 m
Sensing area
8 x 10-4 m2
Sensing area diameter
32 x 10-3 m
Passive guard area 42 x 10
-4
m
2
(a passive guard is required by ISO 9869)
Guard width to thickness ratio 5 m/m
(as required by ISO 9869 D.3.1)
Sensor thickness
5.6 x 10-3 m (6 x 10-3 m at cable exit from sensor)
Sensor thermal resistance
81 x 10-4 K/(W/m2)
Sensor thermal conductivity
0.69 W/(m·K)
Response time (95 %)
180 s
Sensor resistance range
1 to 4 Ω
Required sensor power zero (passive sensor)
(provided that self-calibration is not used)
Standard governing use of the
instrument
Not applicable
Standard cable length (see options)
2 x 5 m
Wiring
0.15 m wires and shield at cable ends
Cable diameter
4 x 10-3 m
Cable markers 2 x sticker, 1 x at sensor and 1 x cable end, wrapped
around the heat flux sensor cable (cable 1). Both
stickers show sensitivity and serial number.
IP protection class
IP67
Rated operating relative humidity range
0 to 100 %
Gross weight including 5 m cable
0.35 kg
Net weight including 5 m cable
0.30 kg
Packaging
box of 320 x 230 x 30 mm
FILM HEATER
Film heater resistance (nominal)
100 Ω ± 10 %
(measured value supplied with each sensor in the
production report)
Film heater rated power supply
9 to 15 VDC
Film heater power supply
12 VDC (nominal)
Film heater area
0.003885 m2
Suggested current sensing resistor
10 Ω ± 0.1 %, 0.25 W, < 15 ppm/°C
SELF-TEST
Power consumption during heating
interval (nominal)
1.5 W
Power consumption daily average
0.02 W at 6 hr interval between tests and 180 s
heating interval
Interval between self-tests
6 hr, optionally 3 or 12 hr
Self-test duration
360 s
Heating interval duration
180 s
Settling interval duration
180 s
INSTALLATION AND USE
Recommended number of sensors
2 per measurement location
Orientation
red side up
Installation
see recommendations in this user manual
Cable extension see chapter on cable extension or order sensors with
longer cable

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Table 3.1 Specifications of HFP01SC (started on previous pages)
CALIBRATION
Calibration traceability
to SI units
Product certificate included
(showing calibration result and traceability, as well as
film heater resistance and surface area)
Factory calibration method
method HFPC01, according to ASTM C1130
On-line calibration method
self-calibration as part of the self-test
Calibration hierarchy
From SI through international standards and through
an internal mathematical procedure
Factory calibration uncertainty < 3 % (k = 2)
compliant with ISO 9869 requirement < 2 % (k = 1)
Recommended recalibration interval
5 years, provided that on-line calibration is used
Factory calibration reference conditions 20 °C, heat flux of 350 W/m
2
, thermal conductivity of
the surrounding environment 0.0 W/(m·K)
Validity of factory calibration
based on experience the instrument sensitivity will not change during storage. During use the instrument
“non-stability” specification is applicable.
Field calibration
is possible by comparison to a calibration reference
sensor. Usually mounted side by side. Preferably
reference and field sensor of the same model and
brand. Typical duration of test > 24 h.
MEASUREMENT ACCURACY
Uncertainty of the measurement statements about the overall measurement
uncertainty can only be made on an individual basis.
see the chapter on uncertainty evaluation.
VERSIONS / OPTIONS
Longer cable in multiples of 5 m, cable lengths above 20 m in
multiples of 10 m
option code = total cable length
ACCESSORIES
No accessories

HFP01SC manual v1624 18/39
3.1 Dimensions of HFP01SC
Figure 3.1.1 HFP01SC heat flux sensor dimensions in x 10-3 m
(1) film heater
(2) heat flux sensor plus passive guard
(3) 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)
5.6
2
5m
80

HFP01SC manual v1624 19/39
4Standards and recommended practices
for use
HFP01SC sensors are used to measure heat flux in soils, as part of meteorological
surface 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 through
the soil, typically at around 0.05 m depth. Usually this measurement is combined with
measurements 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" and
estimate the uncertainty of the measurement of the other (convective and evaporative)
fluxes.
In most meteorological experiments, the main source of energy during daytime is
downward solar radiation. The maximum power of the sun is about 1500 W/m2, around
noon at low latitudes under clear sky conditions. The solar radiation is either reflected or
absorbed by the soil. The absorbed heat is divided between evaporation of water, heating
of 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 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.
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 the
larger 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.

HFP01SC manual v1624 20/39
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 for
the 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 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 mechanical stability to guarantee
stable measurement conditions. Installation at a depth of more than 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 to momentous flux at the soil
surface.
For the above reasons the flux at the soil surface Φsurface is usually estimated from the
flux 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.