Hukseflux

Thermal Sensors

HFP01SC

Self-calibrating heat flux sensorTM
HFP01SC self-calibrating heat flux sensorTM is a heat flux sensor for use in the soil. It offers the best
available accuracy and quality assurance of the measurement. The on-line self-test verifies the stableperformance and good 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 which compensates formeasurement errors caused by the thermal conductivity of the surrounding soil (which varies with soil
moisture content), for sensor non-stability and for temperature dependence.

Introduction

Figure 1 HFP01SC self-calibrating heat flux sensor. The
opposite side has a blue coloured cover.

HFP01SC measures soil heat flux in W/m2. It is
used when the highest level of quality assurance
and a high measurement accuracy are required.
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 aregular interval the film heater is activated toperform a self-test (see figure 4). The self-test
results in a verification of sensor contact to thesoil and in a new sensitivity that is valid for thecircumstances at that moment. The latter iscalled self-calibration. Implicitly also cableconnection, data acquisition and data processing
are tested. The result is a much improved
accuracy & quality assurance of themeasurement relative to measurements withconventional sensors such as model HFP01.
Soil heat flux sensors are preferably left in thesoil for as long as possible, so that the soil
properties become representative of the localconditions. Using self-testing, the user no longer
needs to take sensors to the laboratory to verify
their stable performance. A typical measurement
location is equipped with 2 or more sensors forgood spatial averaging.

Unique features and benefits
•
••
•
•
•
Figure 2 HFP01SC is used in soil heat flux measurements
in high-accuracy surface energy flux experiments•

low thermal resistancelarge guard area (required by the ISO 9869standard)
low electrical resistance (low pickup ofelectrical 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 foroutdoor application)
incorporated film heater for self-testing

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Thermal Sensors

Suggested use•

high-accuracy scientific measurement of soilheat flux, with a high level of data quality
assurance

Measurement and control
Requirements for data acquisition and control:•for heat flux: one millivolt measurement•
for heater current: one current measurement(or voltage over a current sensing resistor)•
for switching the heater current on and off:
one relay with 12 VDC nominal output

Calibration
HFP01SC calibration is traceable to international
standards. The factory calibration method follows
the recommended practice of ASTM C1130.

HFP01SC specifications
Measurand
On-line functionality testingSensing area
Sensor thermal resistance
Measurement range
Sensitivity (nominal)
Rated operating temperature
range
IP protection class
Film heater resistanceFilm heater rated power supply
Interval between self-tests
Heating interval duration
Power consumption
daily average
Standard cable length
Options-30 to +70 oC
IP67
100 Ω ± 10 %9 to 15 VDC
6 hr
180 s
0.02 W
2x5m
longer cable length(10, 15, 20, 30, 40 m)

Options

805m

•

5.6
2

longer cable (2 x), in multiples of 5 m, cable
lengths above 20 m in multiples of 10 m

See also

Figure 3 HFP01SC self-calibrating heat flux plate:(1) film heater, (2) heat flux sensor plus passive guard,(3) 2 x cable, standard length is 5 m (optionally longercable). Total sensor thickness including heater andcovers is 5.6 x 10-3 m (6 x 10-3 m at cable exit fromsensor). Dimensions in x 10-3 m.
0.5(1+X)

0.5(1-X)

2(1-X)

heat flux
self-test including
self-calibration
8 x 10-4 m2
81 x 10-4 K/(W/m2)-2000 to 2000 W/m2
60 x 10-6 V/(W/m2)

Figure 4 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 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 afirst approximation, the division of the total heat flux
between downward flux through the sensor and upward
flux contains the same (1-X) term that alsocharacterises the deflection error. The signal level
during the self-test, multiplied by 2, is used for selfcalibration. The newly measured sensitivity
compensates for the deflection error, and also fortemperature dependence of the sensitivity and nonstability of the sensor.

••

in case a less accurate measurement issufficient, consider model HFP01
view our complete range of heat flux sensors

About Hukseflux
Hukseflux Thermal Sensors offers measurement
solutions for the most challenging applications.
We design and supply sensors as well as test &measuring systems, and offer related services
such as engineering and consultancy. With ourlaboratory facilities, we provide testing services
including material characterisation andcalibration. Our main area of expertise ismeasurement of heat transfer and thermal
quantities such as solar radiation, heat flux andthermal conductivity. Hukseflux is ISO 9001:2008
certified. Hukseflux sensors, systems andservices are offered worldwide via our office in
Delft, the Netherlands and local distributors.

Interested in this product?
E-mail us at: info@hukseflux.com

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Hukseflux

Thermal Sensors

MARKET LEADER IN HEAT FLUX MEASUREMENT

HFP01SC outperforms competing models: how?
HFP01SC is the world’s market leading model for soil heat flux measurement with self-test. Purchasing a HFP01SC is a good investment in accurate and stablemeasurement.

Durable cable with strain relief
World market leader

Low disturbance: small thermal resistanceHFP01SC is 5.6 mm thick, but thanks to its ceramicsplastic composite body it is thermally thin. Its thermal
resistance is comparable to 1.6 mm normal plastic,causing little disturbance of the undisturbed heat flux.

HFP01SC is on the market for more than 15 years
and used by the vast majority of meteorologicalsurface flux measurement networks.

Corrosion-proof plastic coverprotecting the complete sensor

Sensitive area with thermal spreader

High accuracy: passive guard included
A passive guard, i.e. a non-sensitive part aroundthe sensor is essential to avoid errors due toedge effects, HFP01SC includes guard according
to ISO 9869. Competing models often havesensitive parts running to the edge of the sensor,
resulting in large potential measurement errors.

Selft-test: no need to dig it up
HFP01SC’s are typically buried; using
the self-test you do not need to dig itup for re-calibration

Ceramic plastic composite body,equivalent thickness in plastic 1.6 mm
Durable: sturdy “student andinstaller-proof” connection
HFP01SC’s cables to sensor connection isglued, and withstands rough handling andrepeated installations typical for building
applications. Student and installer-proof!
Competing sensors often have wire
connections on weak solder pads.

Best paperwork

Passive guard area, reducing deflection
errors, also used for mounting

Hukseflux has the paperwork covered;
HFP01 is provided with formally traceablecalibration certificates. We calibrate inaccordance with ASTM C1130.

© Copyright by Hukseflux. Version 1808. Page 3/3. For Hukseflux Thermal Sensors go to www.hukseflux.com or e-mail us: info@hukseflux.com