Hukseflux HFP01SC User manual

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USER MANUALHFP01SC
Self-calibrating heat flux sensorTM
Hukseflux
Thermal Sensors

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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 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

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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

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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/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.

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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.

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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 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.

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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.
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