Hukseflux HFP01SC User Manual

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USER MANUAL HFP01SC

Self-calibrating heat flux sensor

TM

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 stat em e nts 2
Contents 3
List of symbols 4
Introduction 5
1 Ordering and checking at delivery 8

1.1 Ordering HFP01SC 8

1.2 Included items 8

1.3 Quick instrument che ck 8

2 Instrument 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

3 Specifications of HFP01SC 15

3.1 Dimensions of HFP01SC 18

4 Standards and recommended practices for use 19
5 Installation of HFP01SC 22

5.1 Site selection and installation 22

5.2 Electrical connection 23

5.3 Requirements for data acquisition / amplification 25

6 Making a dependable measureme nt 26

6.1 Uncertainty evaluation 26

6.2 Typical measurement uncertainties 27

6.3 Contributions to the uncertainty budget 27

7 Maintenance and trouble shooting 30

7.1 Recommended maintenance and quality assurance 30

7.2 Trouble shooting 31

7.3 Calibration and che cks in the field 32

8 Appendices 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 sy m bols

Quantities Symbol Unit
Heat flux Φ W/m²

Voltage output U V
Sensitivity S V/(W/m

2

)
Temperature T °C
Temperature difference ΔT °C, K
Time constant τ s
Time t s
Thermal conductivity λ W/(m∙K)
Thermal resistivity r m∙K/W
Volumic heat capacity c

volumic 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) Q

V

m³/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/ m

2

and 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 se lf-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 thr ough 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/m

2

, is calculated by

dividing the HFP01SC output, a small voltage U, by the sens itivity S

reference

.

The measurement function for HFP01SC is:

Φ = U/S

reference

(Formula 0.1)

The factory-determined sensitivity S

reference

, 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 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 new

sensitivity S

selfcalibration

.

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/S

selfcalibration

(Formula 0.2)

The new sensitivity S

selfcalibration

compensat es for:

• the deflection error caused by non-perfect matching of the thermal conductivity of

sensor and soil, includi ng changes of the thermal conductivity o f the soil caused by
changing moisture co n t ent

• temperature dependence of the sensitivity of the heat flux sensor

• non-stability of the heat flux sensor

Additional quality assurance is offere d b y:

• monitoring of seasonal and yearly patterns of the sensitivity S

selfcalibration

, 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 re d cover. Standard cable length is 5 m (2 cables).

The uncertainty of a mea surement 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 o wn 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

HFP01SC manual v1624 8/39

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 mu ltiples 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 lowe r. 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 ci rcuit; 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.

HFP01SC manual v1624 9/39

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/m

2

, 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 conne ction, 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 n oise 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 t he u ser only needs an
accurate voltmeter that works in the millivolt range. To convert the measured voltage, U,

to a heat flux Φ, the voltage must be divided by the sensitivity S

reference

, a constant that is

supplied with each ind ividual sensor.

HFP01SC manual v1624 10/39

Figure 2.1.1 The general working principle of a heat flux senso r. Th e s en so r in si de
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 volta ge 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 opposit e sen sor
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 calibr a ti on 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/m

2

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 ca n be found in the chapter about uncertainty e valuation.

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

5

4

321

6

HFP01SC manual v1624 11/39

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 I

heater

must accurately be measured.

For the highest accuracy measurements with the best level of quality assurance, the
values S

reference

and R

heater

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 a nd 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 t he soil heat flux during the self­test. 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 hea t 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 S

selfcalibration

, 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/S

selfcalibration

(Formula 2.2.1)

The new sensitivity S

selfcalibration

compensat es for:

• the deflection error caused by non-perfect matching of the thermal conductivity of

sensor and soil, includi ng changes of the thermal conductivity o f the soil caused by
changing moisture co n t ent

• 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 S

selfcalibration

, 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 erro r 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 erro r. 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 for temperature dependence of the sensitivity and non-stability of the sensor.

2

(1-X) (1-X)0.5

(1+X)0.5

HFP01SC manual v1624 13/39

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 ac quisition will result in:

• long response times and

• high or low measured sensitivities S

selfcalibration

during self-calibration.

We recommend defining acceptance intervals for these parameters. For exact definition
of the parameters and calculation of S

selfcalibration

, see the next paragraph.

We suggest to generate an error message if S

selfcalibration

is larger than the factory

determined S

reference

outside a +5 to –20 % acceptance interval around the original

S

reference

.

0.8 S

reference

< S

selfcalibration

< 1.05 S

reference

(Formula 2.3.1.1)

We suggest generating an error message if response times are too long.

|U(360) – U(0)| > 0.1 U

selfcalibration

(Formula 2.3.1.2)

|U

selfcalibration

(170) – U

selfcalibration

(180)| > 0.1 U

selfcalibration

(Formula 2.3.1.3)

2.3.2 Self-calibration

The difference in voltage output, U

selfcalibration

, of the sensor without heating and after
heating for 180 s, multiplied by 2 (because only half of the f lux passes the sensor) is
divided by the heat flux generated by the heater, Φ

selfcalibration

, to calculate the new

sensitivity, S

selfcalibration

.

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.

Φ

selfcalibration

= (U

2

current

·R

heater

)/(R

2

current

·A

heater

) (Formula 2.3.2.1)

U

selfcalibration

= |U(180) - 0.5·(U(0) + U(360))| (Formula 2.3.2.2)

S

selfcalibration

= 2·U

selfcalibration/Φselfcalibration

(Formula 2.3.2.3)

Concluding:

S

selfcalibration

= 2·U

selfcalibration

·R

2

current

·A

heater

/(U

2

current

·R

heater

) (Formula 2.3.2.4)

NOTE: R

heater

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 A

heater

is its nominal

value. The value of R

current

is its nominal value, ass uming use of a 0.1 % resistor.

HFP01SC manual v1624 14/39

Example:

Your HFP01SC product certificate might state: A

heater

= 38.85 x 10-4 m² and R

heater

= 100

Ohm. With a typical value for R

current

of 10 Ohm, you get

S

selfcalibration

= 2·U

selfcalibration

·10²·38.85 x 10-4/(U

current

²·100) = 7.77 x 10-3·U

selfcalibr

/U

current

²

With a 12 VDC power supply, you get a value of Φ

selfcalibration

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
U

selfcalibration

= 9 x 10-3 V

U

current

= 1.09 V
The final result is
S

selfcalibration

= 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, R

heater

, R

current

, A

heater

, S

reference

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 I

heater

180 setting interval

heater off

measure U

self-test: self-calibration

calculate S

selfcalibration

store S

selfcalibration

self-test: quality checks

accept/reject Ssc

accept/reject response time

of U

if accepted, then: use self­calibration

S = S

selfcalibration

else

S = S

reference

generate warning

start measurement of Φ

using new S

selfcalibration

HFP01SC manual v1624 15/39

3 Specifications of HFP01SC

HFP01SC measures the heat flux density through the surface of the sensor. This
quantity, expressed in W/m

2

, 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-calibrat ion, 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 he a t 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 rang e

50 to 70 x 10-6 V/(W/m2)

Sensitivity (nominal)

60 x 10-6 V/(W/m2)
(adapted using self-calibration)

Directional sens itivity heat flux from the red to the blue side gen er a tes a

positive voltage ou tp ut signal

Rated operating environm ent

surrounded by soil

Expected voltage output

application in m ete or ology: -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/S

selfcalibration

Required programming

self-test, including self -calibration

Required readout and c ontrol heat flux sensor: 1 x differential voltage channel or 1

single ended voltage chann el,
input resistance > 106 Ω
heater: 1 x current ch a nnel 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 temperatu r e r a nge

-30 to +70 °C

Temperature dependence

< 0.1 %/°C
(compensated using self-calibration)

Thermal conductivity depe ndence

7 %/(

W/(m·K))(order of magnitude only )

(compensated using self-calibration)

Non-stability

< 1 %/yr

(compensated using self-calibration)

HFP01SC manual v1624 16/39

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 m

2

Sensing area diameter

32 x 10

-3

m

Passive guard area 42 x 10-4 m2

(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 sen s or )

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 a nd serial number.

IP protection cla s s

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 ea c h 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 sensin g r es istor

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 d uration

180 s

INSTALLATION AND USE

Recommended number of sens or s

2 per measurement location

Orientation

red side up

Installation

see recommendation s in this user manual

Cable extension see chapter on cable extens ion or or der s ensors with

longer cable

HFP01SC manual v1624 17/39

Table 3.1 Specifications of HFP01SC (started on previous pages)

CALIBRATION

Calibration trace a b ility

to SI units

Product certificate included

(showing calibra tion result and traceability, a s well as

film heater resistance and surface area)

Factory calibration method

method HFPC01, according to ASTM C1130

On-line calibration m e thod

self-calibration as part of the self-test

Calibration hierarchy

From SI through international standards and through
an internal math e m a tical procedure

Factory calibrat ion uncertainty < 3 % (k = 2)

compliant with ISO 9869 requirement < 2 % (k = 1)

Recommended recalibrati on interval

5 years, provided that on-line calibration is used

Factory calibrat ion reference conditions 20 °C, h ea t f lux of 350 W/m2, thermal conduc tivity of

the surrounding environment 0.0 W/(m·K)

Validity of fac tor y calibration

based on experience the instrument sensitivity will not
change during storage. During use the instrument

“non-stability” specificat ion is applicable.

Field calibration

is possible by compar ison to a calibration reference

sensor. Usually mounted side by side. P referably
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 l engths above 20 m in

multiples of 10 m

option code = total ca b le 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

4 Standards 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 r adiation. The maximum power of the sun is about 1500 W/m

2

, around
noon at low latitudes under clear sky conditions. The solar radiati on i s eit h er re flected 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/m

2

, under clear sky conditions.

The heat flux in the soil at 0.05 m depth is usually between -100 and +300 W/m

2

.

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. O n 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, a nd 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, t h e 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 s t ability 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 Φ

surface

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 t

1

to t2.

Φ

surface

= Φ

0.05 m

+ S (Formula 4.2.1)

The quantity S is called the storage term.

HFP01SC manual v1624 21/39

The storage term is calculated from a space-av e rag ed soil temperature measuremen t ,
using multiple soil temperature sensors, and an estimate of the volumic heat capacity
C

volumic of the soil above the sensor.

S = (T(t

1

) - T(t2))·cvolumic·x/(t1 - t2) (Formula 4.2.2)

Where T(t

1

) - T(t2) is the temperature difference in the measurement interval, x the

depth of installation of the soil heat flux sensors.

A correct estimate of Φ

surface

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 Φ

surface

. When the temperature T is measured closely below the surface, the

response time of the storage term to a changing Φ

surface

is in the order of magnitude of
20 min, while the heat flux sensor buried at twice the depth is a factor 5 slower (square
of the depth). The volumic heat capacity is estimated from the specific heat capacity of
dry soil, c

soil, dry, the bulk density of the dry soil ρ, the water content on mass basis Q, on

a volume basis Q

v

, and cwater, the specific heat capacity of water.

c

volumic = ρsoil·(cso il, dry + Q·dwater) = ρsoil·csoil, dry + ρwater·Q

v

·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 source
of uncertainty in the soil energy balance measurement.

A typical value for dry soil heat capacity is 840 J/(kg·K)

HFP01SC manual v1624 22/39

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 and

representative of the area u nder observation.

Orientation recommended orientation is with the red side of the sensor facin g

upwards. This generates a positive output signal when the heat flux is
downward and also of the heat flux generated by the hea ter.

reversing the sensor orientation will result in a c hange of sign of the
voltage output of the am bient heat flux (not of the flux generated by
the heater). If necessary, this may be compensated by reversing the
wiring at the datalo gg er connection, or in the post processing for
example by giving the sensitivi ty 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, insta ll the sensor from the side of a small h ole . There should
be no air gaps between sensor a nd soil. A 0.1 x 10

-3

m air gap increases
the effective thermal resis tance 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 s o th at after
installing the sensor the original soil structure c a n be restored. The
sensor is installed in the undisturbed face of the ex c avated hole.
Measure the depth from the soil surface at the top of th e hole. With a
knife, make a horizontal cut at the required depth of installation, for
example at 0.05 m below the surface, into the undisturbed face of the
hole. Insert the heat flux sensor into the horizontal cut.
Never run the sensor cable directly to the surface. Bury the sensor cable
horizontally over a distance of at least 1 m, to minimise thermal
conduction through the lead wire. Put the excavated s oil ba ck into its
original position after the sensor and cable are installed.

Fixation / strain r e lief for mechanical stability provide sensor cables with an additional str ain

relief, for example connecting the cable with a tie w r a p to a m e ta l p in
that is inserted firm ly into the soil.

Armoured cable

in some cases cables are equipped with additional armour to avoid

damage by rodents. Make sure the armour does not act as a conductor
of heat or a transp or t c onduit or container of water.

Added temperature
sensors

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

HFP01SC manual v1624 23/39

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 sing le-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 a
brown 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 signa l [‒] v oltage input [‒] or ground

111

Black ground analogue 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 MEASUREMENT SYSTEM

2 Green

heater (brown and green wires

are equivalent)

12 VDC power supply, possibly to a

current sensing resistor

2 Brown

heater (brown and green wires

are equivalent)

12 VDC power supply, possibly to a

current sensing resistor

21

Black ground analogue ground

HFP01SC manual v1624 24/39

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.

CABLE 1

CABLE 2

4

3

1

2

6
5
4

C1+

C1-

G

3
2

1

12VDC

C2+

C2-

HFP01SC manual v1624 25/39

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 the
acquisition / am plification equipment)

Capability to measure the hea ter
current

the heater is powered from 12 VDC , at 0.12 A. The current
should be measured with an un c er tainty of < 1 %
a 10 Ω ± 0.1% current sensor resis tor is often used.

Capability to switch the heater on
and off

a relay must be used, capable of sw itc hing the required 12
VDC at 0.12 A (nomin a l values).

Capability for th e da ta logger or the
software

to store data, and to pe r form division by the sensitivi ty to
calculate the heat flux.
Φ = U/S (Equation 0.1)
to time and control th e s e lf-test
to perform comparison of test results against the acceptance
limits
to reset the sensitiv ity S to S

sc

Data acquisition input resistance for
heat flux sensor

> 1 x 10

6

Ω

Open circuit detec tion
(WARNING)

open-

circuit detection should not be used, unless this is done

separately from the norma l mea s urement by more than 5
times the sensor response time and with a small current
only. Thermopile s ensors are sensitive to the curren t that is
used during open circuit detection . The current will generate
heat, which is measured and will appear as a temporary
offset.

HFP01SC manual v1624 26/39

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 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

• 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 of
quality assurance and accuracy of the measurement. HFP01SC’s self-test partially
compensates for the temperature dependence, non-stability and the deflection error.

Guidelines for uncertainty evaluation:

1) The formal evaluation of uncertainty should be performed in ac cordance with ISO 98-3
Guide to the Expression of Uncertainty in Measurement, GUM.

2) Uncertainties are en te red in measurement equation (equation is usually Formula 0.1:
E = U/S), either as an uncertainty in E (non-representati ven ess, resistan ce er ro r and
deflection error) in U (voltage readout errors) or in S (non-stability, temperature
dependence, calibration uncertainty).

3) In case of special meas u rement 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 cance lling 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 covariance
matrix are zero. Paragraph 5.1 of GUM.

HFP01SC manual v1624 27/39

6.2 Typical measurement uncertainties

Table 6.2.1 typical measurement uncertai n t i es when measuring heat flux with HFP01SC

heat flux sensors.

APPLICATION

TYPICAL MEASUREMENT UNCERT AINTY BUDGET (K=2)

Meteorology

measurements of heat flux in the soil, using the fac tor y sensitivity
S

reference

only, may atta in 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 evalu a tion then assumes that the
self-check including the self-calibration reduces usual uncertainties
related to non-stability, temper a tu r e dependence and deflection by 50
%.

In case the heater resistanc e is not entered as an individual value per
sensor, the electrical power measurement contains a resistance with an
uncertainty of ± 10 %. The total uncertainty then is of the order
of ± 15 %.

Further contributions to the uncerta inty budget: representativenes s of
the measurement loc ation.

Not included: estimates of th e s tora ge 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 of
quality assurance and accuracy of the measurement. HFP01SC’s self-calibration
compensates 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 referen ce conditions is ± 3 % with a
coverage 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 of
operation, 1 % uncertainty should be added in the uncertainty budget, unless the self­calibration is used. We usually assume that the se lf-c heck including the self-calibration
reduces usual uncertainties related to non-stability, by 50 %.

HFP01SC manual v1624 28/39

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 is
called the deflection error. The deflection error may be estimated by experiments or by
numerical 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 the
operational error, we use the term deflection error.

As figure 6.3.4.1 illustrates, the deflection error is largest at the sensor edges, and
smaller at the centre. For this reason sensors are equipped with a passive guard around
the sensitive sensor area.

For sensors that are fully surrounded by a uniform homogeneous material which have a
perfect thermal connection, the deflection error may be expressed as thermal
conductivity dependence D

λ

of the sensitivity S. The order of magnitude of Dλ is constant

for one sensor model. D

λ

used to be part of the HFP01SC produ ct sp e cifications. The

order of magnitude of D

λ

is 7 %/(W/(m·K)). The value of λ

ref

is 0.0 W/(m·K).

S = S

reference·

(1+ Dλ·(λ

environment

- λ

reference

)) (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 λ

environment

usually is not known.

When measuring in soils, we recommend using model HFP01SC to get a higher level of
quality assurance and accuracy of the measurement. We usually assume that the self­check including the self-calibration reduces usual uncertainties related to deflection
by 50 %.

HFP01SC manual v1624 29/39

Figure 6.3.3.1 the deflection error. The heat flux (1) is deflected in particular at the
edges 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 the
environment, 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 is
used. 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

R

heater

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
R

heater

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 %.

2

1

HFP01SC manual v1624 30/39

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 and
sensitivity measured during the self-test. The preferred way to obtain a reliable
measurement 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

INTERVAL

SUBJECT

ACTION

1

1 day

self-test

at least one self-test per day

2 1 week data analysis compare measured data to the maximum possible or

maximum expected heat flux and to other measurements for
example from nearby station s or redundant instruments.
Historical seasonal records can be used as a source for
expected values. Look for any patterns and events that
deviate from what is normal or expected. Analyse the self-test
measurement results. Com pa re to acceptance intervals.
Plot heat flux data a gainst other meteorological measurands,
in particular net-r adiation and soil temperature.

3 6 months inspection inspect cable quality, inspect mounting, inspect location of

installation
look for seasonal pa tterns in measurement data and results of
the 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

HFP01SC manual v1624 31/39

7.2 Trouble shooting

Table 7.2.1 Trouble shooting for HFP01SC

General Inspect the quality of mounting / installationl.

Inspect if the wires are properly attached to the data log ger.
Check the condition of the cable.
Inspect the connection of the shield (typically connected at the datalogger side).
Check the datalogge r pr ogram in particular if the right sensitivity is entered.
HFP01SC sensitivity and serial number are marked on its cable.
Check the electric a l resistance of the sensor between the green [-] an d white [+]
wires of cable [1]. Use a multimeter a t th e 100 Ω range. Measure the sensor
resistance first with one polarity, then revers e the polarity. Take the averag e
value. The typical resistance of the wiring is 0.1 Ω/m. Typical resistance should be
the nominal sens or r esistance 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 resista nce 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.
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.
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. T he signal should read > 100 W/m2 now. Check
the data acquisition by replacing the sensor with a spare sensor.

Check if the sens or reacts to heat: put the multimeter a t its most sensitive range
of DC voltage measuremen t, typically the 100 x 10

-3

VDC range or lower. Expose
the sensor heat, for instance touching it with you r hand, or activating the
HFP01SC heater by putting 9 to 12 VDC a c r oss 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 s ignal, doing the same at the opposite side the sign of
the output reverses.

The sensor
signal is
unrealistically
high or low

Check the cable condition looking for cable breaks.
Check the data acquisition by a pplying a 1 x 10

-6

V source to it in the

1 x 10

-6

V range. Look at the measurement result. Check if it is as expected.
Check the data a c q uisition 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/m

2

.

The sensor

signal shows
unexpected
variations

Check the presence of strong sources of electromagnetic radiation (radar, ra dio).

Check the condition and connection of the shield.
Check the condition of the sensor cable.
Check if the cable is not moving du r in g the measurement.

HFP01SC manual v1624 32/39

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 calib ration 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) are
eliminated.

3) to mount all sensors on the same platform, so that they have the same body
temperature.

4) typical duration of test > 24 h

5) typical heat fluxes used for compariso n: > 20 W/m

2

6) to correct deviations of more than ± 10 %. Lower deviations should be interpreted as
acceptable and should not lead to a revised sensitivity.

HFP01SC manual v1624 33/39

HFP01SC manual v1624 34/39

8 Appendices

8.1 Appendix on cable extension / replacement

HFP01SC is equipped with two cables. Keep the distance between data logger or amplifier
and 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, although
small, will not significantly degrade because the sensor resistance is very low (which
results in good immunity to external sources) and because there is no current flowing (so
no resistive losses). Cable and connection specifications are summarised below.

Table 8.1.1 Preferred specifications for cable extens ion of HFP01SC. Please note that the
sensor has two separate cables.

Cable

2-wire, shielded, with copper conductor (at Huksef lux 3-wire shielded
cable 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/m

Outer diameter

4 x 10

-3

m

Length

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

Outer mantle

with specificati ons 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 plate d waterproof connectors. A lways

connect the shield.

HFP01SC manual v1624 35/39

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 thin
heater apparatus are all allowed. Hukseflux employs a thin heater apparatus, uses a
linear function ac cording to X1.1 and uses a nominal temperatur e of 20 °C, in accordance
with 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 of
operation 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 this
statement is with a coverage factor k = 1.
For a known model, paragraph 5.1.2 allows one heat flow, a typical temper a ture during
use and on a typical building material. ISO 9869 do es 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 CLASSIFICATI O N AND CALIBRATION

ISO STANDARD

EQUIVALENT ASTM STANDARD

no dedicated heat flux calibration standard
available. ISO 9869 recommends use of a
guarded hot plate for characterisation

ASTM C1130 Standard Practice for Calibrating
Thin Heat Flux Transducers

ASTM C 1114-06 Standard Tes t M ethod for
Steady-State Therma l 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 and
through an internal mathematical procedure which corrects for know errors. The formal
traceability of the generated heat flux is through v oltage and current to electrical power
and 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.

HFP01SC manual v1624 36/39

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 1
heat flux sensor signal. Cable 1 internally also has a brown 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

FUNCTION

MEASUREMENT SYSTEM

1 Black shield / ground analogue 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 / ground analogue ground

2 Purple heater low signal

voltage input [

−]

(single ended analogue ground)

2 Yellow heater high signal

voltage input [

+]

(single ended [+] or [−])

2 Red heater power [+] + 12 VDC power supply

121

Black

heater power [

−]

system ground

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

HFP01SC manual v1624 37/39

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 Elect r om ag n et ic Compatibility Directive

hereby declare under our sole responsibility that:

Product model: HFP01SC
Product type: Self-calibrating heat flux sensor

has been designed to comply and is in conformity with the relevant sections and
applicable requirements of the following standards:

Emission: EN 61326-1 (2006)
Immunity: EN 61326-1 (2006)
Emission: EN 61000-3-2 (2006)
Emission: EN 61000-3-3 (1995) + A1 (2001) + A2 (2005)
Report: 08C01340RPT01, 06 January 2009

Eric HOEKSEMA
Director
Delft
September 08, 2015

© 2016, Hukseflux Thermal Sensors B.V.

www.hukseflux.com

Hukseflux Thermal Sensors B.V. reserves the rig ht to change specifications without notice.