Hukseflux HFP01 User manual

HFP01 & HFP03
Heat Flux Plate
Heat Flux Sensor
USER MANUAL
HFP01/ HFP03 manual version 0612
Edited & Copyright by:
Hukseflux Thermal Sensors
http://www.hukseflux.com
e-mail: [email protected]

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Contents
List of symbols 4
Introduction 5
1General Theory 6
1.1 General heat flux sensor theory 6
1.2 Detailed description of the measurement: resistance error,
contact resistance, deflection error and temperature
dependence 8
2Application in meteorology 11
3Application in building physics 14
4Specifications of HFP01 16
5Short user guide 19
6Putting HFP01 into operation 20
7Installation of HFP01 in meteorology 21
8Installation of HFP01 in building physics 22
9Maintenance of HFP01 24
10 Electrical connection of HFP01 26
11 Appendices 27
11.1 Appendix on cable extension for HFP01 27
11.2 Appendix on trouble shooting 28
11.3 Appendix on heat flux sensor calibration 29
11.4 Appendix on heat transfer in meteorology 30
11.5 Appendix on heat transfer in building physics 32
11.6 Appendix on HFP03 33
11.7 CE declaration of conformity 35

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List of symbols
Heatflux ϕW m-2
Thermal conductivity of the
surrounding medium or object on which
the sensor is mounted λW/mK
Voltage output V V
HFP01 sensitivity Esen µV/Wm-2
Thermal conductivity dependence of Esen E
λmK/W
Time ts
Surfacearea Am
2
Electrical resistance ReΩ
Thermal resistance Rth Km2/W
Temperature TK
Temperature dependence TD %/K
Depth of burial d m
Subscripts
Property of the sensor sen
Propertyofair air
Property during calibration cal
Property of the object on which HFP01 is
mounted obj
Property at the soil surface surf

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Introduction
HFP01 is the world’s most popular sensor for heat flux
measurement in the soil and through walls and building
envelopes. By using a ceramics-plastic composite body the total
thermal resistance is kept small.
HFP01 serves to measure the heat that flows through the object
in which it is incorporated or on which it is mounted. The actual
sensor in HFP01 is a thermopile. This thermopile measures the
differential temperature across the ceramics-plastic composite
body of HFP01. Working completely passive, HFP01 generates a
small output voltage proportional to the local heat flux.
Using HFP01 is easy. For readout one only needs an accurate
voltmeter that works in the millivolt range. To calculate the heat
flux, the voltage must be divided by the sensitivity; a constant
that is supplied with each individual instrument.
HFP01 can be used for in-situ measurement of building envelope
thermal resistance (R-value) and thermal transmittance (H-
value) according to ISO 9869, ASTM C1046 and ASTM 1155
standards.
Traceability of calibration is to the “guarded hot plate” of
National Physical Laboratory (NPL) of the UK, according to ISO
8302 and ASTM C177.
A typical measurement location is equipped with 2 sensors for
good spatial averaging. If necessary two sensors can be put in
series, creating a single output signal.
If measuring in soil, in case a more accurate measurement is
needed the model HFP01SC should be considered.
In case a more sensitive measurement is required, model HFP03
should be considered.
In case of special requirements, like high temperature limits,
smaller size or flexibility the PU series could offer a solution.
This manual can also be used for HFP03. Differences between
HFP03 and HFP01 are highlighted in a special appendix on
HFP03.

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Figure 1 Drawing of HFP01 sensor
Figure 2 HFP01 heat flux plate dimensions:
(1) sensor area, (2) guard of ceramics-plastic composite, (3)
cable, standard length is 5 m.
All dimensions are in mm.

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1General Theory
1.1 General heat flux sensor theory
As in most heat flux sensors, the actual sensor in HFP01 is a
thermopile. This thermopile measures the differential
temperature across the ceramics-plastic composite body of
HFP01. Working completely passive, it generates a small output
voltage that is proportional to the differential temperature that
powers the heat flux travelling through it. (heat flux is
proportional to the differential temperature divided by the local
thermal conductivity of the heat flux sensor).
Assuming that the heat flux is steady, that the thermal
conductivity of the body is constant and that the sensor has
negligible influence on the thermal flow pattern, the signal of
HFP01 is proportional to the local heat flux in Watt per square
meter.
Using HFP01 is easy. For readout one only needs an accurate
voltmeter that works in the millivolt range. To convert the
measured voltage Vsen to a heat flux ϕ, the voltage must be
divided by the sensitivity Esen, a constant that is supplied with
each individual sensor.
ϕ= Vsen / Esen 1.1.1
HFP01 is a weatherproof sensor. It complies with the CE
directives.

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Figure 1.1 General characteristics of a heat flux sensor like
HFP01.
When heat (6) is flowing through the sensor, the filling material
(3) will act as a thermal resistance. Consequently the heat flow ϕ
will go together with a temperature gradient across the sensor,
creating a hot side (5) and a cold side (4). The majority of heat
flux sensors is based on a thermopile; a number of
thermocouples (1,2) connected in series. A single thermocouple
will generate an output voltage that is proportional to the
temperature difference between the joints (copper-constantan
and constantan-copper). This temperature difference is, provided
that errors are avoided, proportional to the heat flux, depending
only on the thickness and the average thermal conductivity of
the sensor. Using more thermocouples in series will enhance the
output signal. In the picture the joints of a copper-constantan
thermopile are alternatively placed on the hot- and the cold side
of the sensor. The two different alloys are represented in
different colours 1 and 2. The thermopile is embedded in a filling
material, usually a plastic, in case of HFP01 a special Ceramics-
plastic composite. Each individual sensor will have its own
sensitivity, Esen, usually expressed in Volts output, Vsen, per Watt
per square meter heat flux ϕ . The flux is calculated:
ϕ = Vsen/ Esen.
The sensitivity is determined at the manufacturer, and is found
on the calibration certificate that is supplied with each sensor.

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1.2 Detailed description of the measurement: resistance
error, contact resistance, deflection error and
temperature dependence
As a first approximation, the heat flux is expressed as:
ϕ= Vsen / Esen 1.2.1
This paragraph offers a more detailed description of the heat flux
measurement. It should be noted that the following theory for
correcting deflection errors and temperature dependence is not
often applied. Usually one will work with formula 1.2.1, possibly
corrected with 1.2.2.
When mounting the sensor in or on an object with limited
thermal resistance, the sensor thermal resistance itself might be
significantly influencing the undisturbed heat flux. One part of
the resulting error is called the resistance error, reflecting a
change of the total thermal resistance of the object.
Figure 1.2.1 The resistance error: a heat flux sensor (2)
increases or decreases the total thermal resistance of the object
on which it is mounted (1) or in which it is incorporated. This can
lead either to a larger of smaller (increase of or decrease of
the- ) heat flux (3).

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Figure 1.2.2 The resistance error: a heat flux sensor (2)
increases or decreases the total thermal resistance of the object
on which it is mounted or in which it is incorporated. An
otherwise uniform flux (1) is locally disturbed (3). In this case
the measured heat flux is smaller than the actual undisturbed
flux,( 1).
A first order correction of the measurement is:
ϕ= (Rthobj+Rthsen ) V sen / E sen Rthobj 1.2.2
This correction is often applied with thin or well-isolated walls.
Note: this correction can only be determined for objects with
limited (finite) dimensions. For this reason this correction is not
applicable in soils.
In addition to the resistance error, the fact that the thermal
conductivity of the surrounding medium differs from the sensor
thermal conductivity causes the heat flux to deflect. The
resulting error is called the deflection error. The deflection error
is determined in media of different thermal conductivity by
experiments or using theoretical approximations. The result of
these experiments is laid down as the so-called thermal
conductivity dependence Eλ. The order of magnitude of Eλis
constant for one sensor type. For HFP01, Eλis given in the list of
specifications.
Esen = E sen, cal (1+Eλ(λcal - λmed)) 1.2.3
Note: this correction can only be applied when there is a
substantial amount of (at least 40 mm) medium on both sides of
the sensor. In soils λmed usually is not known. The value of λcal
typically is zero.

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Figure 1.2.3 The deflection error. The heat flux (1) is deflected in
particular at the edges of the sensor. As a result the
measurement will contain an error; the so-called deflection error.
The magnitude of this error depends on the medium thermal
conductivity, sensor thermal properties as well as sensor design.
In addition, the sensitivity of heat flux sensors is temperature
dependent. The temperatre dependence TD reflects the fact that
the sensitivity changes with temperature:
Esen = E sen, cal (1+TD (Tcal - Tsen )) 1.2.4
Combining 1.2.3 and 1.2.4:
Esen =
E sen, cal {(1+Eλ(λcal - λmed))+ (1+TD (Tsen - Tcal ))} 1.2.5
This correction is rarely applied because TD is typically small.
Apart from the sensor's own thermal resistance, also contact
resistances between sensor and surrounding material are
demanding special attention. Essentially any air gaps add to the
sensor thermal resistance, at the same time increasing the
deflection error in an unpredictable way. In all cases the contact
between sensor and surrounding material should be as well and
as stable as possible, so that it is not influencing the
measurement. It should be noted that the conductivity of air is
approximately 0.02 W/m.K, ten times smaller than that of the
heat flux sensor. It follows that air gaps form major contact
resistances, and that avoiding the occurrence of significant air
gaps should be a priority whenever heat flux sensors are
installed.