Hukseflux TP01 User Manual
Copyright by Hukseflux | manual v1627 | www.hukseflux.com | info@hukseflux.com
USER MANUAL TP01
Thermal properties sensor
Hukseflux
Thermal Sensors
TP01 manual v1627 2/35
Warning statements
Putting more than 2 Volt across the sensor wiring
can lead to permanent damage to the sensor.
Putting more than 2 Volt across the heater wiring
can lead to permanent damage to the heater.
Do not use “open circuit detection” when measuring
the sensor output.
If power to the heater is supplied from a 12 VDC
source, you must put a 150 Ω resistor in series with
the heater.
TP01 manual v1627 3/35
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 TP01 8
1.2 Included items 8
1.3 Quick instrument che ck 8
2 Instrument principle and theory 9
2.1 General theory 9
2.2 Thermal conductivity measurement 10
2.3 Soil thermal conductivity for several soil types 11
2.4 Thermal diffusivity measurement 11
2.5 Volumic heat capacity measurement 12
2.6 Measuring the storage term in soil heat flux measurement 12
2.7 Trend monitoring of soil water content 13
2.8 Calibration 13
2.9 Programming 14
3 Specifications of TP01 16
3.1 Dimensions of TP01 19
4 Standards and recommended practices for use 20
5 Installation of TP01 21
5.1 Site selection and installation 21
5.2 Electrical connection 22
5.3 Requirements for data acquisition / amplification 24
6 Making a dependable measureme nt 25
6.1 Uncertainty evaluation 25
6.2 Typical measurement uncertainties 25
6.3 Contributions to the uncertainty budget 26
7 Maintenance and trouble shooting 28
7.1 Recommended maintenance and quality assurance 28
7.2 Trouble shooting 29
7.3 TP01 calibration 30
8 Appendices 32
8.1 Appendix on cable extension / replacement 32
8.2 Appendix on preparation of agar gel for ca librat ion 32
8.3 Appendix on use of TP01 beyond its rated measurement range 33
8.4 EU declaration of conformity 34
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List of sy m bols
Quantities Symbol Unit
Thermal conductivity λ W/(m∙K)
Voltage output U V
Voltage output as a function of heating time U (t) V
Voltage output difference ΔU V
Sensitivity S V/K
Heating power per meter Q W/m
Heater length L m
Temperature T °C
Temperature difference ΔT °C, K
Time constant τ s
Time t s
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) θ
m
kg/kg
Water content (on volume basis) θ
v
m³/m³
Subscripts
property of thermopile sensor sensor
property obtained under calibration reference reference
conditions
property at the (soil) surface surface
property of the surrounding soil soil
property of the heater heater
TP01 manual v1627 5/35
Introduction
TP01 is a sensor for long-term monitoring of soil thermal conductivity. A measurement
with TP01 may also be used to estimate the soil thermal diffusivity and volumic heat
capacity, leading to a better understanding of dynamic (variable heat flux) thermal
behaviour of soils. TP01 is designed for long-term use at one measurement location.
Applied in meteorological surface flux measurement systems, TP01 improves the
estimates of soil heat flux and of the so-called storage term (see the paragraph about
the storage term). The sensor, combining a heater and a temperature-d ifference senso r
with a high sensitivity and an extremely low thermal mass, is a proprietary Hukseflux
design.
The sensor inside TP01 is a temperature-difference sensor consisting of 2 thermopiles. It
measures the radial temperature difference around a heating wire with a record breaking
sensitivity. Both the heating wire and the sensor are incorporated in a very thin plastic
foil.
TP01 measures soil thermal conductivity. It is designed for long-term on-site operation,
buried in the soil. Its rated operating range is 0.3 to 4 W/(m∙K), which covers most
inorganic soil types. The low thermal mass of TP01 also makes it suitable for measuring
the soil thermal diffusivity and the volumic heat capacity.
The thermal conductivity, λ, in W/(m·K), is calculated by dividing the TP01 sensitivity, S,
by the sensor output, a small voltage difference ΔU which is a response to stepwise
heating, and multiplying by the applied electrical power Q per meter heating wire.
The measurement function of TP01 is:
λ = S·Q/ ΔU
(Formula 0.1)
The factory-determined sensitivity S
,
as obtained under calibration reference conditions,
is provided with TP01 on its product certificate. TP01 calibration is traceable to
international standards. The recommended calibration interval of TP01 is 2 years.
Thermal diffusivity and volumic heat capacity are estimated from time response to
stepwise heating. These measurements are optional.
The volumic heat capacity is a linear function of soil water content and you may use TP01
measurements to monitor trends in soil water content. Contrary to many other soil water
content sensors, TP01 is not sensitive to co ntamination by salts and the measurement
still f unctions in electrically conduct ing saline or fertilised soils.
TP01 should be incorporated in the user's measurement and control system. It can be
connected directly to commonly used data logging systems. Typically every 6 hours, the
TP01 heater is switched on to perform a measurement.
A typical TP01 is part of a meteo rological surface flux measurement system in which also
wind, humidity, soil heat flux, soil temperatures at different depths and net-radiation are
TP01 manual v1627 6/35
measured. TP01 then serves to improve the estimate of the so-called storage term,
which is used to model thermal heat transport in the soil. Measurements with TP01 are
often combined with soil temperature profile measurements with sensor model STP01
and measurements with heat flux sensor model HFP01SC.
Soil thermal properties change as a function of depth, in particula r close to the soil
surface. A typical measurement location is equipped with sensors at several depths. For
good spatial averaging at least 2 sensors (> 5 m apart) should be installed at every
depth.
Hukseflux has equipped several testbeds in the electrical power industry, to monitor
dryout, thermal runaway and thermal stability around mock-up high-voltage power lines.
Here the capability to perform a measurement of thermal diffusivity is an important
feature for modelling behaviour under dynamic loads.
Equipped with heavy duty cabling, and potted so that moisture does not penetrate the
sensor, TP01 has proven to be very robust and stable. It survives long-ter m installation
in soils.
Figure 0.1 TP01. Standard cable length is 5 m.
TP01 manual v1627 7/35
See also:
• STP01 soil temperature profile sensor
• for laboratory use, models TP02 and TP08 are available. Turn key measuring systems
are TPSYS02, FTN02 and MTN02.
• Hukseflux sensors for surface flux measurement
• heat flux sensors HFP01 and HFP01SC
Figure 0.2 TP01 thermal properties sensor. The thermopile sensor (1) and heating wire
(2) are both incorporated in a thin plastic foil. The cable (3) is 5 m long in the standard
configuration and may be extended to 50 m.
20
4
20
60
1
3
1.
2.
3.
4.
5.
6.
0.15
TP01 manual v1627 8/35
1 Ordering and checking at delivery
1.1 Ordering TP01
The standard configuration of TP01 is with 5 metres cable.
Common options are:
• longer cable in multip les 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:
• thermal properties sensor TP01
• 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 and heater according to table 5.2.2. Use a
multimeter at the 100 Ω range. The typical resistance of the wiring is 0.1 Ω/m (added
value of 2 wires). 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. Activate the TP01
heater by putting 1 to 2 VDC across the brown and yellow wires. Use a 1.5 V battery. Put
the sensor in soil or another granular material. The signal between the green and white
output should read > 1 x 10
-3
V now. It will vary if the sensor moves.
3. Inspect the instrument for any damage.
4. Check the sensor serial number, and sensitivity on the cable labels (one at sensor end,
one at cable end) against the product certificate provided with the sensor.
TP01 manual v1627 9/35
2 Instrument principle and theory
TP01 measures the thermal conductivity [λ] of the surrounding environment. It has a
rated measurement range of 0.3 to 5 W/(m·K) which makes it suitable for use in most
soils. A requirement for an accurate measurement is that there is good thermal contact
between soil and sensor. You must incorporate TP01 in your own measurement and
control system. For thermal conductivity measurement this system should perform 2 x
voltage readout, and power supply switching. For thermal diffusivity and volumic heat
capacity the sensor response time must be measured.
Advantages of using TP01 are:
• high sensitivity (good signal to noise ratio in low-flux environments, low us e of
power)
• low thermal mass (allows for a quick measurement and the ability to measure
thermal diffusivity and heat capacity)
• robustness, including a strong cable (essential for permanently installed sensors)
• IP protection class: IP67 (essential for outdoor application)
• low electrical resistance (low pickup of electrical noise)
2.1 General theory
The thermopile sensor generates a voltage output, as a reaction to the radial
temperature difference around the heating wire. This can be seen in figure 2.1.1. It gives
a top view of the sensor and the surrounding soil when hea ting . The heating wire
generates a ci rcu lar temperature field. After 180 s, the temperature difference around
the sensor becomes stable.
Figure 2.1.1 Top view of the radial temperature distribution (with isotherms (3)) around
the heating (2) wire of TP01 (1) in two different environments; right high thermal
conductivity, left low thermal conductivity. The thermopiles measure the difference
between the temperature at the hot joints (4) and the c old joints (5).
T
3
T
2
T
1
T
2
T
1
2
1
3 4
5
λ >>λ <<
TP01 manual v1627 10/35
U
t
U ~ 1/
λ
t ~ a
λ < λ
1 1
, a > a
λ = λ
1 1
, a < a
λ > λ
1 1
, a = a
Figure 2.1.2 TP01 signal in different soil types: the signal amplitude varies with [1/λ],
the response time varies with thermal diffusivity [a]
2.2 Thermal conductivity measurement
The measurement principle of TP01 relies on measurement of the radial temperature
difference around a heating wire. The temperature difference is measured by two
thermopiles connected in series, generating a single output. Both the heater and the
thermopile are inc orporated in a thin plastic foil.
The thermal conductivity, λ, in W/(m·K), is calculated by dividing the TP01 sensitivity, S,
by the output, a small voltage difference ΔU which is a response to stepwise heating, and
multiplying by the applied electrical power Q per meter heating wire.
The measurement function of TP01 is:
λ = S·Q/ ΔU
(Formula 0.1)
The voltage difference ΔU is determined by performing a measurement just before the
heating starts and after heating for 180 s.
ΔU = U (180) – U (0) (Formula 2.2.1)
The factory-determined sensitivity S
,
as obtained under calibration reference conditions,
is provided with TP01 on its product certificate. The heating power Q, in W/m, is
determined from a voltage measurement across the heater and taking the heater length
and electrical resistance into account.
TP01 manual v1627 11/35
2.3 Soil thermal conductivity for several soil types
Data for the following graph is taken from IEEE standard 442 – IEEE guide for Soil
Thermal Resistivity Measurements”, figure 3. It gives orders of magnitude of the thermal
conductivity for different soil types as a function of water content.
Figure 2.3.1 Typical soil thermal conductivity values for different soil types as a function
of water content.
2.4 Thermal diffusivity measurement
The low thermal mass of the total TP01 sensor makes it suitable for estimating thermal
diffusivity [a]. Dividing [λ] by the thermal diffusivity [a] gives the volumic heat capacity
[c
volumic
] which varies with water content.
Thermal diffusivity and volumic heat capacity may b e estimated from the response time
to stepwise heating.
The output response of TP01 to stepwise heating is:
ΔU = (S·Q/ λ) · F[a·t] (Formula 2.4.1)
With [t] time, [a] thermal diffusivity, and F a function that equals 1 at large values of
[a·t] and 0 at the start of the heating interval.
Formula 2.4.1 shows that the step response of the sensor signal scales with [Q/ λ] for
the amplitude, and with [a] for the time response.
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