Hukseflux TP01 User Manual

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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

TP01 manual v1627 4/35

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.

TP01 manual v1627 12/35

By curve fitting F you can determine [a]. One of the eas iest ways of doing this is looking
at the 63 % response time of the measurement. The common procedure is to first
determine the thermal conductivity, and during the settling interva l after the heating
interval to determine how much time it takes to arrive at 37 % of the ΔU amplitude.
The thermal diffusivity is measured by comparing the TP01 response time dur ing the
measurement to that under calibration reference conditions. The calibration reference
conditions include a thermal diffusivity of 0.14 10

-6 m2

/s.

a

reference

= 0.14 10-6

The 63 % response time for the determination of the thermal diffusivity is 19 s.

τ

reference

= 19
The measurement equation for the thermal diffusivity is:
a = a

reference · τ reference

/ τ (Formula 2.4.2)

2.5 Volumic heat capacity measurement

The volumic heat capacity is determined from
c

volumic

= λ/a (Formula 2.5.1)

2.6 Measuring the storage term in soil heat flux measurement

In meteorological experiments one of the measured parameters is heat flux in the soil.
For practical reasons this is done at 0.05 m depth. There is n o simple solution for direct
measurement of soil heat flux at the soil surface, which is the parameter that must
eventually be estimated.

The flux at the soil surface Φ

surface

is usually estimated from the flux measured by the
heat flux sensor buried at a depth of around 0.05 m 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 2.6.1)

The quantity S is called the storage term.
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 2.6.2)

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.

TP01 manual v1627 13/35

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 estimate d 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·(c

soil, dry

+ θ

m

·cwater) = ρsoil·csoil, dry + ρwater· θv ·cwater (Formula 2.6.3)

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.

With TP01 the estimate of the volumic heat capacity is much simpler:

c

volumic

= λ/a (Formula 2.5.1)

2.7 Trend monitoring of soil water content

In a certain soil type there are direct and linear relationships between the soil water
content by mass or volume and the volumic heat capac ity. TP01 can therefore be used to
monitor trends of soil water content even without detailed knowledge of soil dry densities
and heat capacities.

θ

m

= ((cvolumic / ρsoil )- c

soil, dry

)) / cwater (Formula 2.7.1)

For estimates on a volume basis, one ha s to multiply by ρ

soil and divide by ρwater:

θ

v

= (cvolumic - c

soil, dry

ρsoil) / ρwater cwater (Formula 2.7.2)

2.8 Calibration

TP01 factory calibration is traceable from SI.

TP01 is calibrated by a thermal conductivity measurement in a calibration reference
material, and by performing an electrical resistance measurement.
The test method is not standardised. The calibration reference material is a verified
intrinsic measurement standard (vocabulary a ccording to ISO Guide 99), an agar gel,
which has thermal properties similar to water. The agar gel is characterised by a
reference thermal needle, which is traceable to length and electrical resistance. The TP01
heater electrical resistance is traceable to current and voltage standards.

TP01 manual v1627 14/35

2.9 Programming

Soil conditions typically change slowly. A typical time between measurements is 6 hr.
You may choose to shorten the interval between measurements to 3 hr.

2.9.1 Programming for thermal conductivity measurement

λ = S·Q/ ΔU

(Formula 0.1)

The power Q in W/m can be determined from the voltage across the heater U

heater

and

the heater resistance

λ = S·U

heater

2

/ (ΔU·R·L)

(Formula 2.9.1.1)

With S the sensitivity, R the heater electrical resistance and L the heater length. These
parameters can be found on TP01’s calibration certificate. For the standard model TP01,
the heater length L is 0.06 m, and the heating time interval is 180 s, so that the
measurement equation becomes

λ = S·U

heater

2

/ (0.06 (U (180) – U (0))·R) (Formula 2.9.1.2)

You may define an acceptance interval for λ, and for U

heater.

2.9.2 Programming for thermal diffusivity measurement

By curve fitting F you can determine [a]. One of the eas iest ways of doing this is looking
at the 63 % response time of the measurement. The common procedure is to first
determine the thermal conductivity, and during the settling interva l after the heating
interval to determine how much time it takes to arrive at 37 % of the ΔU amplitude.

The measurement equation for the thermal diffusivity is:
a = a

reference · τ reference

/ τ (Formula 2.4.2)

= (0.14 x 10

-6

) ·19/ τ

= 7.4 x 10

-3

/ τ

For a reasonably accurate estimate of the thermal diffusivity, the data storage interval
must be < 1 s.

2.9.3 Programming for the volumic heat capacity measurement

c

volumic

= λ/a (Formula 2.5.1)

TP01 manual v1627 15/35

2.9.4 Program summary

In case the user writes his own software program for controlling the TP01, the program
flow in table 2.9.4.1 may be used.

Use a < 1 s data storage interval when the thermal diffusivity is measured.
Table 2.9.4.1 A summary of a program for control of the measurement with TP01

initialisation enter sens or an d system

information

serial number, S, R, L, a

reference , τ reference

every 6 h

360 s thermal

conductivity and thermal
diffusivity measurement

measure U, U

heater,

t

180 s heating interval

heater on

measure U

measure U

heater

180 s settling interval

heater off

measure U

thermal

calculate λ

calculate a

quality checks

acceptance interval λ

acceptance interval a

acceptance interval U

heater

Acceptance interval U(0) – U(180)

if accepted, then

next loop

else

generate warning

calculate derived

parameters such as θ

TP01 manual v1627 16/35

3 Specifications of TP01

TP01 measures the thermal conductivity [λ] of its surrounding environment. It is

designed for long-term monitoring of soils. It has a rated measurement range of 0.3 to 5
W/(m·K) which makes it suitable for use in most (inorganic) soils. It is also used for
measurement of thermal diffusivity and volumic heat capacity. Good thermal contact
between soil and sensor is required. TP01 can only be used in combination with a
suitable measurement and control system.

Table 3.1 Specifications of TP01 (continued on next page)

TP01 SPECIFICATIONS

Sensor type

thermal properties sensor

Measurand

thermal conductivity

Rated operating environm ent surrounded by soil

non-organic, saline and non-saline, fertilised and non-

fertilised soils

Measurand in SI units

thermal conductivity in W/(m·K)

Measurement range

0.3 to 5 W/(m·K)

Optional non-traceable measurand

thermal diffusiv ity

Measurand in SI units

thermal diffusiv ity in m2/s)

Measurement range

(0.05 to 1 ) x 10-6 m2/s

Optional non-traceable measurand

volumic heat capacity

Measurand in SI units

volumic heat capacity in J/(kg·K)

Optional trend mon itoring

soil water content

Measurand in SI units

water content in kg/kg or m3/m3

Measurement principle radial temperature difference meas urement around a

heating wire using a thermopile sensor

Sensitivity

150 x 10-6 V/K (nominal)

Expected voltage output -1 to 1 x 10-3 V (thermopile sensor)

0 to 1.5 VDC (heater)

Required programming

measurement of thermal conductivity

optionally measurement of thermal diffusivity and

volumic heat capacity

Measurement function / required

programming

λ = S·Q/ ΔU

Optional measur ement function/ optional

programming

a = a

reference · τ reference

/ τ

Optional measur em e nt function/ optional
programming

c

volumic

= λ/a

Required readout and c ontrol thermopile sensor and heater: 2 x diff erential voltage

channel or 2 x single ended voltage channel,
input resistance > 10

6

Ω

heater: 1 x switchable power 1 - 2 VDC

Required uncertainty 10 x 10-6 V at 10-3 V

5 x 10-3 V at 2 V

Rated operating temperatu r e r a nge

-30 to +80 °C

Temperature dependence

< 0.1 %/°C

Time constant in a gar gel calibration
reference material

19 s
(nominal)

Non-stability

< 1 %/yr

TP01 manual v1627 17/35

Table 3.1 Specifications of TP01 (started on previous page, continued on next page)

Sensor foil surface dimensions

(60 x 20) x 10

-3

m

Sensor foil thickness

0.15 x 10

-3 m

Connector block dim e nsions

(43 x 24 x 10) x 10-3 m

Thermopile sensor number of

thermocouple pairs

40

Thermopile sensor resistance range

20 to 50 Ω

Standard governing use of the
instrument

not applicable
Standard cable length (see options)

5 m

Wiring

0.15 m wires and shield at cable ends

Cable diameter

5 x 10-3 m

Cable markers

2 x sticker, 1 x at sensor and 1 x cable end, wrapped

around the sensor cable. Both stickers show serial

number.

IP protection cla s s

IP67

Rated operating relative humidity range

0 to 100 %

Gross weight including 5 m cable

0.5 kg

Net weight including 5 m cable

0.4 kg

Packaging

box of 220 x 160 x 30 mm

HEATER

Heater resistance (nominal) 15 Ω

(measured value supplied with ea c h sensor in the
production report)

Heater resistance range

10 to 20 Ω

Heater length

0.06 m

Heater rated power supply

1 to 2 VDC , 0.4 A

Heater power supply

1 VDC (nominal)

Power consumption during heating
interval

0.9 W
(heater powered from 12 VDC , using a 150 Ω series

resistor)

Suggested series resistor

when

powered from 12 VDC

150 Ω ± 5 %, 2 W in series with the heater
Limiting heating power

0.8 W/m

MEASUREMENT

Power consumption daily average 0.007 W

(heater powered from 12 VDC, using a 150 Ω series

resistor, at a 6 hr me a surement interval)

Interval between measur em ents

6 hr, optionally 3 or 12 hr

Duration of measurement

360 s

Heating interval duration

180 s

Settling interval d uration

180 s

INSTALLATION AND USE

Measurement depth

0 to 20 m

Recommended number of sens or s at a measurement site, at every required

measurement depth use > 2 sensors at a distance of

> 5 m

Orientation recommended orien tation is with foil surface vertically

oriented (usua lly this is perpendicular to the soil

surface), so that flow of water is not obstructed.

Installation

see recommendations in the product manual

Cable extension see chapter on cable exten s ion or order sensors with

longer cable

TP01 manual v1627 18/35

Table 3.1 Specifications of TP01 (started on previous pages)

CALIBRATION

Calibration trace a b ility

to SI units

Production certificate included

(showing calibra tion result and traceability, as well as

heater resistance and heater length)

Factory calibrat ion method

method TPSC

Factory calibrat ion uncertainty

10 % (k = 2)

Recommended recalibrati on interval

2 years

Factory calibrat ion reference conditions 20 °C, thermal conductivity of the surrounding

environment 0.6 W/(m·K)

Validity of calibra tion

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

“non-stability” specificat ion is applicable.

Calibration refe r ence material

agar gel, which has th ermal properties identical to

those of water:
thermal conductivity of 0.6 W/(m·K)
thermal diffusivity of 0.14 x 10

-6 m2

/s

both at 20 °C

MEASUREMENT ACCURACY

Uncertainty of the measurement statements about the overall measurement

uncertainty ca n only be made on an individual basis.

see the chapter on uncertainty evaluation.

VERSIONS / OPTIONS

Order code

TP01/cable length in m

Longer cable

in multiples of 5 m, cable lengths a bov e 20 m in

multiples of 10 m

option code = total ca ble length

ACCESSORIES

No accessories

TP01 manual v1627 19/35

3.1 Dimensions of TP01

Figure 3.1.1 TP01 thermal properties sensor. dimensions in x 10-3 m.

(1) thermopile sensor
(2) heating wire
(3) cable (standard length 5 m, optionally longer cabl e in multiples of 5 m, cable above

20 m in multiples of 10 m)

20

4

20

60

1

3

1.

2.

3.

4.

5.

6.

0.15

TP01 manual v1627 20/35

4 Standards and recommended practices

for use

TP01 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.

There are no standardised operating practices for use of TP01 sensors. The next chapters
contain recommendations of the sensor manufacturer.

In meteorological applications a thermal properties sensor measures thermal conductivity
and volumic heat capacity of the soil, typically at several depths.
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.

Figure 4.1 Typical meteorological surface energy balance measurement system with
TP01 installed under the soil.

TP01 manual v1627 21/35

5 Installation of TP01

5.1 Site selection and installation

Table 5.1.1 Recommendations for installation of TP01

Location

Preferably install in a large field which is relatively homogeneous and

representative of the area u nder observation.

Depth Install at the relevant depths for the measurem ent. Preferred depths of

installation diff e r for each application.
To estimate the storage term for soil surface heat flux estimation, a
typical depth of in s tallation is 0.05 m; which is as close to the soil
surface as practically attainable.

Orientation Recommended orientation is with foil surface perpendicular to the soil

surface, so th a t flow of water is not obstructed.
The measurement will work independent of orientation.

Performing a
representative
measurement

At every measurement depth we recommend using > 2 sensors per
location at a distance of > 5 m. This redundancy also improves the
assessment of the measurement accuracy.

Installation

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.
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 that after
installing the sensor the original soil structure c a n be restored. The
sensor is installed in the undisturbed face of the excava ted hole.
Make sure the depth of the hole is close to the required depth of
installation so that the sensor connector block and c a b le can rest upon
the bottom of the hole.
Measure the depth from the soil surface at the top of th e hole. With a
knife, make a vertical cut at the required depth of installation, into the
undisturbed face of the hole. Insert the sensor foil into the 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 sta b ility and in order to avoid exerting too much force

on the sensor foil, provide sensor cables with addit ional strain relief, for
example connecting the ca ble with a tie wrap to one or more metal pins
that are inserted firmly int o 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 and
heat flux sensors

Temperature sensors and heat flux sensors are typically located close to
the sensor.

TP01 manual v1627 22/35

5.2 Electrical connection

A TP01 must be connected to a measurement and control system, typically a so-called
datalogger. TP01’s thermopile sensor is a passive sensor that does not need any power.
The heater is powered from 1 to 2 VDC, using a user-supplied relay to switch it on and off.
The 4-wire connection to the heater makes it possible to perform a heater voltage
measurement that does not depend on the cable length. Two wires carry the heater
current, the others are use d for the measurement. No current flows through the latter
wires, so that there is no voltage drop, and the true voltage across the heater is measured.

Putting more than 2 Volt across the sensor wiring
can lead to permanent damage to the sensor.

Putting more than 2 Volt across the hea ter wiring
can lead to permanent damage to the heater.

If power to the heater is supplied from a 12 VDC

source, you must put a 150 Ω resistor in series with

the heater.

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 thermopile sensor
output is connected to a differential or single-ended voltage input.

Table 5.2.1 Connections of the TP01 cable. The cable internally also has a pink and a
grey wire, which are not used and not visible when supplied from the factory. The wires

extend 0.15 m from the cable end.

WIRE # CLADDING FUNCTION MEASURING SYSTEM

1 White thermopile signal [+] analogue voltage [+]
2 Green thermopile signal [-] analogue voltage [-] or ground
3 Red heater voltage output [+] analogue voltage [+]

4

Brown heater power supply [+] heater power supply 1-2 VD C

5 Blue heater voltage output [−] analogue voltage [-] or ground

6 Yellow heater power supply [−] heater power supply ground w. relay

7

Pink not connected not connected

8

Grey not connected not connected

9

Black ground analog ground

TP01 manual v1627 23/35

Figure 5.2.1 Electrical conn e ct ion of TP01.
Thermopile sensor (2) wires are connected to datalogger inputs 1 and 2. Heater (1) wires
are connected to the power supply at 4 and 6 ( with relay). The heater voltage is
measured across wires 3 and 5. The relay (not included in the TP01 delivery) is used to
switch the heater on and off.

Table 5.2.2 Resistance checks for diagnostics of TP01

WIRE #

CLADDING CLADDING

RESISTANCE ACCEPTANCE

INTERVAL

1-2 White Green 20 to 50 Ω plus 0.1 Ω/m cable resistance
3-4 Red Brown 0.1 Ω/m cable resistance

3-2

Red Green infinite/not connected

3-5

Red Blue 10 to 20 Ω plus 0.1 Ω/m cable resistance

3-6

Red Yellow 10 to 20 Ω plus 0.1 Ω/m cable resistance

3-9

Red Black infinite/not connected

TP01 manual v1627 24/35

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 TP01 are available: contact the supplier of the data acquisition
equipment.

Table 5.3.1 Requirements for data acquisition, amplification and control equipment for
TP01 in the standard configuration

Capability to measure small voltage
signals

preferably: 10 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)
Input resistance > 10

6

Ω

Capability to measure the hea ter
voltage

the heater is powered from 2 VDC
preferably: 5 x 10

-3

V uncertainty

minimum requirement: 20 x 10

-3

V uncertainty
Input resistance > 10

6

Ω

Capability to switch the heater on
and off

a relay must be used. In case you are working from 12 VDC,

you must install a 150 Ω series resistor and the relay must

be capable of switching the required 12 VDC at 0.07 A
(nominal 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 thermal conductivity.
λ = S·Q/ ΔU (Equation 0.1)
to time and control th e measurement
to perform comparis on of measurement results against the
acceptance limits
optionally to determine the response time for measurement
of the thermal diff usivity and volumic heat capacity using a
< 1 s data storage interval.

a = a

reference · τ reference

/ τ (Formula 2.4.2) and

c

volumic

= λ/a (Formula 2.5.1)

Open circuit detec tion
(WARNING)

open-circuit detec tion should not be used, unless this is done
separately from the norma l mea s urement by more than 5
times the sensor resp onse time and with a small curren t
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.

TP01 manual v1627 25/35

6 Making a dependable measurement

6.1 Uncertainty evaluation

A measurement 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.

The measurement uncer t ainty is a function of:

• calibration uncertainty

• 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

• corrections applied

It is not possible to give a single estimat e for TP01 measurement uncertainty.
Statements about the overall measurement uncertainty can only be made on an
individual basis, taking all these factors into account.

6.2 Typical measurement uncertainties

Table 6.2.1 Typical measurement uncertai nt i es when measuring with TP01

APPLICATION

TYPICAL MEASUREMENT UNCERT AINTY BUDGET (K=2)

Meteorology

measurements of soil thermal conductivity may attain uncertainties in
the order of ± 10 % in the rated measurement range of 0.3 to 5
W/(m·K).

measurements of s oil thermal diffusivity may a tta in uncertainties in the
± 20 % range in the rated measur ement range of 0.3 to 5 W/(m·K)

TP01 manual v1627 26/35

6.3 Contributions to the uncertainty budget

6.3.1 Non- representativeness of the measurement location.

The representativeness of the measurement location may be assessed by performing
multiple measurements at various locations in the same area.

6.3.2 Calibration uncertainty

The main source of uncertainly for thermal conductivity is the uncertainty of calibration,
which is defined as ± 10 %. This uncertainly is mainly due to the limited accuracy that
Hukseflux can attain when measuring the thermal conductivity of water (or its thermal
equivalent agar gel). If the user is prepared to accept agar gel as an “inherent
measurement standard” the uncertainty of calibration is ± 5 %.

6.3.3 Non-stability

TP01’s non-stability specification is < 1 %/yr. This means that for every year of
operation 1 % uncertainty should be added in the uncertainty budget.

6.3.4 Temperature dependence

The temperature dependence of the thermopile is < 0.1 %/K. That of the heater is
negligible.

6.3.5 Heater resistance and length

R

heater

is a property of the individual sensor. Its nominal value is 20 to 50 Ω. Its exact
value can be found on the calibration certificate. The TP01 heater length is 0.06 m. The
uncertainties of resistance and length do not play a role in the measurement uncertainty
because when calibrating they are entered as constants and part of one factor: S/(R·L) in
the measurement equation (Formula 2.9.1.1).

6.3.6 Bad contact to the soil

Uncertainty caused by bad contact to the soil is hard to quantify. The suggestion is to
look for yearly patterns, and in the course of time to narrow down acceptance interval
limits of thermal conductivity and diffusivity.

TP01 manual v1627 27/35

6.3.7 Estimate of the time constant

When estimating the thermal diffusivity, the data storage interval must be sufficiently
small so that the time constant of the measureme nt can be compared to the reference
time constant of 19 s.
Although it is possible to attain good results by curve fitting, the easiest approach is to
sample and store date at an interval < 1 s so that the time interval is not the main factor
determining the measurement uncertainty.

TP01 manual v1627 28/35

7 Maintenance and trouble shooting

7.1 Recommended maintenance and quality assurance

TP01 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 thermal conductivity and thermal diffusivity. 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 TP01. If possible the data analysis should be
done on a daily basis.

MINIMUM RECOMMENDED HEAT FLUX SENSOR MAINTENANCE

INTERVAL

SUBJECT

ACTION

1

1 day

measurement

at least one measuremen t per day

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

maximum expected therm a l properties and to other
measurements for example from nearby stations 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
measurement results. Com pa re to acceptance intervals.
Plot thermal conduc tivity and diffusivity data against other
meteorological me a surands, in particular soil water content.

3 6 months inspection inspect cable quality, inspect mounting, inspect locat ion 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

TP01 manual v1627 29/35

7.2 Trouble shooting

Table 7.2.1 Trouble shooting for TP01

General Inspect the quality of mounting / installation.

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, resistance and
heater length are entered. TP01 sensitivity and serial numb er are marked on its
cable, electrical resista nce and heater length on the produc t c ertificate.
Check the sensor serial number, and sensitivity on the cable labels (one at sensor
end, one at cable end) against th e pr odu c t c er tificate provided with the sensor.

The sensor
does not give
any signal

A quick test of the instrument can be done by connecting it to a multimeter.
Check the electric a l resistance of the sensor and h eater 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.
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. Activate

the TP01 heater by putting 1 to 2 VDC a c r os s th e br ow n and yellow wires. Use a

1.5 V battery. Pu t the sensor in soil or another granula r material. The signal
between the green and white output should read > 2 x 10

-3

V now. It will vary if
the sensor moves.
Inspect the instrument for any damage. The sensor foil surface should be smooth
and have no deep scratches.

The sensor
signal is
unrealistically
high or low

Check the cable condition looking for cable breaks.
Check the data a c q uisition of the sensor measurement by apply ing 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 acquisition of the heater voltage measurement by applying a 1 V
source to it in. Look at th e measurement result. Check if it is as expected.
Check the heater voltage outpu t. It should be in the 1 to 2 V range.
Check the data a c q uisition by short circuiting the data acquisition inputs with a
50 Ω resistor. Look at the measured value. Check if the output is close to 0 V.

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.

TP01 manual v1627 30/35

7.3 TP01 calibration

Recalibration of TP01’s is ideally done by the sensor manufacturer.
The recommended calibration interval of heat flux sensors is 2 years.

If the user is prepared to rely on the thermal properties of agar gel, calibration of TP01
can also be done in any laboratory that has the necessary electronic equipment.

The procedure for calibration is as follows:

1) prepare agar gel according to the appendix

2) make sure there is perfect contact between the foil of the sensor and the gel.

3) mechanically fix the sensor

4) perform a calibration by doing a normal measurement in agar gel. Knowing the
thermal properties of the gel, S can be calculated and the 63 % response time can be
timed.

5) measure the heater resistance

6) compare to the original production certificate. Within a typical sensor lifetime of 10
years, changes of more than 5 % of S en R

heater

are unusual.

TP01 manual v1627 31/35

TP01 manual v1627 32/35

8 Appendices

8.1 Appendix on cable extension / replacement

TP01 is equipped with one cable. 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 TP01 cable 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 extension of TP01

Cable

6-wire, shielded, with copper conductor (at Huksef lux 8-wire shielded
cable is used, of which only 6 wires are used)

Extension sealin g

make sure any connections are sealed against humidity ingress

Conductor resistance

< 0.1 Ω/m

Outer diameter

5 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 ne w 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 plated waterproof connectors. Always
connect the shield

8.2 Appendix on preparation of agar gel for calibration

The procedure fo r ca libration of TP01 relies on the use of agar gel. This is a water-based
gel, of which the ingredients can be bought in every pharmacy. In most countries agar is
also available in f ood stores that sell environmentally friendly foods. The agar gel is often
used for growing bacteria. The agar itself does not significantly influence the thermal
properties of water, but reduces the effects of convection.

TP01 manual v1627 33/35

Prepare agar gel by cooking about 4 grams of agar in 1 litre of water, for about 20
minutes, stirring regularly. The solution is put in a pot, and be allowed to cool down and
solidify. This typically takes some hours. Use a pot which may be sealed. Once at room
temperature, make a slit in the gel using a knife. Ins ert the TP01 into the slit. Add some
water with a liquid antiseptic such as Dettol to suppress bacterial growth, and promote
the thermal contact between the gel and the sensor foil. Make sure the water fully covers
the gel in a layer of about 1 x 10

-3

m.

We assume by scientific judgement that the properties of agar gel at 4 x 10

-3

kg/kg are

the same as th ose of pure water:

Thermal conductivity: 0.6 W/(m·K), at 20 °C
Thermal diffusivity: 0.14 10

-6 m2

/s

8.3 Appendix on use of TP01 beyond its rated measurement
range

The rated measurement range of TP01 is 0.3 to 5 W/(m·K). Use beyond this rated range
is possible, but will lead to higher measurement uncertainties. At lower thermal
conductivities the thermal transport through the thermopile metal traces leads to non­linear behaviour. At higher thermal conductivities, the output signal reduces.
Table 8.3.1 lists some measurement results.

Table 8.3.1 Measur ements results with TP01 using the normal measurement equation.
Dry sand (zero moisture content, which will not occur in a real-life situation) and air are
beyond the rated measurement range. The sensor behaviour is strongly non-linear. A
measurement in agar gel is at calibration reference conditions and by definition has zero
deviation.

air

dry

sand

moist

sand

agar

gel

saturated

sand

theoretical

[W/(m·K)] 0.02 0.19 0.30 0.61 2.22

measured

[W/(m·K)] 0.16 0.29 0.32 0.61 2.11

deviation from
ideal

[%] 689 53 8 0 -5

TP01 manual v1627 34/35

8.4 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: TP01
Product type: Thermal properties 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 spec if i c ations without notice.