Hukseflux STP01 User Manual

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USER MANUAL STP01

Soil temperature profile sensor with self-test

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Warning statements

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

Putting more than 15 Volt across the heater wiring
can lead to permanent damage to the heater.

Do not use “open circuit detection” when measuring

the thermocouple sensor output.

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Contents

Warning statements 2
Contents 3
List of symbols 4
Introduction 5
1 Ordering and checking at delivery 9

1.1 Ordering STP01 9

1.2 Included items 9

1.3 Quick instrument check 9

2 Instrument principle and theory 10

2.1 General soil temperature measurement 10

2.2 STP01 soil temperature measurement 10

2.3 STP01 on-line self-test 11

2.4 Optional thermal conductivity measurement 11

2.5 Conformity testing and traceability 12

2.6 Programming 13

3 Specifications of STP01 14

3.1 Dimensions of STP01 17

4 Standards and recommended practices for use 18
5 Installation of STP01 19

5.1 Site selection and installation 19

5.2 Electrical connection 20

5.3 STP01 diagnostics 22

5.4 Requirements for data acquisition / amplification 23

6 Making a dependable measurement 24

6.1 Uncertainty evaluation 24

6.2 Contributions to the uncertainty budget 24

7 Maintenance and trouble shooting 26

7.1 Recommended maintenance and quality assurance 26

7.2 Trouble shooting 27

8 Appendices 29

8.1 Appendix on cable extension / replacement 29

8.2 EU declaration of conformity 30

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List of symbols

Quantities Symbol Unit

Temperature T °C
Temperature difference ΔT °C, K
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
Time constant τ s
Time t s
Resistance R Ω
Depth of installation x m
Distance from the heater r m

Subscripts

property of a sensor sensor
property of the reference temperature Pt100 reference
property at the (soil) surface surface
property of the surrounding soil soil
property of the heater heater

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Introduction

STP01 accurately measures the temperature profile of the soil at 5 depths close to its
surface. It is used for scientific grade surface energy balance measurements. The sensor
is buried and usually cannot be taken to the laboratory for calibration. The on-line self­test using the incorporated heating wire offers a solution to verify STP01’s measurement
stability.

STP01 soil temperature profile sensor offers an accurate temperature difference
measurement at five measurement locations at 0.02, 0.05, 0.1, 0.2 and 0.5 m below the
soil surface. It also has a well specified and fixed distance between the measurement
locations.

STP01 contains 5 matched thermocouples, at locations A to E, and one reference
temperature sensor (Pt100 type) at location E at 0.5 m depth (see figure 0.2). By having
the reference temperature measurement in the sensor and only measuring differential
thermocouple voltages (relative to the reference at 0.5 m), the uncertainty of the
temperature difference measurement is very low: ± 0.02 °C is attainable. Simple copper­conductor signal wire is used in STP01’s cable. As an extra, a heating wire is incorporated
in STP01. Analysis of the temperature change during the heating interval serves as a
self-test.

Soil temperature sensors are preferably left in the soil for as long as possible, so that the
soil properties become representative of natural conditions. Using self-testing, the user
no longer needs to take sensors to the laboratory to verify their stable performance.
The result is a much improved accuracy & quality assurance of the measurement relative
to measurements with conventional sensor types.

Typically every 24 hours, the STP01 heater is switched on to perform a self-test.
When activating the heater for a self-test, this will lead to a local increase in temperature
at the sensors at 0.02, 0.05, 0.1 and 0.2 m depth. The STP01 stability is monitored by
analysis of yearly patterns of this step-response.

The step response of the temperature during the self-test can be used to measure the
soil thermal conductivity at 3 depths; 0.05, 0.1 and 0.2 m. For more background on this
measurement method, see the manual of model TP02 thermal needle. The possibility to
perform this measurement is an experimental option, with an unspecified measurement
accuracy.

STP01 is used for high accuracy, scientific grade measurement of the soil energy
balance, with a high level of data quality assurance. Measurements with STP01 are often
combined with soil thermal conductivity and volumic heat capacity measurements with
sensor model TP01 and measurements with heat flux sensor model HFP01SC.

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STP01 advantages are:

 high accuracy, scientific measurement of soil energy balance, with a high level of data

quality assurance

 high accuracy K/m temperature gradient measurement by accurate positioning of the

thermocouple joints (± 0.001 m), and accurate temperature difference measurement
(± 0.02 K)

 high accuracy and stability of the relative distance between sensors (± 0.0005 m)
 thin, 0.6 x 10

-3

m thickness, construction which leaves the soil structure intact

 simple copper-core signal wire; no special connectors needed
 self-test saves servicing time

Sensors made by Hukseflux are designed for compatibility with the most commonly used
datalogger models. For many models we have example programs and wiring diagrams
available.

Requirements for data acquisition and control are:

 for temperature measurement: four millivolt measurements, one Pt100 measurement
 for the optional self-test: one heater voltage measurement
 for the optional self-test: one relay with 12 VDC nominal output, switching the heater

on and off

Equipped with heavy duty cabling, and potted so that moisture does not penetrate the
sensor, STP01 has proven to be very robust and stable. It survives long-term installation
in soils. For ease of installation with a minimum of disturbance of the local soil, Hukseflux
offers IT01 insertion tool.

Figure 0.1 STP01. Standard cable length is 5 m.

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Figure 0.2 STP01 layout and dimensions

1 soil surface
2 sensor foil (0.6 x 10-3 m thickness, 2.5 x 10-3
m at Pt 100)
3 2 x cable 5 m (see options)
4 copper leads
5 T type thermocouple wire
6 CuNi heater wire
7 Pt100 reference temperature sensor (4-wire
connection)
I and II: connection points of the heating wire
(4-wire connection)
A, B, C, D and E: thermocouple type T joints
Dimensions in x 10-3 m

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Options are:

 longer cable (2 x), in multiples of 5 m, cable lengths above 20 m in multiples of 10 m
 insertion tool IT01

Figure 0.3 IT01 insertion tool is hammered down into the soil. After retracting it leaves
a slit in which STP01 may be inserted.

See also:

 model TP01 soil thermal properties sensor

 soil heat flux sensors HFP01 and HFP01SC

 view our complete product range of surface energy flux measurement products

view our range of pyranometers and net-radiometers

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1 Ordering and checking at delivery

1.1 Ordering STP01

The standard configuration of STP01 is with 2 x 5 metres cable.

Common options are:

 longer cable in multiples of 5 m, cable lengths above 20 m in multiples of 10 m.

specify total cable length.

 insertion tool IT01

1.2 Included items

Arriving at the customer, the delivery should include:

 thermal properties sensor STP01
 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 the tables in
paragraph 5.3. Use a multimeter at the 50 and 500 Ω range. The typical resistance of the
wiring is 0.4 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 thermocouple sensors react to heat: put the multimeter at its most
sensitive range of DC voltage measurement, typically the 100 x 10-3 VDC range or lower.
Measure between the thermocouple common and the other thermocouple joints. Look at
the reaction when you heat one of the joints.

3. Inspect the sensor foil for any damage.

4. Check the sensor serial number on the cable labels (one at sensor end, one at cable
end of both cables) against the product certificate provided with the sensor.

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2 Instrument principle and theory

STP01 accurately measures the temperature profile of the soil at 5 depths close to its
surface. A requirement for an accurate measurement is good thermal contact between
soil and sensor. The sensor is buried and usually cannot be taken to the laboratory for
calibration. The on-line self-test using the incorporated heating wire offers a solution to
verify STP01’s measurement stability. The user must incorporate STP01 in his own
measurement and control system. For soil temperature measurement this system should
perform 4 x voltage, and 1 x Pt100 readout. For the self-test you need power supply
switching, and 1 x voltage readout.

Relevant features of TP01 are:

 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 soil temperature measurement

Usually temperature gradients in the soil are made using separate sensors.
Shortcomings of this method are:

 temperature gradients are calculated by taking the difference between inaccurate

measurements. The end result will have a large uncertainty

 the relative position is not accurately determined
 the sensor construction is often heavy, and conducting a significant amount of heat

Soil temperature and soil thermal properties vary slowly. Usually the data sampling is
done with a 60 s interval and 10 minute averages are stored. You may choose to vary
the data storage interval with depth.

2.2 STP01 soil temperature measurement

STP01 overcomes the traditional shortcomings:

 STP01 uses a string of thermocouples type T sharing one and the same CuNi wire and

the same Cu wire (made from the same sheet). We claim that the thermocouples are

“matched”, i.e. have the same properties. The relative accuracy of such matched

thermocouples is better than 0.05 °C.

 Determined by the manufacturing process, the relative position of the thermocouple

joints is accurate within ± 0.001 m.

 The construction of STP01 consists of plastic foil and relatively thin conductors.