UGT Tensio 140 User manual

Operating Manual
Tensio 140
Version: 18/06/19
Laboratory Tensiometer
Umwelt-Geräte-Technik GmbH
Eberswalder Str. 58 | D-15374 Müncheberg
phone: [phone] - 89 575 | fax: [phone] - 89 573
e-Mail: [email protected] | www.ugt-online.de

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Content
1. Introduction...................................................................................................................................... 3
1.1. Safety Instructions.................................................................................................................4
2. General Information ......................................................................................................................... 5
2.1. What Are Tensiometers? / What Is Tension?..........................................................................5
2.2. What Are Tensiometers Used For? .........................................................................................6
2.3. How Does a Tensiometer Work? ............................................................................................6
2.4. Under Which Conditions Can a Tensiometer Be Used?............................................................7
3. Set Up and Technical Parameters..................................................................................................... 9
3.1. Set Up....................................................................................................................................9
3.2. Technical Parameters.............................................................................................................9
3.3. Pin Assignment....................................................................................................................10
4. Commissioning................................................................................................................................ 11
4.1. Preparation .........................................................................................................................11
4.2. Installation ..........................................................................................................................11
4.3. Pressure Readings................................................................................................................12
5. Maintenance and Repair................................................................................................................. 12
6. Caretaking ....................................................................................................................................... 12
6.1. General Care and Storage Instructions..................................................................................12

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1. Introduction
Thank you for choosing the Tensio 140. This manual can help you understand the Tensio 140’s
features.
We designed this laboratory tensiometer as an innovative measuring device for highly accurate
measurements with a minimum destruction of the soil.
Advantages:
Air bubbles are easy to recognize and eliminate through the acrylic head with vent screw. The
tensiometers can be installed in any position. They are ideally suited to determine the soil moisture
tension in experimental setups in the laboratory. The Tensio 140 is characterized by its long and robust
shaft.
An additional service to this manual is our tutorial on how to refill the Tensio 100 which can be found
at the following link:
https://www.ugt-online.de/en/products/soil-science/tensiometers/tensio-140/
There are several ways to contact UGT if you need a quote or product information please contact
Email: [email protected]
For any technical assistance or feedback or in case of problems with your product please contact
Email: [email protected]
We look forward to your inquiries or questions and are with pleasure at your disposal.
Phone:
+ 49 (0) 33432 –89 575
Fax:
+ 49 (0) 33432 –89 573
If contacting us by email or fax, please include as part of your message your instrument serial number,
your name, address, phone, and a description of your problem or question.
Please read these instructions before operating your Tensio 140 to ensure that it performs to its full
potential.

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1.1. Safety Instructions
Tensiometers are measurement devices for the determination of soil water tension and should
only be used for that purpose. Please note that damage due to improper handling may not be
covered by warranty.
Please attend to the following safety instructions for your own protection and for maintaining the
full functionality of the device:
CAUTION
Freezing of water within the tensiometer may result in permanent
damage of the tensiometer. Make sure to store the tensiometer
in a frost-free environment. At temperatures below 0° C the
tensiometer should not be used for measurements.
Protect from
frost
CAUTION
High pressure may lead to permanent damage of the pressure
transducer. Avoid compression of trapped media during
installation or when putting on protective caps.
Protect from
high pressure
CAUTION
Sudden pressure drops inside the tensiometer may result in
permanent damage of the pressure transducer. Do not open the
screw plug when a negative pressure is applied inside the
tensiometer. This accounts for all available types.
Protect from sudden
pressure drops
CAUTION
Mechanical forces may result in damage of the ceramic and/or the
pressure transducer. Prevent jolting or hard hitting as well as
partial pressure on the ceramic by rocks in the soil.
Protect from
mechanical forces
CAUTION
Grease, talc and solvents may influence the properties of the
ceramic. Aggressive media may affect the whole device and lead
to permanent damage. Do not touch the ceramic with bare hands.
Use only pure water and no detergent for cleaning.
Protect from
chemical influences

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2. General Information
2.1. What Are Tensiometers? / What Is Tension?
Tensiometers are measurement devices for determining the soil water tension. The soil water tension
describes all influences exerted on the water by the soil matrix and is also called the matrix potential
(or simply pressure head). The tension is defined as a negative pressure and therefore is signed
positive if the applied pressure is negative. A pressure of -50 kPa in the pore water accords a tension of
50 kPa.
The tension is closely linked to the water content of a soil. The dryer a soil, the higher is the tension. At
the groundwater surface, the tension is zero. The relationship between water content and soil tension
is soil specific and depends highly on the soil properties.
Exemplary retention curves of different soil types

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2.2. What Are Tensiometers Used For?
The tension corresponds to the force which a plant has to apply via the roots to extract water from
the soil pores. Among other applications, tensiometers can be used for irrigation control and for
evaluations of global warming impacts on ecosystems. The difference between the tensions at two
points within the soil acts as the driving force for soil water transport. Determining this gradient by
using tensiometers enables the evaluation of water movement in the soil regarding flow direction
and velocity. Tensiometer measurements are essential for the comprehensive analysis of the
water balance of a region or an ecosystem.
2.3. How Does a Tensiometer Work?
A Tensiometer consists of an air-tight sealed, water-filled measurement volume, a unit where the
pressure transducer is located, and a porous cup, usually made out of ceramic. The pores of the
ceramic cup are filled with water and connect the water of the measurement volume inside the
tensiometer with the water in the soil outside the tensiometer. By connecting the two water
bodies the pressure conditions inside the tensiometer are always adjusted to the pressure
conditions of the water in the surrounding soil. If the soil dries, water will flow through the pores
of the porous tensiometer cup from the measurement volume into the soil, thereby creating a
negative pressure inside the tensiometer.
Water
Porous Cup
Soil Particle
Air
Water
Schematic representation of the functional principle of a tensiometer

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The water flow equilibrating the pressures goes on until the negative pressure in the
measurement volume corresponds to the negative pressure in the surrounding soil. If the
surrounding soil is wetted again, for example by a precipitation event or irrigation, the negative
pressure decreases in the surrounding soil and water flows from the soil into the measurement
volume, whereby the negative pressure in the measurement volume is reduced again. A
manometer or an electronic pressure transducer measures the pressure inside the measurement
volume. Depending on the tensiometer type and design of the tensiometer the pressure is either
read manually or captured automatically, whereby the reading unit as well as the storage unit can
be integrated in the tensiometer or externally executed. Tensiometers are calibrated in a way that
the measured pressure is zero when the porous cup is right at the level of the groundwater
surface. For the Tensio 100 tensiometers this calibration only applies for vertical installation. The
water-filled volume in the standpipe acts as a hanging water column and applies an additional
negative pressure on the pressure transducer. The tension value at the ceramic tip and the
pressure at the pressure transducer differ by this negative pressure. To compensate the influence
of the hanging water column, the pressure transducer is calibrated accordingly. Since this pressure
difference depends on the height difference between ceramic cup and pressure transducer and
therefore on the installation position, deviant installation angles result in incorrect measurements.
2.4. Under Which Conditions Can a Tensiometer Be Used?
Based on the operating principle the measurement range of conventional, water-filled
tensiometers is physically limited by boiling of the water. The boiling point of water (according to
standard 99.6 °C at 1013.25 hPa ≈ 1 bar) is pressure- and temperature-dependent. The boiling
point inside the tensiometer and thus the upper limit of the measurement range depends on the
ambient atmospheric pressure and ambient temperature. At 20 °C and 950 hPa barometric
pressure the water starts boiling at approx. -926.6 hPa. This value also depends on the
temperature-dependent vapor pressure, which rises with increasing temperature. Thus higher
ambient temperatures and lower barometric pressure reduce the measurement range of a
tensiometer. This point may be delayed by boiling retardation. From this moment on the pressure
change is not longer transferred from the water in the soil.
The second physical limit is defined by the air entry point of the ceramic. This point is reached
when the negative pressure is high enough to suck air through the biggest pore of the ceramic. At
this moment the system is not air-tight anymore and the negative pressure suddenly drops.
Subsequently the tensions are about zero and it is not possible to build up a negative pressure
again.
The upper limit of the measurement range of a tensiometer is defined by which point is reached
sooner. If this tension is exceeded and the measurement range is left, the tensiometer stops
working. This is called “drying up” of the tensiometer. To restore the functionality of the
tensiometer it needs to be refilled with water and the tension in the surrounding soil has to be
within the measurement range of the tensiometer. Some systems, such as the Full Range
Tensiometer of UGT GmbH, are not bound to these limits and cannot fall dry due to innovative
operating principles.
The ceramics of the UGT tensiometers have by default an air entry point of at least 85 kPa. This
value slightly varies for every ceramic and depends next to the maximum pore size also on the
state of the ceramic and on substances dissolved in the water. Therefore, a common minimum

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values is stated as upper limit of the measurement range. This value may be exceeded during
measurements.
Exemplary tension curve with the limits of the measurement range, phase change, and air entry point
Another prerequisite for the correct function of tensiometers is the correct installation. The porous
cup needs to be in good contact with the soil to connect the pore system of the soil and the porous
material. Tensiometers should be installed in moist soil, if possible. When using an auger for the
installation, the diameter of the borehole must be slightly smaller than the diameter of the porous cup.
During installation the mechanical force on the ceramic cup needs to be kept as low as possible.
Obstacles, such as rocks in the soil may result in cracks or fractures in the ceramic cup. This may cause
the system to leak and therefore not to be functional anymore. Please refer to the according chapters
of this manual for detailed and tensiometer-specific information on the installation and maintenance
of the tensiometers.

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3. Set Up and Technical Parameters
3.1. Set Up
3.2. Technical Parameters
Technical Parameters
Tension range
-30… 100 kPa
Operating temperature
0… +40 °C
Response time
< 1 s
Power supply
Power consumption
9… 30 VDC
< 3 mA
Output signal
0… 1 VDC
Linearity error
< 0,5% FSO
Temperature coefficient
< 0,08% FSO/°C
Cable length
2 m
Size of ceramic cup
P80 25mm x 6,5mm
´
2
3
4
5
6
1
1...plug/cable connection
2…pressure transducer
3…acrylic adapter
4…filling screw with seal ring
5…shaft made of V2A
6…ceramic cup

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3.3. Pin Assignment
Color
Signal name
1
brown
+Vs 9… 30VDC
2
white
Signal output 0… 1VDC
3
blue
Case, shield
4
black
-Vs, AGND

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4. Commissioning
4.1. Preparation
Attention!
The tensiometer is delivered empty when shipped over long distances. Follow the listed
steps to prepare your tensiometer for measurements.
Unscrew the shaft with the ceramic cup from the acrylic adapter. Please make sure not to touch the
ceramic cup with your fingers throughout the whole process. Place the ceramic cup in distilled and
degassed water over night or for at least 8 hours.
Fill the rest of the shaft with distilled and degassed water by using a syringe with a hypodermic
needle. For best results insert the needle into the acrylic adapter until it reaches the ceramic cup.
Remove the needle slightly while pushing the water into the shaft. Now, fill the acrylic adapter with
distilled and degassed water by using the syringe and hypodermic needle. Make sure that neither
the needle nor the water jet hits the center of the pressure transducer.
Reattach the shaft with the ceramic cup to the acrylic adapter. Hold the tensiometer with the
ceramic cup pointing to the floor. Tap gently against the shaft to loosen possible air bubbles from
the inner tensiometer walls. Turn the tensiometer slowly about 90°. Existing air bubbles are now
visible in the transparent acrylic part.
For venting keep the bubbles close to the safety screw. Remove the safety screw and insert distilled
and degassed water through the screw opening. Reinsert the safety screw. Check the pressure
readings during this process. Make sure to screw in the sealing as gently as possible and avoid to
apply high pressure to the pressure transducer while sealing the tensiometer. When the sealing ring
seals the hole and no air bubbles are visible, the tensiometer is ready to use.
4.2. Installation
Three ways of installation can be recommended for the tensiometer:
a) horizontal installation
b) vertical installation
c) diagonally offset to the vertical
While preferential vertical flow paths and hence potential failures in the measurement are to be
expected for variant b), and can be almost excluded for variant a) and c).
Attention!
The tensiometer is a sensitive measuring device. Prevent touching the ceramic cup at any
time! In case the ceramic cup has to be touched use a moist paper towel to prevent direct
contact with your fingers.

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4.3. Pressure Readings
The Tensio 140 provides an analog output signal between 0 and 1 V. To transform the output signal
into a pressure value the signal range (0 –1 V) has to be transferred linearly to measurement range
of the pressure transducer (+0.3 bar to –1 bar). A signal of 1 V thus equals a Tension of 1 bar (or
1000 hPa). A signal of 0 V equals an overpressure of 0.3 bar. A Tension of 0 bar (e.g. at saturation
with no back water) is indicated by an output signal of 0.23 V.
5. Maintenance and Repair
•In case of air bubbles in the tensiometer check the tensiometer for leaks (the ceramic cup or
the sealing ring might be damaged) and check if the tension in the soil has been lower than the
measuring range of the tensiometer, which may caused the tensiometer to dry out
•Repair possible damages (repairs are best executed by qualified technicians of our company)
and remove air bubbles by refilling distilled water through the safety screw
•Protect the tensiometer from mechanical forces
6. Caretaking
The Tensio 140 does not need to be calibrated at maintained on a regular basis. However, an
appropriate caretaking and storage can increase functionality and life span of the tensiometer.
6.1. General Care and Storage Instructions
•Always put the tensiometer in the supplied safety/transportation case to prevent it from
damage.
•The sponge of the transportation case should be wet during transportation.
•Protect the tensiometer from frost. Storage and use of the tensiometer at temperatures below
0 ° can lead to damage of the device.
•After use clean the tensiometer cap with a sponge and distilled water to remove dirt.
•To clean the ceramic cup you can push water at a low rate through the cup by applying a light
pressure.
•Do not let the ceramic cup dry when it is dirty.
•If required you can clean the cup carefully by using wet sandpaper.