UGT Full Range Tensiometer User manual

Operating Manual
Full Range Tensiometer
Version: 20/07/20
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.................................................................................................................5
1.2. Scope of Delivery and optional accessories.............................................................................6
2. General Information ......................................................................................................................... 7
2.1. What are Tensiometers? / What is Tension? ..........................................................................7
2.2. What are Tensiometers used for?...........................................................................................8
2.3. How Does a Tensiometer Work? ............................................................................................8
2.4. Product versions..................................................................................................................10
2.5. Technical data......................................................................................................................10
3. Structure and basic functions ......................................................................................................... 11
3.1. structure..............................................................................................................................11
3.2. How to Operate the Full Range Tensiometer? ......................................................................12
4. Installation of the Full Range Tensiometer..................................................................................... 13
4.1. Preparation and Storage ......................................................................................................13
4.2. Installation of the Full Range Tensiometer ...........................................................................14
4.3. Interface and power supply..................................................................................................17
5. Data transfer ................................................................................................................................... 19
5.1. Measurement channels, bus addresses and data polling.......................................................19
5.2. Transformation digital signal in measurement value ............................................................25
6. Maintenance ................................................................................................................................... 27
6.1. Rezero of the Full Range Tensiometer ..................................................................................28

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1. Introduction
Thank you for choosing the Full Range Tensiometer. This manual can help you understand the Full Range
Tensiometer features.
Full Range Tensiometer by UGT covers the complete range of soil water tension relevant for plants,
that can reach up to 1500 kPa (pF 4.2) depending on the FRT version. Unlike other systems used for
this measuring range the Full Range Tensiometer doesn't determine the pressure head indirectly as a
derived variable, but via a direct pressure measurement. Even after drying out beyond the limits of the
measuring range, the measurement is continued independently as soon as the moisture increases
again. Refilling is not necessary. A further advantage compared to other measuring devices for the
complete plant relevant pressure head range is the low energy consumption (20 mA in operational
mode).
The measurement is reliable also in difficult media. Saline sites, such as in arid areas, are not a problem
as a result of the direct pressure measurement when correct calibrated.
The soil temperature is recorded as an additional parameter.
Advantages
•extended measurement range
•fast response
•maintenance free
•low power consumption

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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 Full Range Tensiometer 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 installing the tensiometer with high pressure
surges.
Protect from
high pressure
CAUTION
Sudden pressure drops inside the tensiometer may result in
permanent damage of the pressure transducer.
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 cup. Aggressive media may affect the whole device and
leads to permanent damage. Do not touch the ceramic with bare
hands. Use only pure water and no detergent for cleaning.
Protect from
chemical influences
CAUTION
To remove or insert the tensiometer from or into the protective case
turn the tensiometer only to the right.
Use only clockwise
rotations

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1.2. Scope of Delivery and optional accessories
Basic
Optional equipment
•Full Range Tensiometer
•Protective case
•Cable for power supply and data
transfer, inclusive M12 connector
(standard length: 5 m, customizable)
•Installation auger
•Standpipe (length customizable)
•Datalogger
•Software (UGT-LOG)
Software (FRT-LOG)
Full Range Tensiometer
protective case
Cable for power supply
and data transfer
figure 1 scope of supply and optional accessories

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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.
PWP = permanent wilting point
FC = field capacity
)
figure 2 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. 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?
The operating principle of the Full Range Tensiometer differs slightly from water filled tensiometers.
Both kinds of tensiometers consist of an air-tight sealed, medium-filled measurement chamber, 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 liquid of the measurement chamber inside the
tensiometer with the water in the soil outside the tensiometer. By connecting the two liquid 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 chamber into the soil. In case of water filled tensiometers the measurement
volume is filled with water. Water that leaves the measurement volume via the pores of the ceramic
creates a negative pressure inside the tensiometer. 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.
In case of the Full Range Tensiometer the measurement volume is filled with water that is bonded to a
polymer.
figure 3 Schematic representation of the functional principle of a Full Range Tensiometer

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The measurement principle of the Full Range Tensiometer is based on a known pressure in the
measurement volume that is reduced by the soil tension.
The measurement range of conventional, water-filled tensiometers is physically limited by the boiling
point of water, which 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.
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.
The upper limit of the measurement range of a tensiometer is defined by which point is reached first. If
this tension is exceeded the tensiometer stops working.
Due to the bonding of water to the polymer the physical measurement limit of approximately 1 bar does
not account for the Full Range Tensiometer. The Full Range Tensiometer cannot run dry like water filled
tensiometers and is able to measure soil tensions up to 5 or 15 bar, respectively. For soil tensions out of
the measurement range the Full Range Tensiometer simply does not function properly. When the soil
tension is back in the measurement range, the Full Range Tensiometer simply starts to work properly
again. No refilling of the tensiometer is needed. Additionally, the Full Range Tensiometer also measures
the ambient temperature.

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2.4. Product versions
version 1:
Full Range Tensiometer FRT 5D
•Digital signal output: RS485
•Measurement range tension: -100 - + 500 kPa
(pF 3.7)
version 2:
Full Range Tensiometer FRT
15D
•Digital signal output: RS485
•Measurement range tension: -100 - + 1500 kPa
(pF 4.2)
2.5. Technical data
Technical Parameters
Measurement range Tension
-100 … 500 kPa (FRT 5D)
-100 … 1500 kPa (FRT 15D)
Operating temperature
+1… +39 °C (surrounding soil temperature)
Operating pH-value
pH 2 … pH 10
Power supply / Supply voltage
20 mA / 5 … 20 V
Interface output signal
RS485 / Analog output signal available on request
Dimensions
Ø25 mm, Length 150 mm
range of temperature during
calibration
4-30° C (at 100% water saturation)
Relative error within the
calibration range
+/- 0.5% (based on the entire measuring range)

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3. Structure and basic functions
3.1. structure
Ceramic cup
Stainless steel shaft
Plastic case
(optional with thread for standpipe)
Connector M12 sensor plug 4-terminal

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3.2. How to Operate the Full Range Tensiometer?
The Full Range Tensiometer starts measurements with an initial overpressure (Pref) inside the
measurement chamber, which is then reduced by the soil tension. The soil tension (Ψ) is thus calculated
based on the difference between Pref and the currently measured pressure inside the measurement
chamber:
    
The overpressure in the measurement chamber is temperature dependent. In consequence Pref, which
was determined during the calibration process under specified temperature conditions, has to be
adjusted to the current measurement temperature. The calculation of Pref in equation 1 by using the
measured temperature is done via the following equation:
         
󰇟󰇠
 󰇟󰇠
 󰇟󰇠(Transformation digital signal in measurement
value see chapter 5.3., equation 3 and 4)
 󰇟󰇠(Transformation digital signal in
measurement value see chapter 5.3., equation 3 and 4)
 󰇟°󰇠
 󰇟°󰇠
󰇟󰇠
The parameters CT1, CT2 and P0are determined and preset by UGT prior to delivery. Since they are unique
to each sensor, the parameters are noted in the calibration certificate. Due to aging of the polymer P0
might be subjected to drift and should be calibrated regularly (see chapter 6). In individual cases, it might
be also necessary to re-calibrate the temperature coefficients CT. The calibration can be carried out by
trained UGT staff.
(1)
(2)

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4. Installation of the Full Range Tensiometer
4.1. Preparation and Storage
Preparation
The tensiometer is delivered in a wet state and thus ready to use. If the tensiometer has dried during
the delivery or subsequent storage it has to be saturated prior to the installation. Therefore, place the
ceramic cup of the tensiometer in distilled water for at least 24 hours. Keep the ceramic cup wet during
transportation by using the protective case until installation.
Storage
If the tensiometers are not installed store them in a dry state with the ceramic cup exposed to the open
air at temperatures between +1°C and +39°C to ensure lasting functionality.
Caution!
Freezing of the tensiometers may cause irreparable damage of the ceramic, the pressure transducer
or the polymer. Temperatures over 40°C may change the polymer and cause faulty measurement
readings.



Correct (left) and incorrect (center and right) positioning of the Full Range Tensiometer in a temperature-controlled water bath.

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4.2. Installation of the Full Range Tensiometer
Step 1 Connecting the FRT to a power and data transfer cable
Be careful to not touch the ceramic cup with bare
fingers during installation.
Connect the tensiometer cable to the connector of
the tensiometer and screw the cap hand tight.
Make sure that the cable reaches from the
installation site of the tensiometer to the data
logger/ PC.
The cable can also be installed underground. It is
recommended to lay the cable in such a way that
it is protected from animals. Choose a cable
according to the required length. Excess cable
length may reduce the quality of the tension
measurement. To prevent moisture from
penetrating into the compensating capillary, place
a suitable desiccant at the capillary outlet (usually
in the data logger).
Caution!
A pressure compensation capillary is integrated in the connection cable. Avoid kinking and crushing
the cable. Winding the cable too tightly can lead to insufficient pressure compensation and thus to
incorrect measurement results.


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Step 2: FRT Installation
Caution!
It is recommended to install the tensiometers in wet soil. If you are unconfident about the
installation, contact us for advice or support from our trained staff.
Caution!
Rocks or obstacles in the soil may damage the ceramic cup if the force exerted on the
tensiometer is too high.
The tensiometer can be installed horizontally or
vertically. In both cases make sure the whole
plastic case (including plug) is buried to prevent
temperature conduction via the case which may
influence the measurement.
vertical installation less than 20 cm:
Use an installation auger to prepare the
installation hole in the desired depth. The auger
is designed to keep the disturbance to the
surrounding soil to a minimum and to ensure a
sufficient connection between soil and ceramic
Slurry the ceramic cup. In dry conditions it is
recommended to wet the soil by filling water into
the borehole until the soil in the area of the
ceramic cup is well soaked.

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Now, press the tensiometer carefully with the
ceramic cup first into the borehole.
Vertical installation more than 20 cm:
Use a standpipe for secure positioning of the
tensiometer and for being able to retrieve it
later. Lead the tensiometer cable through the
standpipe and screw the standpipe onto the
thread on the upper end of the plastic case. The
standpipe remains in the soil with the
tensiometer.
Horizontally installation:
Dig up the soil up to the designated
measurement depth that the sensor can be
placed horizontally in the hole. Place the
tensiometer horizontally in the hole and push the
ceramic cup into the side wall of the hole. If
necessary, drill a hole in the side wall for the
tensiometer tip. Make sure that the tensiometer
has a secure position before filling the hole. Then
fill the hole so that there is no risk of subsidence
leading to damage to the Tensiometer.

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4.3. Interface and power supply
The Pin assignment can be found int the calibration certificate.
•Full Range Tensiometers are connected via a ventilated 4-pol connector
•The power supply of the connected sensor is carried out via two pins
•The supply voltage must be in the range of 5 to 20 V DC
•The remaining two pins are the differential lines of the RS485 interface
Pin
Pin color
Output Signal
Type
1
brown
5-12 V DC
VCC
2
white
A+
RS485
3
green
B-
RS485
4
yellow
0V
GND

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Step 3: Connect the data (power) cable
UGT Logger:
Connect the UGT power and data transfer cable via
a ventilated 4-pole connector to a UGT data logger
(example UGTlogger DL-2000).
The Pin assignment can be found in the calibration
certificate.
All bus addresses are addressed during the setup
process by UGT and can be found in the calibration
certificate.
Download the UGT-LOG software to your PC.
Follow the provided logger and software
instruction.
As a service, UGT GmbH can pre-install your project
on a UGT logger.
External Logger:
Connect the UGT power and data transfer cable via
a ventilated 4-pole connector to an external data
logger.
The Pin assignment can be found in the calibration
certificate.
All bus addresses are addressed during the setup
process by UGT and can be found in the calibration
certificate.
Caution!
In order to change or view bus addresses of the
FRT we advise to use FRT log. Experienced users
might want to use dataloggers or terminal
programs for serial ports following the
instructions in chapter 5.
PC interface:
Use FRT LOG to display the FRT values graphically
and change bus addresses.
Connect the UGT power and data transfer cable via
a USB 2.0 converter, a plug to RS-485 to your
computer.
Install the software UGT FRT-LOG.
Follow the provided software instruction.
+
+
+
External Logger

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5. Data transfer
A RS485-Interface is required to connect the Full Range Tensiometer to the UGT Data logger or any other
device with a digital interface. The following chapters describe the communication between the Full
Range Tensiometer and the data logger.
Transmission Parameters:
•Parameters for the serial interface (UART) are 9600 8N1
(9600 baud, 8 data bits, no parity, 1 stop bit)
•The RS485-Interface enables the operation in half duplex mode
5.1. Measurement channels, bus addresses and data polling
The Full Range Tensiometer has four measurement channels. The channel numbers cannot be changed
and are identical for each FRT. The calculation of the tension is done via the values from channel 2
(temperature) and 3 (pressure), which are the channels of main interest for the user.
In order to be able to use more than one FRT (example FRT 1 and FRT 2 and FRT 3), each measurement
channel has to be given a specific bus address. This bus address enables the data logger to get the value
of specific measurement channel of a specific FRT. Those have to be numbered uniquely throughout all
FRTs. Bus addresses can be found in the calibration certificate.
Caution!
In order to change or view bus addresses of the FRT we advise to use FRT log. Experienced users
might want to use dataloggers or terminal programs for serial ports following the instructions
below.
Note:
The 4 measurement channels do not accord with the four cables!
Note:
All bus addresses are addressed during the setup process by UGT. The addresses for
temperature and pressure are numbered serially so all of the FRT can be used with one serial
port. We advise to use the assigned addresses unless there is a conflict with addresses of other
sensors on the same port.

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Example: Addresses and channel numbers for three Full Range Tensiometer:
measurement channel
assignment (unchangeable)
bus address (preset but
changeable)
EXAMPLE
see calibration certificate or
FRT Log
FRT 1
1 = service/ calibration
2 = temperature
1 = temperature
3 = pressure
2 = pressure
4 = service/ calibration
FRT 2
1 = service/ calibration
2 = temperature
3 = temperature
3 = corrected pressure
4 = pressure
4 = service/ calibration
FRT 3
1 = service/ calibration
2 = temperature
100 = temperature
3 = pressure
101 = pressure
4 = service/ calibration