Tensior T8 - Eijkelkamp Agrisearch Equipment

User Manual
T8
Tensiometer
Version January 2003
T8
P.O. Box 4, 6987 ZG Giesbeek
Nijverheidsstraat 30,
6987 EM Giesbeek,
The Netherlands
T +31 313 880200
F +31 313 880299
E eijkelkamp@eijkelkamp.com
I http://www.eijkelkamp.com

Foreword
The success of any technical system is directly depending on a correct
operation. Moreover, the systems must be reliable, durable and require a
minimum of maintenance to achieve target-directed results and keep the
servicing low.
At the beginning of a measuring task or research project the target, all effective
values and the surrounding conditions must be defined. This leads to the
demands for the scientific and technical project management which describes
all quality related processes and decides on the used methods, the technical
and measurement tools, the verification of the results and the modelling.
The continuously optimised and synergic correlation of all segments and it's
quality assurance are finally decisive for the success of a project.
Thus, I would be pleased to receive your critiques and appraisals.
München, 30.09.2002
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Table of contents
1 Content of delivery ....................................................................................7
2 Short instructions .......................................................................................7
3 Safety notes................................................................................................9
4 Description of the T8...............................................................................10
4
4.1 Output signals.....................................................................................10
4.2 Sensor body........................................................................................10
4.2.1 Pressure sensor.........................................................................10
4.2.2 Temperature sensor .................................................................11
4.2.3 IR indicator................................................................................11
4.3 The ceramic cup .................................................................................12
5 Refilling.....................................................................................................13
5.1 When does the Tensiometer needs to be refilled?.............................13
5.2 Refilling in the lab................................................................................13
5.3 Refilling in the field..............................................................................15
5.4 Refilling with vacuum ..........................................................................16
6 Calibration................................................................................................18
6.1 Checking the offset.............................................................................18
6.2 Offset correction for non horizontal installations...............................19
6.3 Correlation of water column and pressure ........................................19
7 Concept and installation...........................................................................20
7.1 Selecting the measuring site................................................................20
7.2 Selecting the installation angle ............................................................20
7.2.1 "Vertical" with downwardly angle .............................................20
7.2.2 "Horizontally" with upwardly angle ...........................................20

7.3 Number of Tensiometers per level ....................................................21
7.4 Extend of the measuring site ..............................................................21
7.5 Length of the refilling tubes ................................................................22
7.6 Jacket tubes.........................................................................................22
7.7 Ideal condition for installation.............................................................22
7.8 Documentation...................................................................................23
7.9 Installation...........................................................................................23
8 Connecting the T8 ...................................................................................25
8.1 Single-ended or differential application...............................................25
8.2 Connecting the IR-indicator................................................................26
8.3 Wire specifications..............................................................................26
9 Protecting the measuring site...................................................................27
9.1 Theft and vandalism............................................................................27
9.2 Frost ...................................................................................................27
9.3 Lightning .............................................................................................27
10 Additional notes..................................................................................29
10.1 Interpretation and maximum measuring range..............................29
10.2 Osmosis .........................................................................................30
10.3 Emptying the T8 ............................................................................30
11 Accessories .........................................................................................31
11.1 Connecting and extension cables...................................................31
11.2 Handheld measuring device ...........................................................31
11.3 Augers............................................................................................31
11.4 Refill- and calibration kits ...............................................................32
12 Contact and communication...............................................................33
13 Technical specifications.......................................................................34
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1 Content of delivery
The delivery of a T8 includes:
• Tensiometer, calibrated and filled, cable with plug and protective cap
• This manual
• Plastic bottle protecting the ceramic cup, must be filled to half with
water to keep the cup moist
• Shaft water protection disk
• Calibration certificate with technical data and output signals
Available accessories: see chapter "Accessories"
If you wish to receive a individual calibration certificate for each Tensiometer,
please contact us and give us the serial number of the Tensiometers.
2 Short instructions
Please read and understand the complete manual prior to first use.
In general, the T8 is filled and ready for installation when supplied.
For installation proceed as follows:
1. Drilling the borehole: An installation angle of 25° to 65° from the vertical
line is ideal for the optimal removal of air from the cup. Mark the required
drilling depth on the auger and on the Tensiometer shaft.
Note: Installation depth = drilling depth / cos α
Drill a borehole, diameter 25 mm, with the required depth. offers
special Tensiometer gouge augers with shaped blade tip.
2. Illuvation of the cup is only needed in clayey soils and only if the borehole is
larger than the diameter of the cup (which is 24 mm). With the special
Tensiometer gouge auger illuvation is unnecessary. In coarse sand and
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pebbly grounds a fine grained illuvation paste would even effect a water
reservoir which would slow down the response.
3. Take off the protective plastic bottle from the Tensiometer cup. Pull the
bottle it off carefully or slowly turn it counter-clockwise!
4. Insert the T8 into the borehole up to the depth mark without using force.
In clayey soils an overpressure might develop: Check the Tensiometer's
pressure reading with an Infield measuring device or a datalogger!
8
Important: Pay attention to the engraved black spot on the
end of the shaft that marks the position of the exit opening of
the external filling inside the cup:
a) Downwards installation: If the position of the cup will be
lower than the end of the shaft, this mark must point
upwards!
b) Upwards installation: If the position of the cup
will be higher than the end of the shaft, this mark
must point downwards!
5. Push down the shaft water retaining disk to a position directly on the soil
surface. Water flowing down the shaft will not disturb measurements.
6. Leave on the protective cap on the plug if the plug is not connected.
7. Connect the Tensiometer signal wires as specified (see chapter 8
Connecting the T8). If the measuring station is prepared , attach
the Tensiometer plug to the suitable connecting cable.
Notes:
- The less air is inside the cup and the better the soil's conductivity is, the faster
the Tensiometer will respond.
- If the soil is dryer than 900 hPa, it does not make sense to refill the
Tensiometer. Refill the Tensiometer as soon as the Tensiometer that is

installed in the next lower level has reached the reading when the upper
Tensiometer had dried out.
3 Safety notes
Tensiometer are instruments for measuring the soil water tension, soil water
pressure and soil temperature and are designed for this purpose only.
Caution
Lightning: Long cables act as antennas and might conduct surge voltage in
case of lightning stroke – this might damage sensors and instruments.
Caution
Frost: Tensiometers are filled with water and therefore, are sensitive to
frost! In wintertime, do not leave Tensiometers inside your car overnight.
Generally, frost should not harm Tensiometers installed in depths below
30 cm.
Caution
Overpressure: The non destructive maximum pressure is 3000 hPa. Higher
pressure, which might occur for example during insertion in wet clayey
soils or triaxial vessels, will destroy the pressure sensor! The applied
pressure should not exceed 2000 hPa!
Caution
Electronics: Any electrical installations should only be executed by qualified
personnel!
Caution
Ceramic cup: Avoid to touch the cup with your fingers. The ceramic
should not have contact with grease or soap as this will influence the
hydrophilic performance.
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4 Description of the T8
4.1 Output signals
The T8 is optimised for in-the-field use. Signals for soil water tension, soil
temperature and filling status are offered.
Water tension and temperature are linear voltage signals 0 ... 2 V. Thus, the
signals are directly connectable to nearly any datalogger or other data
recording device.
The IR indicator works like a switch so the filling status can have two levels:
"0V" (switch is open) for correct filling, or Vin = power supply voltage (switch
is closed) if the Tensiometer needs to be refilled.
4.2 Sensor body
The complete electronic of the T8 is integrated in the sensor body. The
housing consists of glass-fibre reinforced synthetic. All electronic parts are
completely covered with epoxy resin - the optimal and everlasting protection
against moisture.
The T8 is calibrated electronically by
programmable potentiometers. The calibration is
stored in EPROM's.
4.2.1 Pressure sensor
The piezoelectric pressure sensor measures the soil water tension against the
atmospheric pressure. The atmospheric pressure is conducted through a
watertight diaphragm (the white, 2 cm long tube on the cable) and through the
cable to the reference side of the pressure sensor.
The non destructive maximum pressure is 3.000 hPa. Higher pressure will
damage the sensor and absolutely must be avoided!
10
Sensors and refilling tube on sensor body

4.2.2 Temperature sensor
A Pt1000 1/3 DIN B is used as temperature sensor. The Pt1000 has a
maximum deviation of ±0.1 K at 0°C.
The tip of the sensor dips into the Tensiometer cup's water. Thus, the best
possible thermal contact to the soil is achieved.
The correlation of water tension/water content is temperature depended. The
influence is low with water tensions from 0 … 100 hPa ⇒ 0 … 6 hPa/K, but
with tensions over 1000 hPa it is:
Ψ = (R T/M) ln p/po Ψ = water tension R = gas constant (8,31J/mol K)
M = molecular weight p = vapour pressure
po = saturation vapour pressure at soil temperature
(see vapour pressure tables by Scheffler/Straub, Grigull)
4.2.3 IR indicator
With the IR indicator the filling status of a T8 in a downwardly installation can
be checked without removing the Tensiometer from the soil.
If the Tensiometer is installed in an upwardly angle (cup is higher than the
end of the shaft), the indicator will react not before the cup is almost
completely empty.
Measuring principle of the IR indicator: A photo-sensor and a source of IR-light
are integrated in the T8 body. If there is only water between source and sensor
the absorption is high and the photo-sensor will not respond. If there should be
a bubble in-between, the measured light intensity arises and the electronic will
switch to the high output signal, indicating the need to refill. The IR-"switch"
can be burdened with 50 mA. LEDs or relays can be connected.
Benefits of the IR indicator are an optimal service: If for example mounting rails
with integrated LEDs are used, the user can check at a glance if all
Tensiometers work well or which need to be refilled.
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Also, for quality assurance, the status of the signal can be logged for later
evaluation of the readings.
4.3 The ceramic cup
Avoid to touch the cup with your fingers. The ceramic should not have
contact with grease or soap as this will influence the hydrophilic
performance.
To transfer the soil water tension as a negative pressure into the Tensiometer,
a semi-permeable diaphragm is required. This must have good mechanical
stability and water-permeability, but also have gas impermeability.
The T8 cup consists of ceramic Al2O3 sinter material. The special
manufacturing process guarantees homogeneous porosity with good water
conductivity and very high firmness. Compared to conventional porous ceramic
the cup is much more durable.
The bubble point is higher than 6000 hPa. If the soil is dryer than 6000 hPa the
negative pressure inside the cup decreases and the readings go down to 0 hPa.
With these characteristics this material has outstanding suitability to work as
the semi permeable diaphragm for Tensiometers.
4.4 Reference air pressure
The reference atmospheric air pressure is conducted to the pressure
transducer via the air permeable (white) Teflon membrane and through the
cable. The membrane does not absorb water. Water will not pass through the
membrane into the cable, but condensed water inside the cable will leave the
cable through the membrane.
The white membrane on the cable must always have contact to air and
should never be submersed into water.
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5 Refilling
To assure a rapid and reliable measurement of the soil water tension, the cup
must possibly be filled with de-ionised and degassed water and without bubbles
of air. After dry periods or periods with a large number of wet and drying out
successions, the T8 must be refilled.
For all refilling methods, the following items are always required:
- Syringe with valve
- Degassed, de-ionised water
- Measuring device for Tensiometer pressure signal
5.1 When does the Tensiometer needs to be refilled?
The Tensiometer needs to be refilled:
a, For Tensiometers installed "downwardly", the IR indicator will light up.
b, If no signal peaks show up anymore.
c, Tensiometers should not be refilled until the soil is moister than 900 hPa.
If the soil gets dryer than 805 hPa, the readings will stop at the vapour pressure
of water (i. e. 927 hPa at 20°C and atmospheric pressure of 950 hPa). By
diffusion and slight leakage the reading will slowly drop within months.
Still, if the soil dries out more than 6000 hPa, the negative pressure will drop
much faster as air will enter the cup.
5.2 Refilling in the lab
To reach a measuring range of 900 hPa the Tensiometers should be refilled in
the laboratory. With this refilling method a measuring range up to approx. 900
hPa can be achieved (at 1013 hPa atmospheric pressure). A refill kit is required.
Degassing procedure:
1, Set up the refilling kit and connect the vacuum pump. The pump should
achieve at least 30 hPa against vacuum (approx. 930 hPa).
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2, Unscrew the cup in clockwise direction and empty it. Do not touch the
ceramic cup with your fingers. Wrap a clean towel around the cup.
The pressure sensor diaphragm is inside the small boring on the pressure
sensor body. It is very sensitive and may never be touched! It can be
destroyed even by slightest contact! No contamination should get on the
sealing and gasket.
3, If the cup is drained it should be drenched in distilled water overnight by
placing it in a beaker filled with water.
Important: There should be no water inside the cup. The cup must be in an
upright position so only the cup's outside is covered with water. Otherwise
cavities of air might be enclosed inside the ceramic.
4, The drenched cup should be empty. Insert the cup to the adapter and
connect it to the degassing device. Place the cup in water in an upright
position as described above.
5, Fill the other adapter capsule to the half with water and insert the
sensor body. Connect the adapter to the degassing device as well.
6, Start the vacuum pump. With drenched cups, the procedure will take 4 to 5
hours. From time to time, tap on cup and sensor body to loosen bubbles.
Degassing is complete when no air bubbles ascend from ceramic and body
and the cup is completely filled with water.
14
Beaker
Ceramic
cup
Sensor
body
Vacuum
bottle
Pressure reducer
Vacuum pump

7, Before screwing the cup onto the body, connect the sensor to a measuring
device to observe the pressure signal. Carefully and slowly screw the cup on
the sensor body. Make sure that no bubbles are enclosed.
Caution: The pressure must not exceed 2000 hPa!
8, Now, the cup and the pressure sensor are completely degassed. Finally, the
refilling tubes have to be filled as well. To do so, continue with the
procedure as described in chapter 5.3 or 5.4.
5.3 Refilling in the field
For an easy refilling of still saturated cups on site and without removing the
Tensiometer from the soil, the Tensiometers can be refilled externally through
the high-grade steel capillary tubes. If the refilling tubes are longer than 5 m a
vacuum pump might be necessary – see chapter 5.4.
Procedure:
1. Connect the T8 to the measuring device and keep an eye on the pressure
signal at any time.
2. Two steel capillary tubes come out from the T8 shaft: the refilling tube and
the de-airing tube. Pull off the rubber tube from the refilling tube.
Left fig: Downwardly installation – the
marked tube is the refilling tube
Right fig: Upwardly installation – the
marked tube is the de-airing tube
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2. Fill the syringe and the connected rubber tube with water. Attach the
syringe to the refilling tube.
3. Continuously inject water into the filling tube while monitoring the pressure
signal at any time. Water and bubbles arise from the de-airing tube. Stop
injecting water when no more air bubbles appear.
16
Caution: Do not let the pressure exceed 2000 hPa !
4. Remove the syringe. Put a drop of water on both rubber and steel tube's
end to avoid that any air will get into the tubes. Connect both.
With this method a measuring range of approx. 800 hPa can be achieved.
5.4 Refilling with vacuum
To reach the full measuring range the Tensiometer water can be degassed
even more completely by using a vacuum. This method can be applied for
installed or not installed Tensiometers, in any installation angle and for refilling
tubes longer than 5 m.
The refilling kit BKTex contains all required tools:
- manual vacuum pump and vacuum bottle with tube and valve
- syringe with valve
Procedure:
1. Connect the T8 to the measuring device and keep an eye on the pressure
signal at any time. Pull off the rubber tube from the refilling tube.
3, The syringe as well as the tube on the syringe should be filled with degassed
water. Close the valve (!) and connect syringe and refilling tube.
2, Connect vacuum bottle and de-airing tube. Now, evacuate the bottle to the
maximum possible vacuum. This will enlarge the remaining bubble inside the
cup. Briefly open and then close the valve of the syringe to let water flow
into the Tensiometer. The air bubble will be sucked into the vacuum bottle.
Repeat this 2 or 3 times until no bubbles come out anymore.

3, Close the valve of the vacuum tube and remove it. Now press down the
syringe to fill in another amount of approx. 5 ml of water. Remove the
syringe and connect both steel tubes with the rubber connecting tube.
Fig: Downwardly installation – the marked tube is the refilling tube
Fig: Upwardly installation – the marked tube is the de-airing tube
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6 Calibration
Programmable potentiometers are integrated into the T8. They can be
addressed via the two digital lines.
Although the electronics is completely covered with epoxy resin, it is still
possible to calibrate the sensor.
The following parameters can be calibrated:
• Pressure sensor: inclination and offset
• Temperature sensor: offset
• IR indicator: trigger level
The T8 Programming Unit KBT8 is required to address the potentiometers
and to check the inclination.
6.1 Checking the offset
There a two possible ways to check the offset:
1. Put the filled T8 into a beaker with water and immerse the cup for approx.
3 to 4 cm. Wait for the value to stabilise. The measured signal must be
between -5 hPa and +5 hPa (at room temperature). If air bubbles are inside
the Tensiometer, the reading might take a while to reach this value.
2. A more reliable method would be to screw of the cup. Then, shake the
body to remove the water inside the pressure sensor. The pressure value
must be in the range of -5 hPa to +5 hPa. Before screwing the cup on the
T8-body, pressure sensor and cup must be refilled with degassed and deionised
water (see chapter "Filling in the laboratory").
Caution: The pressure sensor diaphragm inside the small boring on the
pressure sensor body is very sensitive and may never be touched! It can be
destroyed even by slightest contact!
Also, no contamination should get on the sealing and gasket.
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6.2 Offset correction for non horizontal installations
The T8 is calibrated for horizontal installation. If it is built in a non horizontal
position, the vertical water column drawing on the pressure sensor has to be
compensated. With vertical installation (completely upright with 0° angle) the
deviation is largest. In this case a water column of 5 cm additionally draws on
the pressure sensor and causes a shift of -5 hPa. This means that a value of
-5 hPa will be indicated instead of the actual value of 0 hPa.
Find the correction value for other angles in the table:
Angle to vertical line 0° 10° 15° 20° 25° 30°
Offset correction in [hPa] = [mV] -5 -4,9 -4,8 -4,7 -4,5 -4.3
Angle to vertical line 45° 60° 70° 75° 80° 90°
Offset correction in [hPa] = [mV] -3,5 -2.5 -1,7 -1,3 -0,9 0
6.3 Correlation of water column and pressure
A precision pressure gauge or a water column is needed. Please also observe
the correlation of pressure and water column if the Tensiometer is used as a
piezometer or water level transmitter.
Pressure = density x gravity x height
Density of water at 20°C: 0,998205 kg/cubic decimetres; at 4°C: 1,0 kg/dm3 .
[Pa] = [N]/[qm] [N] = [kg/m qsec]
[kg] = [Pa qm/m] [Pa] = kg/[qqm qsec]
A water column of 1 meter causes a pressure p of:
p [Pa= N/qm] = 998,205 kg/qqm x 9,81 m/qs x 1m
p = 9792,39 [N/qm x Pa qs/m /qqm x m/qs x m] = 97,92 hPa
In reverse a pressure of 100 hPa at 20°C is caused by a water column of
102,15 cm.
19

7 Concept and installation
7.1 Selecting the measuring site
The installation spot should be representative for the soil horizon! Therefore,
in heterogenic soils, classifying drillings should be made before or during
installation.
On farmed sites with vegetation root spreading and root growth during the
measuring period must be considered. Fine roots will grow around the
Tensiometer cup as this is a poor but still secure source of water. Therefore,
avoid the root zone or move the Tensiometer from time to time depending on
the root growth.
Disturbing effects like waysides, the rim of a field, slopes or dints must be
avoided or considered in the interpretation of the measuring results.
7.2 Selecting the installation angle
An installation position would be ideal if the typical water flow is not disturbed
by the Tensiometer. No preferential water flow along the shaft should be
created. Therefore, Tensiometers are preferably installed at an angle.
7.2.1 "Vertical" with downwardly angle
When installed from the surface, an angle of 25° to 65° from the
vertical line is optimal for refilling. In an absolutely vertical position
air bubbles might hide inside the edges of the cup adapter. Still,
they could be removed completely with the vacuum refilling kit
BKTex. In this position, the refilling tube is the shorter stainless
steel tube with the black mark. Into this tube, water is injected for
refilling.
7.2.2 "Horizontal" with upwardly angle
When installed horizontally from inside a well or pit hole,
the Tensiometer must point upwards! This means the
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cup is in a higher position than the end of the shaft. An upward angle of approx.
5° is ideal for refilling. Note that now de-airing and refilling tube are switched:
the refilling tube is the longer stainless steel tube without the black mark. Into
this tube, water is injected for refilling.
The IR-indicator will react not until at least a half of the cup is empty.
7.3 Number of Tensiometers per level
To verify the results, at least three Tensiometers per soil level and with the
identical soil, vegetation and climatic conditions should be implemented. Thus,
irregular values as well as the variableness of the water situation can be
determined. Guiding principle: More heterogeneous sites and soil structures
require a higher number of Tensiometers.
7.4 Extend of the measuring site
Large distance along with high equidistance between the measuring spots will
reduce the influence of sectional heterogeneity. Therefore, equidistant spots
are mostly used to measure the heterogeneity of the soil, the cultivation or the
observed development.
The following points limit the maximum length for the signal cables:
- Required accuracy: In single-ended applications every 20 m of cable reduces
the accuracy for about 2 hPa or 0,1°C. (Please read chapter "Connecting
the T8" on how to compensate the voltage drop of sensors which are
connected single-ended.)
- Local danger of lightning stroke: Cables act as antennas and therefore
should be as short as possible.
Lengths of more than 100 m are possible but not recommendable. The
available connecting cables are described in the chapter "Accessories".
21

7.5 Length of the refilling tubes
Keep the refilling tubes as short as possible. Caused by the thermal expansion
of air, any air inside the tubes will affect the readings when temperature
changes. Preferably, insulate the refilling tubes or install them isothermal into
the soil.
7.6 Jacket tubes
Jacket tubes are useful with shafts longer than 2 m, in pebbly soils or gravel,
and for horizontal installations from inside a well or pit hole. The jacket tube
should end 30 to 50 cm away from the cup so leaking or condensation water is
not conducted to the cup. The inner diameter of the jacket tube should be at
least 35 mm.
7.7 Ideal condition for installation
For the installation of Tensiometers, the ideal conditions are:
- Frost-free soil.
- Coarse clay or loess.
- In soil types loess, coarse clay, loam or clay as well as in highly organic soils
(humus) preferably with a water tension close to saturation (moist).
- In sandy or pebbly soils with a water tension over 100 hPa (rather dry).
- Low skeletal structure (gravel). The more gravel in a soil the more often
the drilling has to be repeated to reach the required depth.
22

7.8 Documentation
For every measuring spot you should:
- Measure out the position where the pressure sensor will be placed. (A must
for installations below the ground surface).
- Take documenting photos before, during and after installation.
- Save a soil sample.
- Write down installation depth and angle with each sensor identification
(serial number).
- Mark all connecting cables with the corresponding sensor identification on
each end.
7.9 Installation
For the installation of the T8 in the field the following tools are required:
• Tensiometer auger with diameter 25 mm, ideally the
Tensiometer gouge auger with shaped blade tip.
• Rule, spirit level, goniometer, marker pen
• Minute book, maybe camera for documentation of site and soil profile
• Perhaps PE-plastic bags for taking soil samples from the site
Installation procedure:
1. Measure and mark the required drilling depth on auger and Tensiometer
shaft. Drill a hole with the desired depth on the chosen measuring spot.
Read the chapter "Selecting the installation position" for the best installation
angle. Be cautious with the last 20 cm and take out and save this soil.
2. For soils with poor capillary contact or when using augers with a diameter of
over 25 mm, mix a paste of water and crumbled soil material taken out of
the borehole. Fill the paste into the bottom area of the borehole using a
tube (diameter 20 mm).
3. Take off the protective plastic cap from the Tensiometer cup.
23

Important: Only turn of the cap counter-clockwise! Do not store the
Tensiometer without the protective plastic bottle since the cup drains fast!
The bottle must be filled with some water for storage!
4. Connect the Tensiometer to a readout unit. Carefully insert the T8 into the
borehole up to the stop.
24
Do not use any force. The Tensiometer may not be pounded, since cup or
pressure sensor can be destroyed.
Especially in clayey soils the pressure reading must be monitored as high
pressures might build up! The maximum pressure must not exceed 2000
hPa
5. Push the shaft water retaining disk down to cover the soil surface. This
prevents water from running into the borehole and down along the shaft.
6. Keep the protective plastic cap on the plug whenever the plug is not
connected!
7. Connect the signal cables as described in the chapter "Connecting the T8".
8. The Tensiometer will respond to changes in the soil water tension faster if
there is no air inside the system and the soil water conductivity/mobility is
high.
9. Write down serial number, position, installation angle and depth.

8 Connecting the T8
The T8 is fitted with an 8-pin plug. The plug can be connected to a Infield 7
handheld measuring device. The Infield 7 displays and stores the soil water
tension, the soil temperature and the filling status.
With the extension cables offered as accessories the T8 can be
connected to a Datalogger or any other data acquisition devices.
Please observe the max. supply voltage of 20 VDC. Never connect the signal
outputs of the T8 to a supply voltage source!
The Tensiometer plug must be covered with the added protective cap
anytime the T8 is not connected to an extension cable.
8.1 Single-ended or differential application
In general a datalogger with differential inputs should be preferred. This means
both the plus signal as well as the minus signal of each sensor are measured. On
many dataloggers less channels are required for single-ended measurements,
which means only the plus signals are measured and related to the common
sensor supply ground. In this case, the voltage drop, depending on the current
consumption and the cable resistance, influences the reading and must be
compensated:
Current consumption: 7 mA; cable resistance: 82 Ω/km
⇒ Voltage drop inside a cable with length 10 m:
UError = R x I = 0,82 Ω x 0,007 A = 5,66 mV
⇒ Error for water tension (1 mV equal to 1 hPa): 5,66 mV = 5,66 hPa
⇒ Error for temperature (1°C equal to 20 mV): 5,66 mV = 0,283 °C
Therefore, the characteristic lines are lifted for +5,66 hPa resp. +0,283 °C per
10 m cable. The incline is not influenced. For each 10 meters of cable,
withdraw 5,66 hPa for pressure (Note: the water tension signal is regarded
positive) and 0,283 °C for temperature from the measured signal value to get
the correct reading.
25

8.2 Connecting the IR-indicator
Some datalogger types require a pull-down resistor (for example 10 kOhm)
against ground for the IR-indicator. Other logger types already have an internal
pull-down resistor, or a pull-up resistor to 5 V, for example for connecting a
reed switch. The external pull-down resistor must be smaller than the logger
internal pull-up resistor or at least be in a relation so the logger will recognize
the zero level.
8.3 Wire specifications
26
white = V in Power supply plus, 6 … 20 V DC
brown = GND Power supply minus
green = Signal + Soil water tension plus
red = Signal Temp Temperature plus
yellow = Signal - Minus for soil water tension and temperature
grey = Signal IR IR-indicator, switched with Vin pink = Calibration SDA (digital)
blue = Calibration SCL (digital)

9 Protecting the measuring site
9.1 Theft and vandalism
The site should be protected against theft and vandalism as well as against any
farming or field work. Therefore, the site should be fenced and signposts could
give information about the purpose of the site. Cables should be protected
against rodents.
9.2 Frost
Tensiometer are filled with water and therefore are endangered by frost!
With temperatures below–5°C do not leave filled Tensiometers in your car,
in a measuring hut, etc.
Do not fill the Tensiometers with Ethanol, as this is corrosive for some
materials (i. e. PMMA) and might destroy these..
Also it is not recommendable to fill the Tensiometers with Decalin, monoethylene-glycol,
di-ethylene-glycol, etc. These could harm any of the materials,
destroy the ceramic cup or leak into the soil.
Specially designed Tensiometers which can be filled with Ethanol are available
on request. They can be used to temperatures down to - 20°C.
9.3 Lightning
Measuring instruments in the field are always endangered by over-voltages.
Whenever technically feasible, over-voltage and false polarity protection are
realised. If you have questions about an optimal integration of the T8 into a
measuring system, please contact our system engineers.
Still, there is no absolute certain lightning protection! Lightning strokes are not
predictable. The more extensive a site is, the more import is the installation of
a protection system which could be passive with ground spears – preferably
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with groundwater contact, or active with each sensor and logger individually
equipped with a lightning protection module. This normally is quite costly.
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10 Additional notes
10.1 Interpretation and maximum measuring range
Since no pressure smaller than vacuum can appear in the Tensiometer, the
atmospheric pressure (reference pressure) limits the measuring range of the
Tensiometers. Additionally the measuring range is reduced by the vapour
pressure of the Tensiometer's filling water, depending on the temperature.
At 20 °C the vapour pressure is 23.4 hPa. The maximum measuring range at an
atmospheric pressure of 950 hPa is calculated as:
950 hPa - 23.4 hPa = 926.6 hPa
The vapour pressure exponentially rises with temperature. A higher ambient
temperature and lower atmospheric pressure reduces the measuring range.
The above considerations are ruled out by the boiling point shift, enabling a
Tensiometer to measure beyond the vapour pressure. This status is
reproducible, but cannot be quantified.
If the soil gets dryer than 850 hPa the reading will halt at the value of the steam
point (i. e. 927 hPa at 20°C and 950 hPa atmospheric pressure) By diffusion and
minor leakage this value will drop within months.
If the soil gets dryer than 10000 hPa the reading will drop much faster as air
will enter the cup. Evaluating this will give further information, especially if
measurements are taken with TDR probes at the same time, as with this the
extrapolation of the pF-curve in the range of 850 … 10000 hPa determined.
Vapour pressure influence on water tension/water content relation:
If temperature rises from 20°C to 25°C in a soil sample with constant water
content, the water tension gets smaller about 8.5 hPa.
Temperature 4°C 10°C 16°C 20°C 25°C 30°C 50°C 70°C
Change of pressure per 0,6 0,9 1,2 1,5 1,9 2,5 7,2 14
Kelvin in [hPa]
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10.2 Osmosis
The ceramic used has a pore size of 1 µm. Thus, ions can hardly be kept out.
An influence of the measured value by osmotic effect is negligible under normal
circumstances. If the T8 is dipped into saturated NaCl solution 10 hPa are
measured. Either consider this fault or fill the water with this solution (after it
has been degassed).
10.3 Emptying the T8
Freezing of the Tensiometer water will destroy the pressure sensor and
possibly the cup. Therefore, frost endangered Tensiometer must be
removed from the soil or emptied.
In non-freezing depths Tensiometers can remain in the ground in winter.
Procedure for emptying:
1. Remove the connecting rubber tube from the refilling tube.
2. Connect the empty syringe to the refilling tube.
3. Completely suck out the Tensiometer water.
4. Connect rubber tubes and filing tube.
Until now, we received no tensiometer with freezing damage while installed in
the soil deeper than 30 cm, although the soil was frozen.
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11 Accessories
The following equipment is available:
11.1 Connecting and extension cables
Connecting cables for connections to dataloggers, etc.. One end with female
plug M12/IP67 for connection to a Tensiometer, the other end with loose
wires:
article: CC-8/5 (length 5 m)
CC-8/10 (length 10 m)
CC-8/20 (length 20 m)
Extension cables with one each male and female plug M12/IP67:
article: EC-8/10 (length 10 m)
EC-8/20 (length 20 m)
Individual lengths on request.
11.2 Handheld measuring device
Infield 7 handheld measuring device for manual readings (with or
without calibration function). article: INFIELD7.
11.3 Augers
Tensiometer gouge auger with specially shaped blade. The tip of the blade has
the same shape and diameter as the Tensiometer cup, so no illuvation with clay
paste is necessary. article: TB-25
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11.4 Refill and calibration kits
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BKTex
Refill kit for external refillable Tensiometer, including
manual vacuum pump, 250 ml coated glass bottle,
refill syringe, pipes and valves. article: BKTex
BKT468
Refill kit for Tensiometer, incl. base stand with
clamps, adapter that fits to T4, T6 and T8-sensor
bodies, 500 ml bottle, pressure gauge, tubes, beaker,
refilling syringe. article: BKT468
KBT8
Calibration kit for T8 with calibrator for digital balance, offset, incline; incl.,
precision manometer, valve, mounting kit, vacuum tube, requires a precision
voltmeter. article: KBT8
All glass vacuum bottles are plastic covered and implosion proof.

12 Contact and communication
Please take advantage of our consulting service – we want you to achieve
optimal results with our instruments.
With pleasure we would like to receive your comments, inspirations, critics.
P.O. Box 4, 6987 ZG Giesbeek
Nijverheidsstraat 30,
6987 EM Giesbeek,
The Netherlands
T +31 313 880200
F +31 313 632167
E eijkelkamp@eijkelkamp.com
I http://www.eijkelkamp.com
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13 Technical specifications
Dimension, material
Ceramic
Length 60 mm, diameter 24 mm
Sensor body
Material PA6
Shaft
Material PMMA, diameter 25 mm
Cable
Standard cable length
with shafts < 120 cm 1,5 m from sensor body
with shaft > 121 cm 0,6 m from end of shaft
Plug 8-pin, thread M12,
protective rate IP67 (waterproof)
Measuring range
Soil water tension
electronically -1000 hPa ... +1000 hPa
physically
-1000 hPa ... +850 hPa
Temperature -30 ... +70°C
Signal
Soil water tension 0 ... 2 V = -1000 hPa ... +1000 hPa
Temperature
0 ... 2 V = -30 ... +70°C
IR-indicator
Switch open: refilling ok
Switch closed (Vin): refilling required;
max. burden 50mA
Accuracy
Soil water tension ± 5 hPa
Temperature
± 0,2 K (-10...+30°C); ± 0,4 K
Supply
Power supply 6 ... 20 V
Current consumption 7 mA
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