How are Tensiometers installed?

Drill the hole with the special UMS soil auger type TB20 (for T1 Tensiometer and T3 Tensiometer) or TB25 (for T4 Tensiometer and T8 Tensiometer). For T5 Tensiometers the auger kit TBt5 is abailable and with which even the sensor body can be inserted into the soil.
 
These augers have a tapered tip, which means the part of the hole around the shaft will be wide enough for an easy insertion, but the bottom of the hole around the ceramic cup will be very tight for an optimum contact of ceramic and soil.
 
Thus. the use of slurry paste on the ceramic is not necessary!
 
Also in stony soils and gravel Tensiometers will responde much quicker without slurry. A slurry paste on the ceramic needs time to get in equilibrium with the surrounding water tension, and this will slow down the reaction of the Tensiometer.
 
T4, T5 and T8 Tensiometer can be installed in any angle. T1 and T3 Tensiometer cannot be installed horizontally. But in any case the shafts should not be straightly vertical to prevent preferential water to flow down along the shaft.
 
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T4e or T8: Oscillating readings due to temperature variations

A recent study by Prof. Wolfgang Durner showed that aboveground refilling tubes must be protected from heating up and solar radiation.
 
As a prevention we now offer an insulating protection cover to push over the aboveground part of Tensiometer and refilling tubes. The protection cover is supplied for free on request.
 
The revealed thermal effect is:
  • As long as Tensiometer and tubes are accurately filled it will work perfectly. But as soon as some air is trapped inside the aboveground part of the tube, the air will expand when heated up by solar energy. This causes a drop of the water tension and the adequate amount of water will flow from the cup into the ground.
  • You will notice a noise on the readings corresponding to the solar radiation. The readings will oscillate around the actual tension. The effect becomes more apparent with low potentials.
  • Furthermore, under permanent solar exposure the tubes get sticky and brownish.
 
As a prevention we now offer an insulating protection cover to push over the aboveground part of Tensiometer and refilling tubes. The protection cover is supplied for free on request.
 
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Can Tensiometers remain installed during winter time?

T1, T3 and T5 cannot remain installed during frost seasons as they are not frost resistant. Ceramic cup as well as pressure transducer might brake due to overpressure.
T4, T4e, T6 and T8 Tensiometer may remain installed as long as the installation depth is at least 30 cm. Then, frost will ingress slowly enough into the soil and the overpressure due to freezing is relieved through still open ceramic pores. 
 
Beware!
Never leave your not installed but filled Tensiometers in a place where frost can occur - for example overnight in your car! Then, the ceramic will freeze on it's outside first, the pores are closed and when the water inside the cup freezes the pressure transducer will definitely break.
 
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Indicator indicates bubble after refilling

The indicator is very sensitive and also respondes to also small bubbles, even if the reading is correct.
 
If the deairing tube is not at the highest point it may occur, that a small bubble remains,removable applying vacuum to enlarge this bubble.
 
First the cock is closed to have vacuum inside the tensiometer and enlarge this remaining bubble, which flows into the dearing tube.
Then the cock is opened for a some seconds two or three times, water gets succed in.
 
With this method all bubbles can be removed.
 
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Bef├╝llung mit BKTex

When do I have to refill my Tensiometer?

If the soil still is moist and below 85 kPa, but the reaction time gets long in comparison to the freshly filled Tensiometer. This can be noticed if the signal graph  has less sharp peaks but soft turns.
OR
After the first rainfall after a dry period, but only if the rainfall was sufficient enough and the soil is moister than 85 kPa. Check the readings of Tensiometers which are installed in a greater depth to estimate if the soil is moist enough.
BUT
It does not make sense to refill a Tensiometer when the soil is still to dry, as the water would be withdrawn from the soil right away.
 

Bad response time of a T5 Tensiometer

A T5 Tensiometer should react in seconds. If it does not do so, this can have different reasons:
 
1, Air - either inside sensor body or trapped in pores of the ceramic.
What to do: With a vacuum pump which can achieve a vacuum of a few kPa against vacuum degas both sensor body and shaft over night.
or: Use the lockable syringe which is available as an accessory.
Please check the manual for refilling instructions or contact UMS for assistance.
2, Clay particles block the ceramic cup.
What to do: Fill the cup with water and apply an overpressure of approx. 1 bar to the inside. With a wet sandpaper, grain size 120, or a wet corundum grinder, abrade the ceramic surface just a little.
 
3, The cup is clogged by microorganism.
What to do: Fill the cup with REHALON and rinse the ceramic with an overpressure of 1 bar.
 
4, The cup is clogged by algae growth.
What to do:  Fill the cup with REHALON and rinse the ceramic with an overpressure of 1 bar.
 
5, The "x-" cups of T5x Tensiometers have an extended measuring range but in general have a slower reaction time due to the finer pores. The standard T5 ceramics respond quicker but have a bubble point of 2 bar which limits the measuring range to max. 1,5 bar.
What to do: Always order standard high-speed T5 Tensiometers unless you really require the extended measuring range.

Should I wear gloves when refilling?

Not necessarily, but the ceramic should never be touched by hand. Ceramics are highly hydrophilic and must be able to absorb water. With skin grease or perspiration the surface of the ceramic might become hydrophobic resulting in a reduction of the water flow capability and the bubble point.
 
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Do I have to refill the Tensiometer as soon as the indicator detects a bubble?

The indicator can detect bubbles with a size of at least 1 ml. If the soil is moister than 50 kPa, this is no problem as the bubble changes only little with changing soil moisture.
 
But if the soil gets drier the bubble will expand quickly and an equivalent amount of water will run into the soil to achieve an equilibrium of soil and Tensiometer water tensions. As dry soils, especially sandy soils, have a poor water conductibility the response time of the Tensiometer will slow down significantly.
 
Therefore, the indicator works as an alert for the data evaluation, especially in sandy soils, and supports the quality assurance.
 
 

Into which tube should I fill in water?

If the Tensiometer is installed in a downwardly angle (cup lower than end of shaft) connect the refilling syringe to the shorter tube with the black mark.
 
If the Tensiometer is installed in a horizontal or upwardly angle (cup higher than the end of shaft) connect the refilling syringe to the longer and unmarked tube.
 

When should I stop to fill in water?

Fill in water until no bubbles come out of the open tube anymore, and then fill in another 10 ml.

How tight should the Tensiometer be reassembled?

Screw the parts together hand-tight only: As soon as the O-ring has contact and the pressure (please always read the pressure during reassembling) rises significantly only screw on for another quarter turn!
 
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Which soil volume is measured by Tensiometers?

In principle pressure potentials spread spherically and are not limited to a certain volume. In soils the water conductibility - which is always depending on the soil, current water tension, etc. - is the primal factor that affects the sphere of influence.
 
Additionally, the dynamic situation has to be considered. If, for example, a Tensiometer is installed in the root zone and the roots draw up water, the Tensiometer will measure this potential. With increasing distance between Tensiometer and roots the influence of the water conductivity rises.
 
After rain the different pores are refilled successively from the surface to lower depths. But in an extreme condition like dry, fine-grained sand, the water flow is buffered (hydrophobic effect) and the seepage water front will only move in a few centimeters per day. The Tensiometer will measure this accordingly, hence the measured volume is reduced to a few millimeters or centimeters.
 
In contrary, moist coarse clay has a very high water conductibility - the measured volume can span over a range of several meters.
 
In general the sphere of influence is much larger in depths, e. g. 2 meters, where the soil is moist and potential differences are little, and smaller close to the surface where water potentials might strongly vary within short distance.
 
Tensiometers always measure the potential at the spot where they are installed. They do not measure how the potentials spread out in the soil.

How to I make degased water?

Degased water means that any dissolved air in the water is removed as good as possible. To do this, fill a syringe (for example 50 ml syringe) to the half with water. Distilled water is sufficient. Now block the syringe, for example with your thumb, and completely draw it up - a negative pressure inside the syringe is created and the dissovled gas accumulates in bubbles. Turn and tip on the syringe to create one big bubble. Point the syringe upwards and press out the bubble. Repeat this procedure once or twice, and then use the syringe for refilling the Tensiometers.
By the way: this procedure is just identical with the physical process of boiling water.
 
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Contact

Georg von Unold
Tel: +49 89 12 66 52 - 15
Fax: +49 89 12 66 52 - 20
E-Mail: gvu@ums-muc.de
 
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