SWT4 User Manual v3.1
Sensor Description
13
2.1.4 The ceramic cup
To transfer the soil water tension as a negative pressure into the
Tensiometer, a semi-permeable barrier is required. This must have
good mechanical stability, be permeable to water and impermeable to
gas (when wet).
The Tensiometer cup consists of ceramic Al
2
O
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sintered aluminium. A
special manufacturing process guarantees homogeneous porosity with
good water conductivity and very high hardness. Compared to
conventional porous ceramic the cup is much more durable.
The bubble point is at least 1500 kPa (15 bar, 15,000 hPa). If the soil is
dryer than -1500 kPa air can enter, the negative pressure inside the
cup decreases, and the readings go down to 0 kPa.
With these characteristics this material has outstanding suitability to
work as the semi-permeable diaphragm for Tensiometers.
The cup has a lifetime guarantee against breakage.
Ceramic cup: Do not touch the cup with your fingers. Grease,
sweat or soap residues will influence the ceramic's hydrophilic
performance.
2.2 External refilling (SWT4R only)
The SWT4R is equipped with 2
stainless-steel capillary tubes
which are led through the cable
gland and the shaft. Each one has
an opening that ends inside the
ceramic cup. This allows easy
refilling of the SWT4R while it
remains installed in the ground.
Tube 1 ends at the very tip of the
cup, tube 2 ends next to the
pressure transducer opening.
Outside, both tubes are connected together with a short rubber tube.
Any air bubbles will ascend and accumulate either in the ceramic’s tip
around tube 1 (upwards installation angle) or next to opening 2
(downwards installation angle). Accordingly the air is removed either
through tube 1 or 2 (please see chapter 3.2).
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