16
Installation
SWT4 User Manual v3.1
3.1.4 Protection of refilling tubes (SWT4R only)
A recent study by Prof. Wolfgang Durner showed that refilling tubes
must be protected from heating up and solar radiation.
If a bubble grows inside a refilling tube, temperature changes will lead
to an expansion of the air, resulting in a variation of the reading.
Therefore, refilling tubes should be as short as possible and should be
thermally protected, either by providing an insulating protection or by
burying the tubes.
Thermal effect:
As long as the Tensiometer and its tubes are
freshly and completely filled it will work
perfectly. Any air trapped inside the upper
parts of the tube will expand when heated up
by solar energy. This causes a drop of the
water tension and some water will flow from
the cup into the ground.
Thus, readings will fluctuate around the actual
reading during solar radiation, specially with
low water potentials. Furthermore, under
permanent solar exposure the tubes get sticky
and brownish.
Slide the supplied thermal insulation tube over the shaft end and the
refilling tubes as shown in above photo!
3.1.5 Use of Tube Jackets and Conduit
Tube Jackets:
These 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 tube jacket should end 30 to 50
cm away from the cup so that no leakage or
condensation is conducted to the cup. The
inner diameter of the jacket should be at least
35 mm.
Thermal inuslation
tube
Tube Jackets
