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Service and maintenance
SWT4 User Manual v3.1
4 Service and maintenance
4.1 Refilling
To assure a rapid and reliable measurement of the soil water tension,
the cup must be filled possibly bubble-free with degassed water. After
dry periods or periods with a large number of wet and drying out
successions, the SWT4 or SWT4R must be refilled.
The following items are required for all refilling methods:
Syringe with valve (one supplied with each order)
Degassed, de-ionized or distilled water
Measuring device for checking the pressure signal
Simple method to degas water:
The best way to degas water is by using a syringe.
Draw up water into the syringe until it is 2/3 filled. Close the valve
or block the syringe with your finger.
Now draw up the syringe as far as possible to create a vacuum
inside. Rotate the syringe to create one big bubble.
Remove your finger or open the valve and squeeze out the bubble.
Repeat this procedure a few times.
4.1.1 When do Tensiometers need to be refilled?
Tensiometers need to be refilled if:
the curve of the readings apparently gets flatter (for example a rain
event has no sharp peak but is round),
the maximum of -85 kPa is not reached anymore.
Refilling is only reasonable if the soil is moister than -90 kPa,
or as soon as a Tensiometer installed in a lower level shows wetter
readings than the reading at which the upper Tensiometer stopped
working...
If the soil gets dryer than -85 kPa, the readings will remain constant at
the vapor pressure of water (for example: the reading will be 92.7 kPa
at 20°C and atmospheric pressure of 95 kPa). By diffusion and slight
leakage the reading will slowly drop within months.
