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Description of the T8
2.1.5 Filling status indicator
The filling status indicator indicates the filling status of a T8 in a
downwards installation (cup lower than end of the shaft).
If the Tensiometer is installed in an upward angle (cup higher than
the end of the shaft), the water will still cover the indicator even if
air is already inside the cup. Thus, the indicator will react not
before the cup is almost completely empty (s. fig. 5.3 on page 38).
The filling status is determined by measuring the heat flux with the
internal temperature probe. For a probe dipped into water the heat
flux rate is lower than in air. If an air bubble appears around the
temperature probe, the heat flux rate rises and the insufficient filling
status is detected. This method offers a rough but reliable reference
for checking the filling and has the advantage of staying stable over
the whole lifespan of the Tensiometer. It replaces the previously use
IR-photo diode. The measurement of the soil water tension is not
influenced.
Benefits of the filling status 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.
Also, for quality assurance, the status of the signal can be logged for
later evaluation of the readings.
2.1.6 The ceramic cup
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 at least 1500 kPa (15 bar). If the soil is dryer than
1500 kPa 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.
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