The Ceramic Tip; Analog Output Signals - Delta-T Devices SWT5 User Manual

2.1.4 The ceramic tip

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 a ceramic, sintered Al
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 of a SWT5 cup is about 200 kPa (20 bar, 2,000 hPa),
and for a SWT5x it is about 500 kPa (50 bar, 5,000 hPa).
If the soil gets dryer than these values, 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.
Ceramic cup: Do not touch the cup with your fingers. Grease, sweat
or soap residues will influence the ceramic's hydrophilic
performance.
Do not allow the SWT5 ceramic to dry out by leaving it unprotected
in air: By drying out the bubble point might be reached, the reading
will go to 0 kPa and air can enter the cup, which will then require
refilling.

2.2 Analog output signals

The pressure transducer offers the soil water tension as a linear output
signal, with 1 mV corresponding to 1 kPa (10 hPa).
As the pressure transducer is a Wheatstone full bridge, it has to be
connected correctly.
See also Connecting SWT5 and SWT5x on page 19.
Please also read the user manual for your display unit or data-logger
before connection.
14  Sensor Description
O . A special
2 3
SWT5 User Manual v2