
This sensor element is distinguished through its nominal resistance R
0
of 100 ohm.
Humidity is measured by use of a capacitive sensor element. Thus, the humidity
sensor forms a capacitor with its environment. If there are any changes in humidity,
there are also changes in capacitance of the capacitor because of the difference in
permittivity. Figure 2.3 shows the capacitive sensor principle. The capacity is
calculated with equation 2.1.
Figure 2.3: Capacitive Sensor Principle
[http://upload.wikimedia.org/
wikipedia/commons/2/20/PlateCapacitorII.svg, 10.10.08]
C
0
=
0
r
A
d
(2.1)
•C
0
= Capacity [F]
•
0
= Permittivity Constant [Fm
−1
]
•
r
= Permittivity [Fm
−1
]
• A = Area [mm
2
]
• D = Distance [mm
2
]
The higher the rate of water, the higher the permittivity and therefore the higher
the capacitance. This change is detected by the connected electronics of the sensor
and allocated as an electrical output signal. The output signal is proportianal to
the changes in capacitance.
15
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