Endress+Hauser nivocompact FTC 831, Installation And Operating Manual

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Function
A metal plate at the end of the probe, inside the insulation, and the area
around it (e.g. silo wall) constitute the two electrodes of a capacitor with a
high frequency voltage between them.
The limit value is based on the principle of a discharge circuit.
As long as the end of the probe is in air with a dielectric constant of
ε
r
= 1,
then the discharge time constant is
τ
= R x C
A
where R is the resistance of
the circuit and C
A
the capacitance of the capacitor formed by the probe
end and its surroundings.
If a material with a dielectric constant
ε
r
≥
2.0 moves into the HF electrical
field at the end of the probe, then the capacitance C
A
increases and with it
the time constant
τ
.
This change in the time constant is evaluated and the Nivocompact FTC
831 is activated according to its switching mode.
Safety Switching
The built-in feature for minimum/maximum fail-safe switching allows the
Nivocompact to be used in all applications requiring high operational safety:
Maximum Fail-Safe:
The circuit is blocked if the probe is covered
or the power supply fails.
Minimum Fail-Safe:
The circuit is blocked if the probe is uncovered
or the power supply fails.
A red LED on the electronic insert indicates the switching status.
See also Fig. 16 in Section »Safety Switching«, Page 20.
Fig. 5
The operation of the Nivocompact FTC 831
with rope probe is totally unaffected by
material build-up on the walls of the silo.
Nivocompact FTC 831 Function
8