8 Application examples
8.1 Level measurement in a cylindrical tank with
overfill protection/dry run protection
The level is detected via a sensors and transmitted to the signal
conditioning instrument by means of a 4 … 20 mA signal. Here, an
adjustment is carried out converting the input value delivered by the
sensor into a percentage value.
Due to the geometrical form of the cylindrical tank, the vessel volume
does not increase linear with the level. This can be compensated by
selecting the linearization curve integrated in the instrument. This
curve states the relation between percentage level and vessel volume.
If the level should be displayed in litres, also a scaling must be carried
out. For this purpose, the linearized percentage value is converted into
a volume, for example with the unit litres.
The filling and emptying is controlled vial relay 1 and 2 integrated in
the signal conditioning instrument. During filling, relay mode "Overfill
protection" is adjusted. The relay is hence switched off (safe
currentless condition) when the max. level is exceeded, when falling
below the min. level it is switched on again (switch on point < switch off
point). During emptying, mode "Dry run protection" is used. This relay
is hence switched off when falling below the min. level (safe
currentless condition), when falling below the min. level it is switched
on again (switch on point > switch off point).
Fig. 11: Example of a level measurement, cylindrical tank
A cylindrical tank has a capacity of 10000 litres. The measurement is
carried out with a level sensor operating according to the principle of
the guided microwave. The filling by a tank car is controll via relay 1
and a valve (overfill protection). The discharge is carried out via a
pump and is controlled by relay 2 (dry run protection). The max.
Functional principle
Example
VEGAMET 391 • 4 … 20 mA/HART signal conditioning instrument 41
8 Application examples
36032-EN-091116
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