VEGA VEGAMET 391 4 … 20 mA_HART - Manual (Page 39)

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8 Application examples
VEGAMET 391 • 4 … 20 mA/HART
36032-EN-130701
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.
Filling and emptying are controlled via relay 1 and 2 which are
integrated in the signal conditioning instrument. During filling, relay
mode "Overfill protection" is set. The relay is thus switched off (safe
currentless condition) when the max. level is exceeded, and switched
on again when the min. level is underrun (switch-on point < switch-off
point). During emptying, mode "Dry run protection" is used. This relay
is thus switched off when the min. level is underrun (safe currentless
condition), and switched on again when the min. level is exceeded
(switch-on point > switch-off point).
Rel. 1 Rel. 2
Rel. 1: 90%
Rel. 2: 5%
100%
0%
Fig. 13: Example for 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.
volume should be at 90 % level, these are 9538 litres with a standard
vessel (according to bearing chart). The min. level should be adjusted
Functional principle
Example