Operating instructions

39
8 Application examples
VEGAMET 625 • Double channel HART
28970-EN-150129
8 Application examples
8.1 Level measurement in a cylindrical tank with
overll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lling,relay
mode"Overll protection"isset.Therelayisthusswitchedo(safe
currentless state) when the max. level is exceeded, and switched on
againwhenthemin.levelisunderrun(switch-onpoint<switch-o
point).Duringemptying,mode"Dry run protection"isused.Thisrelay
isthusswitchedowhenthemin.levelisunderrun(safecurrentless
condition), and switched on again when the min. level is exceeded
(switch-onpoint>switch-opoint).
Rel. 1Rel. 2
Rel. 1: 90%
Rel. 2: 5%
100%
0%
Fig. 11: 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llingbyatankcariscontrollviarelay
1andavalve(overll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 stand-
ard vessel (according to bearing chart). The min. level should be
Functional principle
Example