l Input signal is between lower and upper switching point -> Relay
remains switched off
l Input signal is smaller than the lower switching point -> Relay
remains switched off
8.3 Tendency recognition
The function of the tendency recognition is to recognize a defined
change within a certain time period and to transfer this information to a
relay output.
The information for tendency recognition is generated out of the
measured value change per time unit. The output variable is always
the measured value in percent. The function can be configured for
rising and falling tendency. The actual measured value is determined
and summed with a sample rate of a second. After the max. reaction
time, the average value is generated out of this sum. The real
measured value change results then of the newly calculated average
value less the previously calculated average value. If this difference
exceeds the defined percentage value, the tendency recognition
responds and the relay deenergises.
Note:
The activation and configuration of the tendency recognition requires
PACTware with the suitable DTM. Setting parameters via the
integrated indicating and adjustment unit is not possible.
l Measured value change higher: Measured value change per
time unit, at which the tendency recognition should respond
l Max. reaction time: Time after which a new measured value
generation is carried out and the measured value change is
recalculated
l Hysteresis: is automatically always 10 % of the value of
"Measured value change more than"
l Reaction in case of failure: In case of a failure, the relay goes
into the defined condition
Note:
After switching on or a failure, always two complete cycles must be
executed until a measured value difference can be calculated and a
tendency can be outputted.
The level in a basin should be monitored on rising tendency. If the rise
is higher than 25 % per minute, an additional emptying pumpt should
be switched on. The max. reaction time should be one minute. In case
of a probable failure, the pump should be switched off.
Select in the DTM navigation section the menu items "Meas. loop -
Outputs - Relay".
l E.g. set for relay 1 the mode "Rising tendency"
Functional principle
Principle of operation
Parameter
Example
Setup
VEGAMET 624 • 4 … 20 mA/HART signal conditioning instrument 39
8 Application examples
28969-EN-091116
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