Siemens SITRANS FC430 Operating Instructions Manual page 134

Functions
8.1 Process values
● Lower Limit Alarm
● Alarm Hysteresis
The system reports a process alarm when the process value exceeds the Upper Limit Alarm
or the Lower Limit Alarm. Likewise, the system reports a process warning when the process
value exceeds the Upper Limit Warning or the Lower Limit Warning. Process value alarms
and warnings are displayed in the HMI as well as at the communication interfaces.
Hysteresis
The hysteresis functions as follows:
Figure 8-1
A: Upper Alarm Limit with hysteresis
The alarm is triggered when the process value overshoots the Upper Alarm Limit (1). The
alarm is cleared when the process value undershoots the Upper Alarm Limit minus
hysteresis (2).
B: Lower Alarm Limit with hysteresis
The alarm is triggered when the process value undershoots the Lower Alarm Limit (1). The
alarm is cleared when the process value overshoots the Lower Alarm Limit plus hysteresis
(2).
C: Lower Alarm Limit without hysteresis
The alarm is triggered when the process value undershoots the Lower Alarm Limit (1). The
alarm is cleared when the process value overshoots the Lower Alarm Limit (2).
D: Upper Alarm Limit without hysteresis
The alarm is triggered when the process value overshoots the Upper Alarm Limit (1). The
alarm is cleared when the process value undershoots the Upper Alarm Limit (2).
Note
Flow direction warning
The limit function can be used to signal the flow direction by setting the Lower Limit Warning
for the Process Value to 0. A warning will occur in case of negative flow
All alarms and warnings can be signaled on the output if Status Mode is set to Alarm Item,
see Status output (Page 147).
132
Hysteresis
Operating Instructions, 05/2015, A5E03361511-AF
FC430 with HART