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 Individual alarms,
see Digital output (Page 144).
Limit behavior on the outputs
Process Alarms can trigger Fail Safe behavior on the Signal Output, whereas Process
Warnings are only used as information available in HMI and on the communication. Process
value will bring the Signal output to Fail Safe mode if:
- Signal Output is configured to Current, Pulse or Frequency
- Fail Safe Mode is configured to react on a failure
- Process Alarm occurs on a process value selected on the output
The alarm behavior is described in detatail in Device status symbols (Page 167).
Hysteresis is used to adjust the tolerance by undershooting or overshooting the limit as
described below.
Process value derivations
The front-end of the device measures time and derives the values of certain process variables
from those measurements. The time period of vibration of the two measuring tubes is inversely
proportional to their frequency, which is used to determine density. The average difference in
phase of the two measuring tubes is dependent upon the mass flow rate of the process
medium. In this measurement context, phase difference is expressed not in degrees of rotation
but as an absolute time measurement. For this reason the result of zero offset correction is
displayed in µs, being the unit of the true measurement.
Parameter assignment
8.2 Functions
SITRANS FC330
Operating Instructions, 01/2019, A5E44030648-AB
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