Vega VEGAFLEX 80 series Safety Manual - 3 Planning
6
3 Planning
VEGAFLEX 80 series • Two-wire 4 … 20 mA/HART
42960-EN-170410
3 Planning
3.1 Safety function
The sensor generates on its current output a signal between 3.8 mA
and 20.5 mA corresponding to the process variable. This analogue
signal is fed to a connected processing system to monitor the follow-
ing conditions:
•
Exceeding a defined limit value of the process variable
•
Falling below a defined limit value of the process variable
•
Monitoring of a defined range of the process variable
For the design of the safety function, the following aspects must be
taken into account with regard to the tolerances:
•
Due to undetected failures in the range between 3.8 mA and
20.5 mA, an incorrect output signal can be generated which devi-
ates from the real measured value by up to 2 %
•
Increased measurement deviations can occur at the boundaries of
the measuring range (see Technical Data in the operating instruc-
tions)
3.2 Safe state
The safe state of the current output depends on the safety function
and the characteristics set on the sensor.
Character-
istics
Monitoring upper limit val-
ue
Monitoring lower limit value
4 … 20 mA
Output current ≥ Switching
point
Output current ≤ Switching
point
20 … 4 mA
Output current ≤ Switching
point
Output current ≥ Switching
point
Possible fault currents:
•
≤ 3.6 mA ("fail low")
•
> 21 mA ("fail high")
3.3 Prerequisites for operation
•
The measuring system should be used appropriately taking pres-
sure, temperature, density and chemical properties of the medium
into account. The application-specific limits must be observed.
•
The specifications according to the operating instructions manual,
particularly the current load on the output circuits, must be kept
within the specified limits
•
Existing communication interfaces (e. g. HART, USB) are not used
for transmission of the safety-relevant measured value
•
The instructions in chapter "Safety-related characteristics", para-
graph "Supplementary information" must be noted
•
All parts of the measuring chain must correspond to the planned
"Safety Integrity Level (SIL)"
Safety function
Safety tolerance
Safe state
Fault signals in case of
malfunction
Instructions and restric-
tions