Functional Safety; Safety Function - Siemens SITRANS P DS III Operating Instructions Manual

Functional safety

8.1

Safety function

The safety function of SITRANS P refers to the measurement of pressures. (Ordering option
-Z C20 and -Z C23) Add an additional safety accuracy of 2% of the maximum measuring
range to the application-specific measurement error.
Total tolerance (safety function) = ± [application-specific measuring error + 2% safety
accuracy].
Pressure transmitter safety accuracy: the maximum effect of a random non-critical individual
error on the measured value.
The diagnostics function will respond within 4 seconds in the worst-case scenario.
The safety accuracy together with the application-specific measurement error allows the
system operator to include a backup for process monitoring. Even if a random individual
error occurs that is within the safety accuracy, the system can still be safely shut down.
As there is also always a possibility of dangerous faults occurring, these are categorized and
listed in the manufacturer declaration for the device (SIL declaration of conformity, functional
safety pursuant to IEC 61508 and IEC 61511).
Example:
A silo is to be securely monitored to check that the level does not exceed 10 meters.
Application-specific measurement error: 0.1%
Safety accuracy: 2.0%
Total tolerance: 2.1%
2.1% of 10 meters is 21 centimeters. If process monitoring is set to 9.79 meters, safe
shutdown is guaranteed even in the event of a random individual error within the safety
accuracy.
Note
Use of remote seals
If remote seals are used, the application-specific measurement error is the product of the
transmitter and remote seal measurement errors.
SITRANS P DS III with HART
Operating Instructions, 06/2013, A5E00047092-09
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