38477BA2B
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9 Functional Safety Volume 1
9.3 Safety Function
The safety function of the measuring system comprises the mea-
surement and detection of levels caused by the presence of product
being measured in the measuring path between radiation source
and measuring system.
9.4 Safety Requirement
Safety integrity level Operating mode with low
demand rate
Operating mode with high or continuous
demand rate
SIL PFD PFH
4 ≤10
-5
to <10
-4
≤10
-9
to <10
-8
3 ≤10
-4
to <10
-3
≤10
-8
to <10
-7
2 ≤10
-3
to <10
-2
≤10
-7
to <10
-6
1 ≤10
-2
to <10
-1
≤10
-6
to <10
-5
Safe failure fraction Hardware fault tolerance
SFF HFT = 0 HFT = 1 HFT = 2
none: <60% not allowed SIL 1 SIL 2
low: 60% to <90% SIL 1 SIL 2 SIL 3
medium: 90% to <99% SIL 2 SIL 3
high: ≤99% SIL 3
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