Endress+Hauser Liquiline M CM42 - Page 65

Functional Safety Manual
for the Memosens transmitter Liquiline M CM42 SIL
Version: Page:
2.0 65 of 72
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So after the proof test has been successfully finished, the systems PFD
avg
value has been
"improved" and you can determine when the next proof test has to be carried out.
Here we use the Memosens cable CYK10 in a 1oo1D setting for the example.
Assumptions for this example:
Proof test is done after two years of operation, because system is not allowed to have a
higher PFD
avg
than 1.80 × 10
-4
at all times.
Initial PFD
avg
of new cable: PFD
avg
(0) = 0
PFD
avg
of a two year old cable: PFD
avg
(2 years) = 1.80 × 10
-4
assuming

DU
= 2.05 × 10
-8
1/h (= 20.5 FIT) and
where PFD
avg
(t) = 1/2 × t × 
DU
, t in hrs.
Then you do the proof test (follow the CM42 menu guidance) successfully.
Proof test coverage is (see Memosens cable safety manual): 90%.
New values after the proof test has been successfully finished:
New PFD
avg
value after two years and after a successful proof test
PFD
avg
(2 years; proof test successful) = 1.80 × 10
-4
× (1.00 – 0.90) = 0.18 × 10
-4
PFD
avg
value after two additional years (no additional proof test done yet):
P F D
avg
(4 years) = 0.18 × 10
-4
+ 1.80 × 10
-4
= 1.98 × 10
-4
Further questions:
What is the time period t, after which the PFD
avg
(t) value of this once "proof tested
system" reaches again 1.80 × 10
-4
?
Find t, where PFD
avg
(t) = 1.80 × 10
-4
=> 1.80 × 10
-4
= 0.18 × 10
-4
+ 0.50 × 
DU
× t
=> T in years: T = 0.9 × 2.0 years = 1.8 years = 21.6 months
And therefore the proof test interval T after the first "incomplete" proof test with a proof
test coverage of 90%, will be smaller than two years.