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4
1.2.2 Transducer Materials
A broad selection of transducer materials is available
for the Model 961/962. Metallic transducers include 316
SS, Hastelloy® C, and Monel®. The 316 SS transducer
has a NACE construction option for sour gas service,
and can also be built per ASME B31.1 and B31.3 piping
codes. Thermoplastic transducers include Kynar® and
CPVC. These corrosion resistant plastic transducers
feature a stiffening tube that runs the length of the
transducer for extra rigidity. Kynar-faced 316 SS flange
options are offered with the Kynar transducers.
1.3
Determining Safety Integrity Level (SIL)
The FMEDA report provides the failure rates and infor-
mation for the PFDavg and Architecture Constraint.
The architecture constraint was determined using the
2H approach according to 7.4.4.3 of IEC 61508-2, 2010
edition. The Model 961/962 meets the hardware archi-
tecture constraints for up to SIL 2 at HFT=0 (or SIL 3
at HFT=1) when the failure rates listed in the FMEDA
report are used.
Table 1
SIL vs. PFDavg
Safety
Integrity Level
(SIL)
4
3
2
1
2.0
Applicable Models
This manual is applicable to the following models of
the Echotel ultrasonic level switches with current shift
output:
• Model 961 Single-Point Level Switches
• Model 962 Dual-Point Level Switches
3.0
Mean Time To Repair (MTTR)
SIL determinations are based on a number of factors
including the Mean Time To Repair (MTTR). The analysis
for the Echotel Model 961/962 ultrasonic level switch
is based on a MTTR that is dependent on the end-user
practices.
Target Average
Probability of Failure
on Demand (PFDavg)
≥10
to <10
-5
-4
≥10
to <10
-4
-3
≥10
to <10
-3
-2
≥10
to <10
-2
-1
51-650 Model 961/962 Echotel SIL Safety Manual
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