Transmitters versus switches
Transmitters give a continuous signal: the logic solver can compare redundant channels, detect out-of-range signals and drift, and operators see the value. Switches only change state at the setpoint and a stuck switch is invisible until tested. Most modern SIFs use transmitters for these diagnostic advantages.
Independence from control
The SIF sensor should not be the control loop's sensor. A failure of a shared transmitter could be the initiating cause and defeat the protection at the same time.
Installation and common cause
- Separate process tappings and impulse lines for redundant transmitters; a shared tapping can plug for all of them.
- Heat tracing and winterisation for impulse lines in cold climates; hydrate and freezing failures are typical common causes.
- Diversity (different measuring principles) lowers β where common causes are likely, for example radar and differential pressure for level.
Specific devices
- Level: radar and guided wave radar are common; check echo curves and probe build-up. Independent level for overfill protection must not share the tank gauge.
- Temperature: thermowell response time can dominate the response budget in fast reactions.
- Flame scanners: self-checking types are needed for continuous operation; sight glass fouling is a classic failure.
- Analysers: sample systems add lag and failure modes; their PFD and response time must include the sample conditioning.
In First Out AI
Sensor groups carry the device description and voting; the proof test draft generates specific steps for pressure, level, temperature and flow transmitters, flame scanners, gas detectors, analysers and switches.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.