What are the instrumentation requirements for Safety Instrumented Systems (SIS)?

SIS instrumentation for IEC 61508/61511 compliance requires: certified sensors and final elements with known failure rates (SFF, MTTF, proof test intervals), redundancy architectures (1oo2, 2oo3) based on SIL requirements, systematic failure avoidance (proven-in-use, third-party certification), diagnostic coverage (partial stroke testing, valve position feedback), separation from BPCS, reliable communication (hardwired or certified safety bus), and comprehensive documentation. SIL 1-3 have increasing requirements for probability of failure on demand. Regular proof testing verifies dangerous undetected failures. Bypass management is critical during testing.