The equipment that measures, transmits and commands — the plant's senses and brain. It does not touch the fluid's energy: it converts process state into signal and signal into decision.
Usual groups
(handbooks per type in Phase 1)
- Sensors and transmitters (input devices in ISO 14224) — pressure, temperature, flow and level: the quality of every loop starts here;
- Control logic (control logic units) — PLCs, DCSs and dedicated controllers;
- Fire and gas detection (fire & gas detectors) — safety functions, typically within instrumented systems (SIS, IEC 61511).
The loop's final element — the control valve — lives in the Valves family.
Reliability: calibration drift, impulse-line plugging and hidden failures of safety functions dominate — SIS instruments have diagnostic coverage and proof-test intervals dictated by SIL (IEC 61511), not by shutdown convenience.