Loop Check
What a definition is not
A definition is SPEQ’s plain-language decode of how a term is used in practice, cited to the documents that define it. It is a practitioner reference, not legal or regulatory advice, it does not replace the definition in the source, and where a regulator’s wording differs the regulator’s wording governs.
The point-to-point verification that an instrument signal travels correctly through the whole control loop — sensor, wiring, I/O card, controller, display and any alarm or interlock it drives — before the system is operated. It confirms the loop is wired and configured as designed, not that it performs to a process specification.
A loop check verifies that a signal travels correctly through an entire control loop — sensor, wiring, terminations, I/O card, controller, and the display or record at the end — rather than confirming that the instrument itself is accurate. The two are routinely confused. A correctly calibrated transmitter wired to the wrong channel produces a perfect calibration certificate and a reading attributed to the wrong vessel.
It is performed by injecting or simulating a known value at the sensor end and confirming that the expected value appears at every point in between, including at the record. Because it tests continuity of identity as well as of signal, it catches crossed loops, inverted ranges, misconfigured scaling and mislabelled tags — the defects that are almost impossible to detect later, because the system behaves plausibly.
The same reasoning applies to outputs and interlocks: a loop check on a valve or a trip is the verification that the command reaches the element and the element does what was commanded. Skipping those is how an interlock that was never wired passes qualification.
- —Tests the whole path — sensor to record — not the instrument’s accuracy.
- —A perfectly calibrated instrument on the wrong channel still reads for the wrong vessel.
- —Catches crossed loops, inverted ranges, misconfigured scaling and mislabelled tags.
- —Applies to outputs and interlocks, not only to measurements.
- —The defects it catches are the ones that look plausible afterwards.
ISPE Baseline Guide Vol. 5 (2nd ed., 2019); ISPE GAMP 5 (2nd ed., 2022) for the automation-system verification it supports.
Frequently asked questions
What is Loop Check?
The point-to-point verification that an instrument signal travels correctly through the whole control loop — sensor, wiring, I/O card, controller, display and any alarm or interlock it drives — before the system is operated. It confirms the loop is wired and configured as designed, not that it performs to a process specification.
Which regulations cover Loop Check?
ISPE Baseline Guide Vol. 5 (2nd ed., 2019); ISPE GAMP 5 (2nd ed., 2022) for the automation-system verification it supports.