Sample time
Sample time is how often the controller executes. Between scans the output holds, so a slow sample time is extra dead time. Fast loops on a slow scan cannot be retuned out of it.
What it means
Sample time is the interval between controller executions: every scan, every second, every few hundred milliseconds. Between executions the controller is blind and the output simply holds where it was last set.
On a continuous process a slow sample time behaves like dead time. The controller hears about a change later than it could have, and that delay consumes the same stability budget that process dead time does.
The working rule is that sample time should be a small fraction of the loop's response time. A fast loop on a slow scan is being asked to control on stale information, and no amount of re-tuning recovers the delay the scan itself introduces.
Questions that come up
What is PID sample time?
How often the controller executes. Between scans the output holds, so a slow sample time behaves like extra dead time. No retune recovers delay the scan itself introduces.
Related
Where this comes up
Written by PIDSnap control engineering. Published 2026-08-01, last reviewed 2026-10-22.