Dead time

Dead time is the delay between a change at the output and the first sign of a response in the measurement. It is the number that sets how fast reset can safely repeat.

What it means

Dead time is the interval during which absolutely nothing happens. Move the valve, and for some period the measurement does not change at all — the fluid has to travel, the heat has to conduct, the sample has to reach the analyser.

It is the single most important number in tuning, because it sets a hard limit on how well any controller can perform. A controller cannot correct for a disturbance it has not been told about, and dead time is exactly the period during which it has not been told. Loops with long dead time relative to their time constant are difficult regardless of tuning.

Dead time is also the reason reset can be set too fast. If integral action repeats faster than the dead time, the controller keeps adding correction while the previous one is still in transit, and the loop cycles by construction.

Also called

On a plant you will hear this called transport delay. They mean the same thing.

Questions that come up

What is dead time in a control loop?

The delay between a change at the output and the first movement of the PV. Fluid has to travel, heat has to conduct. Reset faster than the dead time cycles by construction.

Glossary

Where this comes up

Written by PIDSnap control engineering. Published 2026-08-01, last reviewed 2026-10-22.