Time constant
The time constant is lag after dead time: roughly how long the PV takes to reach 63% of the new value once it starts moving. Dead time is the wait; this is the climb.
What it means
The time constant is the measure of lag that comes after dead time. Where dead time is a period in which nothing happens at all, the time constant describes how the measurement climbs once the response begins: roughly, the time to reach about 63% of the way to the new value.
The distinction matters because the two limit tuning in different ways. Dead time sets how fast integral action can safely repeat. The time constant, relative to the dead time, decides how hard the loop is to control at all — a loop with a long time constant and a short dead time is forgiving, while a loop with a long dead time relative to its time constant is difficult regardless of tuning.
"Lag time" is sometimes used loosely for this, but it is worth keeping the two apart: lag (the time constant) is how the process responds once it starts, dead time is the wait before it starts.
Questions that come up
What is a time constant in process control?
How quickly the process responds once it starts — roughly the time to 63% of the new value after dead time. Lag, not the wait before anything happens.
Related
Where this comes up
Written by PIDSnap control engineering. Published 2026-08-01, last reviewed 2026-10-22.