Glossary · Automation fundamentals, platforms and components
Time constant
Also known as: Tau (τ)
German: Zeitkonstante
In control engineering, the time constant (τ) of a first-order system is the time its output needs after a step input to reach about 63.2 % of its final change. After about five time constants, the response is practically complete.
- Automation components
In one sentence
The time constant (τ) is the time a first-order system needs to reach about 63 % of its final value after a step change.
Example
A temperature sensor in a thermowell has a time constant of 20 s, so it needs about 100 s to show a sudden process temperature change almost fully.
How it applies
- Engineering: Time constant, Process gain and dead time describe process dynamics for controller tuning. The Sampling time should be clearly shorter than the relevant time constant.
- Measurement: Sensor time constants delay measured values. A slow sensor can hide fast process changes or delay protective reactions, which matters in response time calculations.
- Documentation: Data sheets should state sensor response times with the measurement conditions (for example t90 in water at a given flow). Loop documentation records identified time constants for later retuning.
Time constant vs. dead time
During dead time, the output does not react at all. The time constant describes how quickly the output then approaches its new value. Processes with long dead time relative to the time constant are hard to control.