Question

Dead time in a process refers to:

• A. Time taken to reach steady state
• B. Time delay between input change and output response
• C. Time constant of the system
• D. Time for the controller to react

Hint:

Remember: Dead time = delay before the system starts reacting. It’s the flat delay portion in the system’s step response curve.

Answer 1

The Correct Option is B

Dead time, also known as transportation lag or time delay, is a key concept in process control. It refers to the interval of time between when an input or disturbance is applied to a system and when the output begins to respond.
Mathematically, in the Laplace domain, dead time is represented by a term like $e^{-s\tau}$, where $\tau$ is the dead time.
It is important to distinguish dead time from other time-based characteristics:
- It is not the time taken to reach steady state — that relates to the system's dynamics.
- It is not the time constant — that defines how fast the system responds after the delay.
- It is not related to controller reaction time — the controller may act instantly, but the process may still have inherent delay.
Dead time makes control more challenging, especially in fast systems, because the controller cannot act on something it has not yet detected in the output.
Hence, dead time is best defined as the time delay between input change and output response.