Dead time

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, lag time. They mean the same thing.

Related

Where this comes up

• Control valve hunting
• PID loop oscillating
• Valve position oscillates but the process variable barely moves
• Sluggish control loop
• Flow loop unstable
• Pump speed loop hunting
• Noisy PV and valve chattering
• DeltaV PID parameters explained
• Ovation PID parameters explained
• SIMATIC PCS 7 PID parameters explained
• Rockwell PIDE PID parameters explained
• Yokogawa Centum PID parameters explained
• Honeywell Experion PID parameters explained
• Reset units converter
• Bump test procedure
• Loop type tuning cheat sheet
• Tuning flow control loops
• Tuning temperature control loops
• Tuning pump and fan speed loops
• Tuning pH control loops
• Process gain
• Cascade control
• Lambda tuning
• Bump test