When the load changes, either the process value changes or the valve position must be changed to compensate for the load change.

Dead Time: A delay in the loop due to the time it takes material to flow from one point to another

Also called: Distance Velocity Lag

Transportation Lag

Most loop combine dead time and lag.

The dynamics differ from one loop to another.

However, they usually result in a response curve like this:

L is Lag—the largest lag in the process loop.

D is "Pseudo Deadtime"—the sum of the deadtime and all lags other than the largest lag.

Almost all processes contain disturbances.

Disturbances can enter anywhere in the process.

The effect of the disturbance can depend on where it enters the loop.

Most disturbances cannot be measured.