Answer / carl_ellis

The problem is in the controller's PID tuning, which is
tuned to be too sluggish.

All of the loop's control components work properly:
- the transmitter reports the process variable
- the controller responds to an error (albeit too sluggishly)
- the control valve responds to the controller's output, as
evidenced by switching to manual mode

The PV response as reported by the transmitter depends on
the controller's appropriate response to error. If the
controller's response to error is sluggish, then the loop's
PV response will be sluggish.

Manual mode offers a means of overriding the PID's
calculated output value and driving the output to where it
needs to be.

This question qualifies as one of the best interview
questions I've read on this site. I'll be sure to use
this question when I interview candidates.

Carl Ellis
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