If transmitter fails but controller is active, what happens to loop?

Multiple Choice Questions Instrumentation MCQ
1

If transmitter fails but controller is active, what happens to loop performance in most industrial control systems?

A. Stable Control
B. Improves control
C. No control
D. Full control

View Answer:

:white_check_mark: Correct Answer: C No control

:brain: What’s happening in the loop?

A basic control loop needs three key elements:

  • Transmitter → provides PV (feedback)
  • Controller → calculates action
  • Final element → executes control (valve, drive, etc.)

:backhand_index_pointing_right: If the transmitter fails, the controller loses the PV (Process Variable).

:gear: Why this leads to No Control

The controller works based on error:

Error=SP−PV

:backhand_index_pointing_right: If PV is missing or invalid:

  • Controller cannot calculate error correctly
  • Output becomes unreliable or frozen
  • Control action becomes meaningless

:magnifying_glass_tilted_left: Real Industrial Behavior

Depending on system design, one of these happens:

  • Controller goes to manual mode
  • Output holds last value (freeze)
  • Output goes to fail-safe (0% or 100%)

:backhand_index_pointing_right: In all cases:
Closed-loop control is lost → effectively NO CONTROL

:cross_mark: Why other options are wrong

:cross_mark: Stable Control

Needs valid feedback → not possible without transmitter

:cross_mark: Improves Control

Impossible without measurement

:cross_mark: Full Control

Opposite situation → requires all elements working

:bullseye: Final Understanding

:backhand_index_pointing_right: A control loop without feedback is like:

Driving a car with your eyes closed

You may still press the accelerator (controller active),
but you have no idea what’s happening in the process.

:light_bulb: Practical Insight (Important)

In real plants:

  • Transmitter failure is treated as a critical fault
  • Safety systems (SIS) or alarms will trigger
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