Instrumentation – Flapper–nozzle mechanism: In which type of controller is a flapper–nozzle assembly used as the key sensing/modulating element?

Introduction / Context:
Flapper–nozzle mechanisms are classic building blocks in industrial control systems. Recognizing where they are applied helps in selecting instruments and diagnosing loop behavior in older and modern retrofitted plants.

Given Data / Assumptions:

• Focus on the physical principle of operation (back pressure from nozzle).
• Controller media: air (pneumatic), oil (hydraulic), or electricity (electronic).

Concept / Approach:
A flapper–nozzle pair converts small mechanical motions into pneumatic pressure signals by throttling the flow of compressed air. As the flapper approaches the nozzle, back pressure rises sharply, creating a sensitive pneumatic signal for relay, pilot valves, and pneumatic PID controllers.

Step-by-Step Solution:
Identify energy domain → compressed air.Recognize function → motion-to-pressure transduction using a restriction (nozzle) and moving element (flapper).Hence, the assembly is intrinsic to pneumatic controllers.

Verification / Alternative check:
Standard pneumatic PID controllers and I/P (current-to-pressure) converters often contain flapper–nozzle stages or similar pneumatic relay internals.

Why Other Options Are Wrong:

• Electronic: uses electrical sensors (bridge circuits, op-amps), not flapper–nozzle air stages.
• Hydraulic: employs liquid pressure/flow; not flapper–nozzle with compressed air.
• None/Electro-hydraulic only: contradicts the well-known pneumatic application.

Common Pitfalls:
Assuming any mechanical motion transducer is “hydraulic”; the flapper–nozzle is specifically pneumatic.

Final Answer:
Pneumatic controller