Instrumentation and Automatic Controls: Sensors, Loops, and Tuning
What this domain tests
This domain covers sensors and transmitters, control loop elements, controller modes, and tuning. Items ask you to identify the measured variable, choose a control mode for a process, interpret a loop failure, and recognise calibration and safety-instrumented-system concepts.
Core rules and concepts
- Loop elements: sensor, transmitter, controller, final control element (valve, damper, VFD), and the process. Feedback loops compare measurement to setpoint and act on the error.
- Control modes: on-off, proportional (offset remains), integral (removes offset), derivative (responds to rate of change). PID combines them. Derivative is rarely used on noisy flow loops.
- Proportional band is the inverse of gain; a narrow band gives high gain and can oscillate.
- Calibration: a 4-20 mA transmitter spans its range; 4 mA is 0% and 20 mA is 100%. A live zero allows detection of a broken wire.
- Safety instrumented systems are separate from basic process control; they trip to a safe state on demand. Testing proves the trip works.
Worked example
A temperature transmitter spans 0-200 C and outputs 4-20 mA. What current corresponds to 50 C, and what does a reading of 3.6 mA indicate?
Reasoning: fraction = 50/200 = 0.25. Current = 4 + 0.25 x 16 = 8 mA. A reading of 3.6 mA is below the live zero of 4 mA, which indicates a fault such as an open circuit or a failed transmitter, not a temperature of -5 C. The controller should alarm or drive to a safe state rather than act on the bad signal.
Traps that cost marks
- Ignoring the live zero and treating 0 mA as 0% of range.
- Using proportional-only control and expecting zero offset.
- Adding derivative action to a noisy flow loop, which amplifies noise.
- Assuming a control loop and a safety trip are the same; they must be independent.