CCST Instrumentation & Process Control 3 — Questions and Answers
Question 1: A cascade control system consists of two controllers. Which controller's output directly manipulates the final control element?
- Primary (master) controller
- Secondary (slave) controller (Correct answer)
- Both controllers in parallel
- A separate override controller
Correct answer: Secondary (slave) controller
In cascade control the secondary (slave) controller receives its setpoint from the primary and directly outputs to the final control element such as a valve.
Question 2: Which sensor type operates on the principle of electrical resistance change proportional to mechanical strain?
- LVDT
- Strain gauge (Correct answer)
- Piezoelectric crystal
- Capacitive sensor
Correct answer: Strain gauge
A strain gauge changes its electrical resistance when deformed, and the resistance change is proportional to the applied strain.
Question 3: The gain of a proportional controller is increased. What is the expected effect on steady-state offset for a step load disturbance?
- Offset increases
- Offset decreases but is not eliminated (Correct answer)
- Offset is completely eliminated
- Offset remains unchanged
Correct answer: Offset decreases but is not eliminated
Higher proportional gain reduces offset because the controller produces a larger corrective output per unit of error, but proportional-only control can never fully eliminate steady-state offset.
Question 4: A Coriolis flowmeter measures flow based on which physical principle?
- Pressure drop across a restriction
- Time-of-flight difference of ultrasonic pulses
- Inertial force caused by mass flow through vibrating tubes (Correct answer)
- Magnetic induction in a conductive fluid
Correct answer: Inertial force caused by mass flow through vibrating tubes
Coriolis meters measure the twist (phase shift) in vibrating tubes caused by the inertial Coriolis force, which is directly proportional to mass flow rate.
Question 5: What does 'turndown ratio' describe in the context of a flowmeter?
- Maximum allowable operating pressure
- Ratio of maximum to minimum measurable flow with acceptable accuracy (Correct answer)
- Frequency at which the meter must be re-calibrated
- Percentage of full scale at which the output signal saturates
Correct answer: Ratio of maximum to minimum measurable flow with acceptable accuracy
Turndown ratio (also called rangeability) expresses the ratio of the maximum to minimum flow rate within which the meter meets its accuracy specification.
Question 6: In a split-range control scheme, two control valves are operated by a single controller output. How is the output typically split?
- Both valves respond to the full 4-20 mA range simultaneously
- One valve operates over 4-12 mA and the other over 12-20 mA (Correct answer)
- The signal alternates between valves on a time basis
- One valve is opened first; the second opens only on high output
Correct answer: One valve operates over 4-12 mA and the other over 12-20 mA
In classic split ranging, the controller output is divided so valve A strokes over the lower half (4-12 mA) and valve B strokes over the upper half (12-20 mA) of the signal range.
Question 7: A level transmitter using a displacer element measures level based on which principle?
- Hydrostatic head pressure
- Buoyancy force change on a submerged displacer (Correct answer)
- Capacitance change between sensing rod and vessel wall
- Radar reflection time from liquid surface
Correct answer: Buoyancy force change on a submerged displacer
A displacer level transmitter senses the change in buoyancy force as liquid level rises or falls around the suspended displacer, converting force change to a level signal.
CCST Instrumentation & Process Control 3
A cascade control system consists of two controllers.
Which controller's output directly manipulates the final control element?