CPT Instrumentation & Equipment Maintenance 2 — Questions and Answers
Question 1: What is the primary purpose of a thermocouple in process instrumentation?
- Measure flow rate
- Measure temperature by generating a voltage (Correct answer)
- Control valve position
- Measure pressure differential
Correct answer: Measure temperature by generating a voltage
A thermocouple generates a small voltage proportional to the temperature difference between its hot and cold junctions, which is used to infer temperature.
Question 2: A control valve fails to respond to the positioner signal. Which component should be checked first?
- The downstream piping
- The instrument air supply pressure (Correct answer)
- The process fluid temperature
- The upstream block valve
Correct answer: The instrument air supply pressure
Insufficient instrument air supply pressure is the most common reason a pneumatic control valve fails to respond to its positioner signal.
Question 3: What does the term 'dead band' refer to in a control instrument?
- The frequency range where the signal is lost
- The range of input change that produces no output change (Correct answer)
- The time delay between setpoint and measurement
- The minimum pressure required to open a valve
Correct answer: The range of input change that produces no output change
Dead band is the range over which an input signal can change without causing a corresponding change in output, and it causes hysteresis in the control loop.
Question 4: Which maintenance strategy involves replacing components based on their predicted service life before failure occurs?
- Corrective maintenance
- Reactive maintenance
- Preventive maintenance (Correct answer)
- Emergency maintenance
Correct answer: Preventive maintenance
Preventive maintenance involves scheduled replacement or servicing of equipment at predetermined intervals to prevent unexpected failures.
Question 5: A pressure transmitter reads higher than expected. After zeroing, the reading is still off-scale. What is the most likely cause?
- Plugged impulse line (Correct answer)
- Incorrect range calibration
- Low power supply voltage
- Loose terminal connections
Correct answer: Plugged impulse line
A plugged impulse line traps pressure and prevents the transmitter from sensing actual process pressure, causing an erroneously high or stuck reading.
Question 6: What is the function of a seal pot in an impulse line system?
- Prevent backflow into the transmitter
- Maintain equal liquid head on both impulse lines for accurate DP measurement (Correct answer)
- Store instrument air
- Filter particulates from the process fluid
Correct answer: Maintain equal liquid head on both impulse lines for accurate DP measurement
Seal pots maintain equal and constant liquid levels in both high- and low-side impulse lines, ensuring the hydrostatic head cancels out for accurate differential pressure measurement.
Question 7: When calibrating a 4–20 mA transmitter, what output signal corresponds to 0% of the measurement range?
- 0 mA
- 2 mA
- 4 mA (Correct answer)
- 10 mA
Correct answer: 4 mA
In a 4–20 mA loop, 4 mA represents 0% of the measurement span, and 20 mA represents 100%, with the 4 mA offset used to detect open-circuit faults.
CPT Instrumentation & Equipment Maintenance 2
What is the primary purpose of a thermocouple in process instrumentation?