CCT Troubleshooting & Maintenance of Instruments 2 — Questions and Answers
Question 1: A pressure transmitter consistently reads 2% higher than the reference standard across its full range. This is most likely caused by:
- Span error
- Zero offset error (Correct answer)
- Nonlinearity error
- Hysteresis error
Correct answer: Zero offset error
A consistent offset across the full range indicates a zero shift, not a span or linearity problem.
Question 2: During maintenance of a thermocouple installation, you notice the extension wire has been routed near a high-voltage cable. What is the primary concern?
- The extension wire will overheat and fail
- Electromagnetic interference will corrupt the millivolt signal (Correct answer)
- The thermocouple junction will degrade faster
- Oxidation of the extension wire will increase resistance
Correct answer: Electromagnetic interference will corrupt the millivolt signal
Thermocouple signals are low-level millivolt signals highly susceptible to EMI from nearby high-voltage conductors.
Question 3: A differential pressure flow meter shows erratic readings only when the process flow is high. The most probable cause is:
- Clogged impulse lines
- Air entrainment at high flow velocities (Correct answer)
- Incorrect damping setting
- Loose wiring in the transmitter
Correct answer: Air entrainment at high flow velocities
At high flow velocities, dissolved gases can come out of solution and cause bubble-induced signal noise in the impulse lines.
Question 4: When troubleshooting a 4-20 mA loop, a technician measures 0 mA with the loop powered. The most likely fault is:
- Transmitter span set too high
- Open circuit in the loop wiring (Correct answer)
- Shorted 250-ohm burden resistor
- Transmitter zero set too low
Correct answer: Open circuit in the loop wiring
Zero milliamps in a 4-20 mA loop indicates a broken circuit, since a functioning loop always carries at least 4 mA.
Question 5: A pH electrode exhibits sluggish response during calibration, taking several minutes to stabilize. The most likely cause is:
- Incorrect buffer solution temperature
- Dehydrated or fouled glass membrane (Correct answer)
- Wrong reference electrode type
- Insufficient electrolyte in the reference junction
Correct answer: Dehydrated or fouled glass membrane
A glass pH electrode that has dried out or is coated with process deposits will respond very slowly to changes in hydrogen ion activity.
Question 6: A technician performs a five-point calibration on a level transmitter and finds that the as-found data shows a bow-shaped curve deviating from the ideal line. This indicates:
- Zero error
- Span error
- Nonlinearity (Correct answer)
- Hysteresis
Correct answer: Nonlinearity
A bow or S-shaped deviation from the ideal straight line across multiple calibration points indicates a nonlinearity error.
Question 7: After replacing a control valve positioner, the valve overshoots its setpoint repeatedly. The technician should first adjust:
- The valve packing gland tension
- The positioner gain or proportional band (Correct answer)
- The supply air pressure regulator
- The feedback linkage length
Correct answer: The positioner gain or proportional band
Overshooting in a positioner is a control response issue, and adjusting the proportional gain directly controls how aggressively the positioner responds.
CCT Troubleshooting & Maintenance of Instruments 2
A pressure transmitter consistently reads 2% higher than the reference standard across its full range.
This is most likely caused by: