CCT Troubleshooting & Maintenance of Instruments 3 — Questions and Answers
Question 1: A vortex flowmeter produces no output signal when flow is present and verified by an independent method. After checking power and wiring, what should be inspected next?
- The pipe upstream straightening vanes
- The low flow cutoff setting (Correct answer)
- The pipe schedule and wall thickness
- The grounding of the signal cable shield
Correct answer: The low flow cutoff setting
Vortex meters have a configurable low flow cutoff below which output is suppressed to zero, and this setting may be masking real flow.
Question 2: During preventive maintenance on a magnetic flowmeter, a technician finds the electrode resistance to ground is significantly lower than the manufacturer's specification. This likely indicates:
- A failed transmitter input card
- Coating or buildup on the electrodes
- Liner damage allowing process fluid contact with the metal body (Correct answer)
- Incorrect process fluid conductivity
Correct answer: Liner damage allowing process fluid contact with the metal body
Abnormally low electrode-to-ground resistance suggests process fluid is electrically connecting the electrode to the grounded meter body through a liner crack.
Question 3: A pneumatic temperature controller in a furnace is hunting continuously. After verifying the sensor and process, what is the most appropriate corrective action?
- Increase the integral (reset) rate
- Reduce the proportional band (increase gain)
- Increase the derivative (rate) time
- Reduce the integral (reset) rate (Correct answer)
Correct answer: Reduce the integral (reset) rate
Continuous hunting (cycling) in a pneumatic controller is most often caused by excessive integral action that keeps driving the output past the required value.
Question 4: A Coriolis mass flowmeter is installed on a gas application but was previously calibrated on water. How will this affect the reading?
- The meter will over-range and go offline
- The density measurement will be accurate but mass flow inaccurate
- The mass flow reading will still be accurate because Coriolis is density-independent (Correct answer)
- Both mass flow and density readings may be inaccurate without recalibration
Correct answer: The mass flow reading will still be accurate because Coriolis is density-independent
Coriolis meters measure mass flow directly by sensing the Coriolis force, which is independent of fluid density, so mass flow accuracy is unaffected by fluid change.
Question 5: A technician finds that a RTD measures correctly at 0°C ice point but reads 2°C low at 100°C. This best describes:
- A zero offset error
- A positive span error
- A negative span error (Correct answer)
- A hysteresis error
Correct answer: A negative span error
When a sensor is accurate at zero but reads low at the top of range, the span (sensitivity) is too low, indicating a negative span error.
Question 6: Which diagnostic step is most important before disassembling a failed pressure transmitter that was used on a chemical process line?
- Documenting the last calibration date
- Verifying the replacement transmitter is in stock
- Reviewing the Safety Data Sheet for the process fluid (Correct answer)
- Checking the transmitter for damage during removal
Correct answer: Reviewing the Safety Data Sheet for the process fluid
Before handling any process instrument that contacted chemicals, the SDS must be reviewed to identify exposure hazards and required personal protective equipment.
Question 7: An annubar (averaging pitot tube) flowmeter shows a reading 15% lower than expected. Process conditions are verified correct. The most likely cause is:
- Incorrect K-factor programmed in the transmitter
- High-pressure ports facing downstream
- Plugged sensing ports from process debris (Correct answer)
- Incorrect pipe inner diameter entered in the configuration
Correct answer: Plugged sensing ports from process debris
Partially blocked sensing ports reduce the measured differential pressure, causing a proportionally lower flow reading than the actual flow.
CCT Troubleshooting & Maintenance of Instruments 3
A vortex flowmeter produces no output signal when flow is present and verified by an independent method.
After checking power and wiring, what should be inspected next?