Troubleshooting & Maintenance of Instruments Flashcards

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?

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.

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:

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.

A pneumatic temperature controller in a furnace is hunting continuously. After verifying the sensor and process, what is the most appropriate corrective action?

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.

A Coriolis mass flowmeter is installed on a gas application but was previously calibrated on water. How will this affect the reading?

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.

A technician finds that a RTD measures correctly at 0°C ice point but reads 2°C low at 100°C. This best describes:

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.

Which diagnostic step is most important before disassembling a failed pressure transmitter that was used on a chemical process line?

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.

An annubar (averaging pitot tube) flowmeter shows a reading 15% lower than expected. Process conditions are verified correct. The most likely cause is:

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.