CCT Troubleshooting & Maintenance of Instruments 5 — Questions and Answers
Question 1: A strain-gauge based load cell outputs a signal that drifts significantly with ambient temperature changes even after zero balancing. The root cause is most likely:
- Loose mechanical coupling to the structure
- Poor temperature compensation of the Wheatstone bridge resistors (Correct answer)
- Incorrect excitation voltage
- A broken solder joint in one arm of the bridge
Correct answer: Poor temperature compensation of the Wheatstone bridge resistors
Temperature-induced drift in a load cell results from mismatched temperature coefficients in the bridge resistors, causing the bridge to go out of balance as temperature changes.
Question 2: During inspection of a ultrasonic level transmitter, the technician finds heavy foam on the liquid surface. What effect will foam have on the measurement?
- No effect, as ultrasonic waves pass through foam
- The transmitter may read a level higher than actual liquid surface
- The transmitter may read a level lower than actual liquid surface or lose the echo (Correct answer)
- Foam increases signal strength, improving accuracy
Correct answer: The transmitter may read a level lower than actual liquid surface or lose the echo
Foam absorbs and scatters ultrasonic pulses, potentially causing the sensor to lose echo or detect the foam surface instead of the true liquid level.
Question 3: A pneumatic transmitter output is pegged at 15 psi (maximum) regardless of process input. After verifying supply air at 20 psi, what should the technician inspect next?
- The zero adjustment screw
- The flapper-nozzle assembly for foreign material or damage (Correct answer)
- The bourdon tube for cracks
- The process isolation valve for proper opening
Correct answer: The flapper-nozzle assembly for foreign material or damage
In a pneumatic transmitter, a blocked or damaged flapper-nozzle can cause the output to peg at maximum because the nozzle back-pressure cannot bleed down properly.
Question 4: A technician is troubleshooting a grounding problem on an analog instrument loop that exhibits 60 Hz noise on the signal. The most effective corrective action is:
- Add a ferrite bead to the signal wire
- Eliminate multiple ground points and ground the shield at one end only (Correct answer)
- Replace the transmitter with a HART digital instrument
- Increase the loop resistance to filter the noise
Correct answer: Eliminate multiple ground points and ground the shield at one end only
Ground loops caused by multiple shield grounding points create circulating currents at power line frequency — grounding the shield at one point only eliminates this loop.
Question 5: A calibration procedure requires a 5-point upscale and downscale traverse. What instrument characteristic is this procedure specifically designed to detect?
- Repeatability
- Hysteresis (Correct answer)
- Resolution
- Sensitivity
Correct answer: Hysteresis
Comparing upscale readings to downscale readings at the same applied stimuli reveals hysteresis, which is the difference in output depending on the direction of approach.
Question 6: After calibrating a digital pressure transmitter and writing the new configuration, the technician should perform which step before returning the transmitter to service?
- Install new gaskets on all process connections
- Loop the output and verify the full-scale mA signal with a reference meter (Correct answer)
- Restart the associated PLC controller
- Replace the LCD display module
Correct answer: Loop the output and verify the full-scale mA signal with a reference meter
After configuration changes, looping the output end-to-end and verifying the live 4-20 mA signal with a reference meter confirms the entire measurement chain is working correctly.
Question 7: A thermowell is found to be significantly corroded and thinned during an annual inspection. The primary safety concern with continuing its use is:
- Increased thermal lag causing measurement error
- Mechanical failure under process pressure allowing process fluid release (Correct answer)
- Thermocouple contamination from the corroded material
- Vibration-induced resonance at normal flow velocities
Correct answer: Mechanical failure under process pressure allowing process fluid release
A thinned thermowell wall has reduced pressure-bearing capacity and risks catastrophic mechanical failure, which could release hazardous pressurized process fluid.
CCT Troubleshooting & Maintenance of Instruments 5
A strain-gauge based load cell outputs a signal that drifts significantly with ambient temperature changes even after zero balancing.
The root cause is most likely: