Troubleshooting & Maintenance of Instruments Flashcards

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:

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.

During inspection of a ultrasonic level transmitter, the technician finds heavy foam on the liquid surface. What effect will foam have on the measurement?

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.

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?

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.

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:

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.

A calibration procedure requires a 5-point upscale and downscale traverse. What instrument characteristic is this procedure specifically designed to detect?

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.

After calibrating a digital pressure transmitter and writing the new configuration, the technician should perform which step before returning the transmitter to service?

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.

A thermowell is found to be significantly corroded and thinned during an annual inspection. The primary safety concern with continuing its use is:

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.