CCT Troubleshooting & Maintenance of Instruments 4 — Questions and Answers
Question 1: A technician is troubleshooting a level transmitter on a condensable vapor service that has been reading high. The impulse lines are not heat-traced. The most likely cause is:
- Transmitter drift over time
- Condensate accumulation in the low-pressure impulse line (Correct answer)
- Process density change
- Plugged high-pressure impulse line
Correct answer: Condensate accumulation in the low-pressure impulse line
On condensable vapor service, liquid can accumulate in the low-pressure leg, adding a false head pressure that makes the transmitter read higher than actual level.
Question 2: During a loop check, the control room reads 25% on a 4-20 mA transmitter while a calibrator at the field device reads 8 mA. What is indicated?
- The transmitter is functioning correctly
- There is a resistance fault between the field device and control room
- The control room input card is malfunctioning (Correct answer)
- The calibrator is reading incorrectly
Correct answer: The control room input card is malfunctioning
8 mA corresponds to 25% of the 4-20 mA span, so both readings are consistent and indicate normal transmitter operation.
Question 3: A flow control loop is stable but the process variable consistently lags the setpoint changes by several minutes. What tuning parameter most directly addresses this?
- Increase derivative time
- Increase proportional gain
- Increase integral (reset) rate (Correct answer)
- Decrease the controller output range
Correct answer: Increase integral (reset) rate
Increasing the integral rate causes the controller to eliminate offset more quickly, reducing the time it takes for the PV to reach setpoint after a change.
Question 4: A technician notices a digital HART transmitter shows a good primary variable but has a fixed output of 4 mA regardless of process changes. The most likely cause is:
- Loss of process fluid
- The transmitter is in multidrop mode (Correct answer)
- A failed analog output circuit
- The HART communicator drained the signal
Correct answer: The transmitter is in multidrop mode
HART multidrop mode fixes the analog output at 4 mA and communicates process data digitally only, which can be mistaken for a failed analog output.
Question 5: When performing maintenance on a turbine flowmeter, inspection reveals worn or chipped rotor blades. What is the expected effect on meter performance?
- The meter will read high at low flow rates
- Pulse output frequency will decrease, causing the meter to underread (Correct answer)
- Meter accuracy will improve due to reduced turbulence
- The meter will fail to produce any output signal
Correct answer: Pulse output frequency will decrease, causing the meter to underread
Damaged turbine blades reduce the rotor's rotational efficiency, causing fewer pulses per unit volume and an under-reading of actual flow.
Question 6: A technician is calibrating a pressure switch and finds the switch actuates 5 psi higher on increasing pressure than it releases on decreasing pressure. This difference is called:
- Span error
- Deadband or differential (Correct answer)
- Repeatability error
- Hysteresis in the sensing element only
Correct answer: Deadband or differential
The difference between the actuation point on increasing pressure and the release point on decreasing pressure is the switch deadband or differential.
Question 7: What is the primary reason for performing a 'as-found' calibration check before making any adjustments to an instrument?
- To verify the reference standard is within tolerance
- To document the instrument's condition before intervention and support traceability (Correct answer)
- To determine if the instrument needs to be replaced
- To satisfy the scheduled maintenance interval
Correct answer: To document the instrument's condition before intervention and support traceability
As-found data documents the instrument's pre-adjustment condition for traceability, quality records, and assessing whether the instrument has drifted outside tolerance.
CCT Troubleshooting & Maintenance of Instruments 4
A technician is troubleshooting a level transmitter on a condensable vapor service that has been reading high.
The impulse lines are not heat-traced.
The most likely cause is: