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ISA CCST Level III study materials
ISA CCST Level III study resources

ISA CCST Level III practice questions

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Question 1Calibration, Maintenance, Repair, and Troubleshooting

A transmitter calibrated for a 4–20 mA output reads 12.0 mA. What percentage of calibrated span does this output represent?

  1. 50% Correct answer.
  2. 40%
  3. 75%
  4. 25%

Explanation

The calibrated span is 16 mA, from 4 mA at 0% to 20 mA at 100%. An output of 12 mA is 8 mA above the live zero, so the span fraction is 8/16 = 0.50, or 50%. The value is not a simple ratio of 12/20 because the 4 mA live zero must be subtracted first.

Question 2Calibration, Maintenance, Repair, and Troubleshooting

Which calibration record provides the strongest evidence that a field instrument was compared with a known reference?

  1. An operator's verbal statement confirming that the loop was checked and looked acceptable
  2. A signed note that the instrument was adjusted until the plant DCS reading looked normal
  3. A traceable calibrator certificate from a national metrology institute Correct answer.
  4. A photograph showing the calibrator connected to the transmitter and its test leads

Explanation

Traceability is established by an unbroken, documented chain of calibrations from the working reference to a national or international standard. A traceable calibrator certificate identifies that chain and supports the validity of the comparison. Adjustment notes, photos, and verbal statements do not quantify the reference used.

Question 3Calibration, Maintenance, Repair, and Troubleshooting

A two-wire 4–20 mA transmitter shows no output, and the technician suspects that loop power is absent. Which field measurement most directly verifies that the transmitter is receiving loop power?

  1. Resistance from the transmitter housing to earth
  2. DC voltage across the transmitter loop terminals Correct answer.
  3. Current at the DCS input card with the loop disconnected
  4. A 20 mA injected signal at the DCS input

Explanation

A two-wire transmitter derives its operating power from the DC voltage impressed across its loop terminals. Measuring that terminal voltage under loop conditions confirms whether supply and wiring are delivering power to the device. The other checks address grounding, DCS input calibration, or current with the loop broken, not transmitter supply voltage.

Question 4Calibration, Maintenance, Repair, and Troubleshooting

A pressure transmitter calibrated for 0–100 psig reads 0 psig at true zero, 48 psig at 50 psig, and 96 psig at 100 psig. Which calibration adjustment is indicated?

  1. Replace the transmitter
  2. Increase damping
  3. Adjust span only Correct answer.
  4. Adjust zero only

Explanation

The zero reading is correct, but the output is low by a proportional amount across the range: 48 instead of 50 and 96 instead of 100. That pattern is a span or gain error, so the span adjustment is indicated while the zero remains correct. Zero adjustment would shift the entire curve rather than correct the slope.

Question 5Calibration, Maintenance, Repair, and Troubleshooting

A three-wire RTD transmitter reads several degrees high, and the error changes when the lead wires are moved. Which maintenance action is most appropriate?

  1. Increase the transmitter damping to smooth the reading and hide the variation
  2. Replace the RTD with a thermocouple and change the transmitter input type
  3. Check RTD lead resistance matching and the compensating-lead connection Correct answer.
  4. Short the RTD input terminals to force a zero reading and verify the output

Explanation

A three-wire RTD uses the third lead to compensate for the resistance of the connecting wires. If the lead resistances are mismatched or the compensating lead is loose, the measured resistance changes and produces a variable temperature error. Checking lead resistance and terminations addresses that cause directly.

Question 6Calibration, Maintenance, Repair, and Troubleshooting

During a control valve stroke test, the valve reaches 0% and 100% but hesitates near mid-travel while the positioner output changes smoothly. Which item should be inspected first?

  1. Valve stem stiction or binding in mid-travel Correct answer.
  2. Instrument air dew point and dryer performance
  3. Controller tuning constants and derivative settings
  4. DCS trend resolution and scan rate

Explanation

A smooth positioner output with a mechanical hesitation at mid-travel points to local valve binding, such as stem stiction, packing friction, or linkage interference. The controller and DCS are upstream of the positioner command and would not explain a localized mechanical delay. Air quality problems usually affect the actuator more broadly rather than one mid-travel region.

Question 7Calibration, Maintenance, Repair, and Troubleshooting

A 4–20 mA signal is stable on the bench but shows a fluctuating offset after installation with the cable shield grounded at both ends. What is the most likely cause?

  1. A ground loop is injecting current into the signal circuit Correct answer.
  2. The loop load resistor is too small for the transmitter
  3. The transmitter span is set too wide for the installed flow range
  4. The DCS input has excessive damping and a slow filter setting

Explanation

Grounding the shield at both ends creates a parallel path between points that may be at different earth potentials. The resulting ground-loop current can induce noise or a fluctuating offset in the low-level signal circuit. Removing one shield ground or using a single-point grounding scheme is the usual corrective action.

Question 8Calibration, Maintenance, Repair, and Troubleshooting

A differential-pressure flow transmitter spans 0–100 inH2O and the control system applies square-root extraction. If the indicated flow is 25% of span, what differential pressure is being measured?

  1. 12.5 inH2O
  2. 25 inH2O
  3. 6.25 inH2O Correct answer.
  4. 50 inH2O

Explanation

For a square-root flow measurement, flow percentage equals the square root of differential-pressure percentage. Therefore the differential pressure is 25% squared, or 6.25% of span, which is 6.25 inH2O on a 100 inH2O span. The square-root relationship must be applied before converting percentage to engineering units.

Question 9Calibration, Maintenance, Repair, and Troubleshooting

A fast flow control loop begins to oscillate continuously after the proportional gain is increased, while the integral time is left unchanged. Which tuning change is most likely to restore stable control without making the loop excessively slow?

  1. Increase controller gain further and decrease integral time
  2. Decrease controller gain and increase integral time slightly Correct answer.
  3. Increase the valve stroke time and leave the controller tuning unchanged
  4. Place the controller in manual and leave it there

Explanation

Excessive proportional gain is a common cause of sustained oscillation in a fast flow loop. Reducing gain lowers the loop's aggressive response, and slightly increasing integral time reduces reset action enough to damp cycling without making the loop very slow. The other choices either worsen the oscillation or abandon automatic control.

Question 10Calibration, Maintenance, Repair, and Troubleshooting

A transmitter with a ±0.1% of span accuracy limit shows an as-found error of 0.14% of span. The reference used for the check has an uncertainty of ±0.05% of span. Which conclusion is most defensible?

  1. The transmitter must be adjusted until its residual error is no larger than the reference uncertainty
  2. Inconclusive unless measurement uncertainty is included near the tolerance limit Correct answer.
  3. The transmitter definitely fails because 0.14% is greater than 0.1%
  4. The transmitter definitely passes because the reference uncertainty is smaller than the tolerance

Explanation

The observed error is above the nominal tolerance, but the reference uncertainty is large enough that the true error could fall on either side of the limit. A defensible pass/fail decision must apply a documented decision rule that combines the tolerance with measurement uncertainty. Declaring a definite pass or fail from the raw comparison alone overstates what the check proves.

Question 11Calibration, Maintenance, Repair, and Troubleshooting

A technician connects a HART communicator across the transmitter terminals of a two-wire 4–20 mA loop but cannot establish digital communication. The loop is powered by a 24 VDC supply and the transmitter is operating. Which condition most commonly prevents HART communication in this configuration?

  1. The transmitter output is below 4 mA.
  2. The communicator is connected in series with the loop.
  3. Loop resistance is below the minimum required for HART signaling. Correct answer.
  4. The loop power supply is isolated from earth ground.

Explanation

HART superimposes a frequency-shift keyed digital signal on the loop current and needs sufficient loop impedance, typically at least 250 ohms, to develop a detectable voltage. Below that minimum, the digital signal is too small for the communicator to decode reliably.

Question 12Calibration, Maintenance, Repair, and Troubleshooting

A pressure switch is calibrated to trip at 50 psig rising pressure and reset at 45 psig falling pressure. During calibration, the technician notes the switch trips at 50 psig but resets at 42 psig. Which adjustment or condition explains the wider reset differential?

  1. The supply pressure is fluctuating during the test.
  2. The switch is wired normally closed instead of normally open.
  3. The reset adjustment is set too low, widening the deadband. Correct answer.
  4. The trip adjustment is set too high, raising the setpoint.

Explanation

Deadband is the difference between trip and reset pressure; a reset at 42 psig instead of 45 psig makes the differential 8 psi rather than 5 psi. The trip point is correct, so the reset adjustment is set too low.

Question 13Calibration, Maintenance, Repair, and Troubleshooting

A valve positioner is intermittently sticking and the maintenance schedule calls for checking the instrument air supply. Which maintenance action is most appropriate for the filter-regulator assembly on the air line to the positioner?

  1. Lubricate the positioner with oil through the air line.
  2. Drain the bowl and replace the filter element on schedule. Correct answer.
  3. Remove the filter element to increase flow.
  4. Increase the regulator setpoint to the maximum supply pressure.

Explanation

The filter-regulator removes moisture and particulate from instrument air; routine draining and element replacement prevent clogging and contamination that can cause a positioner to stick. Maintaining clean, dry air is a basic preventive maintenance step.

Question 14Calibration, Maintenance, Repair, and Troubleshooting

A type K thermocouple transmitter reads about 20°F low when the thermocouple is at ambient temperature. The technician finds that copper extension wire was used from the thermocouple head to the transmitter instead of type K extension wire, and the connection points are at different temperatures. Which error mechanism best explains the low reading?

  1. The copper extension wire has higher resistance, causing a low reading from lead resistance.
  2. The type K thermocouple has a negative temperature coefficient at ambient.
  3. The copper extension creates a parasitic thermocouple junction that defeats cold-junction compensation. Correct answer.
  4. The transmitter is configured for a type J thermocouple instead of type K.

Explanation

Extension wire must match the thermocouple alloy; copper introduces an unwanted junction at the head, so the transmitter’s cold-junction compensation no longer sees the true reference temperature. This produces a predictable offset when the connection points are at different temperatures.

Question 15Calibration, Maintenance, Repair, and Troubleshooting

A differential-pressure level transmitter is installed on a closed tank with a wet leg on the high side. With the tank empty and the wet leg filled with condensate, the transmitter reads 20 inH2O. The transmitter is calibrated for a 0–100 inH2O span and should output 4 mA at empty tank. What calibration adjustment is required?

  1. Apply zero suppression of +20 inH2O so the empty tank reads 4 mA. Correct answer.
  2. Apply zero elevation of –20 inH2O so the empty tank reads 4 mA.
  3. Reverse the high and low impulse lines to remove the offset.
  4. Increase the span to 120 inH2O so the empty tank reads 4 mA.

Explanation

The wet leg adds a constant +20 inH2O pressure on the high side at empty tank, so the lower range value must be raised by 20 inH2O to make that condition read 4 mA. This is zero suppression, not span change.

Question 16Calibration, Maintenance, Repair, and Troubleshooting

An I/P transducer is calibrated for a 4–20 mA input and 3–15 psig output. At 12 mA input, the output pressure is 8.0 psig instead of 9.0 psig, and the output does not change when the nozzle is cleaned. Which adjustment should be checked first?

  1. Replace the I/P transducer immediately.
  2. Adjust the input current meter calibration.
  3. Increase the air supply to 30 psig.
  4. Check and adjust the nozzle-flapper gap or zero/span linkage. Correct answer.

Explanation

A clean nozzle with low midscale output points to flapper alignment or feedback linkage as the likely cause; zero and span calibration should be checked before replacing parts. The nozzle-flapper mechanism directly sets the pressure output for a given input current.

Question 17Calibration, Maintenance, Repair, and Troubleshooting

A newly installed Coriolis mass flowmeter shows a nonzero mass flow reading when the process line is completely filled and isolated with zero flow. The technician has confirmed the meter is full and the isolation valves are closed. Which step is required before performing the zero calibration?

  1. Change the meter's K-factor to zero.
  2. Increase the drive gain to maximum.
  3. Open the bypass valve to create flow through the meter.
  4. Confirm the meter is full, isolated, and at stable operating temperature and pressure. Correct answer.

Explanation

Coriolis zero calibration requires no flow, a full pipe, and stable temperature and pressure; residual flow or thermal gradients can create a false zero. The meter must be allowed to reach equilibrium before the zero routine is run.

Question 18Calibration, Maintenance, Repair, and Troubleshooting

A pH analyzer is calibrated with pH 4.00 and pH 7.00 buffers. After calibration, the analyzer reads pH 7.00 correctly but reads pH 4.30 in the pH 4.00 buffer. Which calibration parameter is most likely out of specification?

  1. The temperature compensation is set to 25°C.
  2. The electrode slope is too low. Correct answer.
  3. The pH 4.00 buffer is expired.
  4. The reference electrode is poisoned.

Explanation

Slope is determined by the span between two buffers; a correct reading at pH 7.00 but a compressed reading at pH 4.00 means the electrode span is too small. The analyzer should be cleaned and recalibrated, and the electrode slope checked against specifications.

Question 19Calibration, Maintenance, Repair, and Troubleshooting

A 4–20 mA pressure transmitter signal becomes noisy only when a nearby variable-frequency drive is running. The transmitter is two-wire, loop-powered, and the signal cable shield is grounded at both the transmitter and the control cabinet. Which corrective action is most likely to reduce the noise?

  1. Disconnect the shield completely.
  2. Ground the shield at one end only and use a twisted pair. Correct answer.
  3. Increase the transmitter damping to maximum.
  4. Replace the transmitter with a four-wire unit.

Explanation

A shield grounded at both ends forms a ground loop that can pick up VFD common-mode currents; single-point grounding plus a twisted pair reduces magnetic pickup. The shield should normally be grounded at one end only to avoid circulating currents.

Question 20Calibration, Maintenance, Repair, and Troubleshooting

A control valve with a smart positioner hunts continuously around the setpoint at steady process conditions after a technician performed an auto-tune with default settings. The stem moves freely with air disconnected, the air supply is stable, and the position feedback is repeatable. Which adjustment is most appropriate?

  1. Reverse the valve action in the positioner.
  2. Replace the position feedback sensor.
  3. Increase the air supply pressure to the positioner.
  4. Reduce the positioner proportional gain or increase the deadband. Correct answer.

Explanation

Hunting that begins after an auto-tune with default settings indicates excessive positioner loop gain; lowering gain or adding deadband stabilizes the valve. Mechanical friction and feedback repeatability have already been ruled out.

Question 21Calibration, Maintenance, Repair, and Troubleshooting

A differential-pressure transmitter is isolated and its high and low sides are equalized, but the output reads 2.5 inH2O instead of 0 inH2O. What is the appropriate first corrective action?

  1. Perform a zero trim on the transmitter. Correct answer.
  2. Increase the loop supply voltage.
  3. Replace the transmitter sensor module.
  4. Adjust the span so that 100% reads correctly.

Explanation

With the high and low sides equalized, the differential pressure is truly zero. A nonzero indication under that condition is a zero offset, so a zero trim is the correct first correction; span adjustment would not remove an offset at the zero point.

Question 22Calibration, Maintenance, Repair, and Troubleshooting

A type K thermocouple is replaced with a type J thermocouple, but the transmitter configuration is left as type K. What effect will this have on the indicated temperature?

  1. The reading will be in error because the transmitter uses the type K table and cold-junction compensation. Correct answer.
  2. The indication will be accurate because both are base-metal thermocouples.
  3. The transmitter will automatically identify the new thermocouple type and correct the reading.
  4. The reading will be low because type J produces less voltage than type K at all temperatures.

Explanation

Thermocouple type determines the mV-versus-temperature relationship and the reference table used for linearization. Leaving the transmitter configured for type K while a type J junction is installed produces a temperature error even if the cold-junction measurement itself is correct.

Question 23Calibration, Maintenance, Repair, and Troubleshooting

A technician must verify a panel-mounted 4–20 mA indicator that is not used for control. Which method applies a known input to the indicator with the least risk to the live loop?

  1. Connect a voltage source directly across the loop terminals without a current-limiting resistor.
  2. Isolate the indicator and use a loop calibrator to drive it with a known mA signal. Correct answer.
  3. Connect a loop calibrator in parallel with the indicator to drive it with a simulated current.
  4. Short the indicator terminals and adjust its zero potentiometer.

Explanation

A loop calibrator is a precision current source used to drive the indicator through its full input range. The indicator must be isolated from the live loop and fed a known mA signal so the indicated value can be compared with the applied value.

Question 24Calibration, Maintenance, Repair, and Troubleshooting

A control valve is specified as air-to-open and fail-closed. During a loss-of-instrument-air test, the valve moves to about 40% open and stays there. Which condition is the most likely cause?

  1. The positioner is configured for direct action instead of reverse action.
  2. The valve packing is too tight.
  3. The I/P transducer nozzle is clogged.
  4. The actuator spring is broken or disconnected. Correct answer.

Explanation

An air-to-open actuator depends on the spring to close the valve when air pressure is lost. If the spring is broken or disconnected, the valve can stop at an intermediate position instead of reaching its fail-closed position.

Question 25Calibration, Maintenance, Repair, and Troubleshooting

A pH analyzer is calibrated with pH 4.00 and pH 7.00 buffers. After calibration, it reads pH 7.00 correctly but reads pH 4.15 in the pH 4.00 buffer. Which adjustment is most likely needed?

  1. Adjust the temperature compensation to the buffer temperature.
  2. Replace the reference electrode.
  3. Recalculate the slope using fresh pH 4.00 and pH 7.00 buffers. Correct answer.
  4. Increase the sample flow through the analyzer.

Explanation

A pH calibration uses the pH 7 buffer primarily for offset and a second buffer for slope. If pH 7 is correct but pH 4 is off, the slope is wrong, so the analyzer should be recalibrated with fresh buffers.

Question 26Calibration, Maintenance, Repair, and Troubleshooting

A magnetic flowmeter installed on a full pipe with flow present indicates 0% flow. The transmitter has power, the coil circuit is energized, and the empty-pipe detector is not active. Which cause is most likely?

  1. The electrode cable shield is grounded at both ends.
  2. The flow tube is installed in a vertical upward-flow line.
  3. The process liquid conductivity is below the meter's minimum requirement. Correct answer.
  4. The transmitter damping is set too high.

Explanation

A magnetic flowmeter needs a conductive liquid to develop an electrode signal. If conductivity is below the meter's minimum, the transmitter can show zero flow even though the pipe is full and the coils are energized.

Question 27Calibration, Maintenance, Repair, and Troubleshooting

A two-wire 4–20 mA transmitter is powered by a 24 VDC supply. The loop includes a 250 Ω sense resistor and 600 m of cable with a loop resistance of 3.5 Ω per 100 m. The transmitter needs at least 12 V at its terminals. What is the maximum additional series resistance that can be added at 20 mA?

  1. about 120 Ω
  2. about 440 Ω
  3. about 220 Ω
  4. about 330 Ω Correct answer.

Explanation

The cable resistance is 600 m × 3.5 Ω/100 m = 21 Ω. At 20 mA, the 250 Ω resistor and cable drop 5.42 V, leaving 6.58 V above the 12 V minimum; 6.58 V / 0.020 A gives about 330 Ω.

Question 28Calibration, Maintenance, Repair, and Troubleshooting

A pressure transmitter is calibrated annually and has a tolerance of ±0.25% of span. The last four as-found errors at 50% span were +0.10%, +0.15%, +0.20%, and +0.24% of span. Which action is most technically defensible?

  1. Replace the transmitter because any upward drift indicates imminent failure.
  2. Shorten the interval because the drift trend will likely exceed tolerance before the next calibration. Correct answer.
  3. Lengthen the interval because the errors are small compared with span.
  4. Keep the 12-month interval because the latest error is still within tolerance.

Explanation

The as-found errors are increasing by roughly 0.04% to 0.05% of span per year. The next annual check is likely to reach or exceed the ±0.25% tolerance, so shortening the interval provides margin before the transmitter becomes out of tolerance.

Question 29Calibration, Maintenance, Repair, and Troubleshooting

A pressure transmitter has a local indication of 50.0 psig, but the DCS shows 44.0 psig. The transmitter range is 0–100 psig and the DCS analog input is configured for 4–20 mA. Which step best isolates whether the error is in the transmitter output or the DCS input?

  1. Replace the DCS analog input card first.
  2. Measure the loop current with a calibrated mA meter and compare it with the expected 12.0 mA. Correct answer.
  3. Adjust the DCS input scaling until the displays agree.
  4. Change the transmitter range to 0–50 psig.

Explanation

The local display and DCS disagree, so the next step is to compare the actual loop current with the expected value. If the current is 12.0 mA, the DCS input or scaling is suspect; if it is not, the transmitter output is suspect.

Question 30Calibration, Maintenance, Repair, and Troubleshooting

A 100 Ω platinum RTD with a nominal sensitivity of 0.385 Ω/°C is wired to a transmitter in a two-wire configuration. Each lead has 1.5 Ω of resistance. At a true temperature of 100°C, what approximate temperature error does the lead resistance cause?

  1. +7.8°C Correct answer.
  2. +1.5°C
  3. +3.9°C
  4. −7.8°C

Explanation

At 100°C the RTD resistance is about 138.5 Ω, and the two leads add 3.0 Ω. The transmitter interprets 141.5 Ω as about 107.8°C, so the lead resistance causes an error of roughly +7.8°C.

Question 31Calibration, Maintenance, Repair, and Troubleshooting

A technician is calibrating a 4–20 mA pressure transmitter with a calibrated range of 0–200 psig. What pressure should the technician apply to produce a 12 mA output?

  1. 200 psig
  2. 150 psig
  3. 100 psig Correct answer.
  4. 50 psig

Explanation

A 12 mA output is 50% of the 4–20 mA span, so the applied pressure should be 50% of the 0–200 psig range, which is 100 psig. The transmitter output is linear with applied pressure over its calibrated range.

Question 32Calibration, Maintenance, Repair, and Troubleshooting

Before calibrating a pressure transmitter that is installed on a live process line, which step must the technician take first?

  1. Disconnect the loop wiring from the DCS.
  2. Isolate and vent the transmitter. Correct answer.
  3. Adjust the transmitter zero potentiometer.
  4. Connect the HART communicator to the loop.

Explanation

The transmitter must be isolated from process pressure and any trapped pressure vented before a calibration source is applied. Otherwise the process pressure will bias the calibration and may create a safety hazard. Zero adjustment and HART communication are performed after safe isolation.

Question 33Calibration, Maintenance, Repair, and Troubleshooting

A technician needs to measure the current in an operating 4–20 mA loop without opening the circuit. Which instrument is appropriate?

  1. A HART communicator in digital trim mode
  2. A clamp-on milliammeter Correct answer.
  3. A multimeter set to measure resistance
  4. A loop calibrator set to source mode

Explanation

A clamp-on milliammeter measures loop current by sensing the magnetic field around the conductor, so the circuit remains closed. A source-mode calibrator injects current rather than measuring it. A multimeter on resistance cannot measure current and a HART communicator in trim mode is used to adjust the transmitter output.

Question 34Calibration, Maintenance, Repair, and Troubleshooting

A technician performs a five-point calibration check on a 4–20 mA transmitter. The as-found outputs are 4.0 mA at 0%, 8.1 mA at 25%, 12.2 mA at 50%, 16.3 mA at 75%, and 20.4 mA at 100%. Which type of error is present?

  1. Span error Correct answer.
  2. Deadband
  3. Hysteresis
  4. Zero shift only

Explanation

The zero output is correct at 4.0 mA, but the error grows from 0.1 mA at 25% to 0.4 mA at 100%. That proportional increase is a gain or span error. A zero shift would offset all points by the same amount, while hysteresis and deadband produce different response patterns.

Question 35Calibration, Maintenance, Repair, and Troubleshooting

A technician uses a HART communicator to perform a digital trim on a transmitter, but the 4–20 mA output does not change. The transmitter is in a live loop and the communicator reads the primary variable correctly. What is the most likely cause?

  1. The loop has an open circuit.
  2. The transmitter has failed its self-test.
  3. Loop current mode is disabled. Correct answer.
  4. The HART communicator is connected across the power supply.

Explanation

Digital trim adjusts the transmitter's digital-to-analog conversion, but if loop current mode is disabled the 4–20 mA output will not respond to trim changes. An open loop would prevent communication and loop current. A failed self-test would normally be annunciated rather than selectively blocking trim.

Question 36Calibration, Maintenance, Repair, and Troubleshooting

A pneumatic valve positioner receives a 3–15 psig signal and is supposed to stroke the valve from 0% to 100%. The technician finds that the valve reaches 50% travel at 12 psig. What is the most likely cause?

  1. The positioner span is incorrectly adjusted. Correct answer.
  2. The actuator spring is broken.
  3. The air supply is too high.
  4. The positioner zero is set too low.

Explanation

With a 3–15 psig input, 50% travel should occur at 9 psig, the midpoint of the signal range. Reaching 50% at 12 psig indicates that the positioner span is compressed or shifted. A zero error would shift the starting point, while a broken spring or incorrect supply pressure would produce different symptoms.

Question 37Calibration, Maintenance, Repair, and Troubleshooting

A differential-pressure flow transmitter is configured for square-root extraction. The technician applies 25% of the DP span and expects a 50% flow output, but the transmitter outputs 25%. What is the most likely cause?

  1. Square-root extraction is disabled. Correct answer.
  2. The loop resistor is the wrong value.
  3. The flow element is installed backwards.
  4. The transmitter has a zero shift.

Explanation

For a DP flow measurement, flow varies with the square root of differential pressure, so 25% DP corresponds to 50% flow. If the output is 25%, the square-root extraction is not being applied and the transmitter is acting linearly. A zero shift would offset the output, and a reversed flow element or wrong resistor would not produce this exact linear relationship.

Question 38Calibration, Maintenance, Repair, and Troubleshooting

A differential-pressure transmitter is installed with a three-valve manifold. With both block valves open and the equalizer valve open, the transmitter should indicate zero DP and output 4 mA. Instead, it outputs 20 mA. Which condition most likely explains this reading?

  1. The low-side block valve is closed. Correct answer.
  2. The equalizer valve is leaking.
  3. The high and low impulse lines are both blocked.
  4. The transmitter zero is shifted high.

Explanation

With the equalizer open, both sides of the DP cell should see the same pressure, so DP should be zero. A 20 mA output indicates a full-scale DP, which can occur if the low side is isolated from the high side by a closed block valve while high pressure is still applied. A zero shift would not normally drive the output to full scale under equalized pressure.

Question 39Calibration, Maintenance, Repair, and Troubleshooting

An orifice-plate flow transmitter is calibrated for 0–100 inH2O differential pressure corresponding to 0–500 gpm. The transmitter uses square-root extraction. If the measured differential pressure is 30 inH2O, what is the indicated flow rate?

  1. About 300 gpm
  2. About 274 gpm Correct answer.
  3. About 387 gpm
  4. About 150 gpm

Explanation

Flow is proportional to the square root of differential pressure, so flow = 500 × √(30/100) = 500 × 0.5477 ≈ 274 gpm. A linear interpretation would give 150 gpm, which is incorrect for a square-root flow measurement. The other choices do not match the square-root relationship.

Question 40Calibration, Maintenance, Repair, and Troubleshooting

A thermocouple transmitter is calibrated accurately with a millivolt source at the transmitter terminals. In service, the indicated temperature is consistently higher than the actual process temperature. The thermocouple is 50 feet from the transmitter, and the technician used copper instrument wire instead of thermocouple extension wire. What is the most likely cause of the error?

  1. The transmitter cold-junction compensation is set to the wrong thermocouple type.
  2. Copper wire adds an unwanted thermocouple junction. Correct answer.
  3. The millivolt source used for calibration was out of tolerance.
  4. The thermocouple is installed with reverse polarity.

Explanation

Thermocouple extension wire is made of alloys matching the thermocouple so that intervening junctions do not generate additional emf. Copper wire creates a copper-to-thermocouple-alloy junction at the transmitter, adding an unwanted voltage that corrupts the reading. Reverse polarity would drive the reading lower, and wrong cold-junction compensation would affect the reading regardless of copper wire.

Question 41Calibration, Maintenance, Repair, and Troubleshooting

A technician is troubleshooting a 4–20 mA analog signal that shows erratic readings whenever a nearby variable-frequency drive is energized. The signal cable is shielded twisted pair. Where should the cable shield be connected?

  1. At the control-system cabinet ground only, with the field end left floating Correct answer.
  2. At both the field transmitter and the control-system cabinet ground
  3. At neither end, because the shield must remain isolated to prevent ground loops
  4. At the field transmitter ground only, with the control-system end left floating

Explanation

Analog signal cable shields are normally grounded at one end only, typically at the control-system or cabinet end, to drain interference without creating a shield-to-shield ground loop. Grounding both ends allows circulating currents to flow in the shield, which can couple noise into the signal pair.

Question 42Calibration, Maintenance, Repair, and Troubleshooting

A four-wire RTD is connected to a temperature transmitter. What is the primary advantage of the four-wire configuration over a two-wire configuration?

  1. It raises the RTD sensitivity by a factor of ten
  2. It removes lead-wire resistance error from the measurement Correct answer.
  3. It allows the RTD to operate without cold-junction compensation
  4. It permits the transmitter to be loop-powered from the same pair

Explanation

In a four-wire RTD circuit, two leads carry the excitation current and the other two are high-impedance sense leads, so the voltage measurement excludes the lead-wire voltage drops. This makes the reading essentially independent of lead resistance and its temperature coefficient.

Question 43Calibration, Maintenance, Repair, and Troubleshooting

Before using a digital pressure calibrator to certify a plant transmitter, a technician checks the calibrator's calibration certificate. Which attribute is most important for the certificate to establish?

  1. The manufacturer's recommended calibration interval
  2. The date the calibrator was last powered on
  3. Traceability to a national standard with stated uncertainty Correct answer.
  4. The serial number of the transmitter under test

Explanation

A calibration certificate supports traceability when it identifies the reference standard, the calibration method, and the uncertainty chain back to a national metrology institute. The power-on date, recommended interval, and the transmitter serial number do not establish that the calibrator's readings are traceable.

Question 44Calibration, Maintenance, Repair, and Troubleshooting

A closed tank uses a differential-pressure transmitter for level measurement. The high side connects to the bottom tap and the low side connects to a wet leg filled with condensate. During commissioning, the wet leg is accidentally drained. How will the transmitter output respond, and what is the correct remedy?

  1. Output cannot be zeroed; replace the transmitter because the diaphragm is damaged
  2. Output drives high; refill the wet leg and re-zero at a known level Correct answer.
  3. Output stays correct because square-root extraction compensates; no action is needed
  4. Output reads low; vent the low side and re-range the transmitter

Explanation

The wet leg applies a constant hydrostatic pressure to the low side that the transmitter subtracts from the high-side pressure. If the wet leg drains, that subtraction is lost, the differential pressure rises, and the indicated level goes high; the leg must be refilled and the transmitter re-zeroed at a known level.

Question 45Calibration, Maintenance, Repair, and Troubleshooting

A spring-return diaphragm actuator on a control valve has a bench set of 6–18 psig. The technician slowly increases air pressure and observes that the stem just begins to move at 6 psig, but at 18 psig the stem has traveled only 80% of full stroke while the actuator still has spring travel remaining. Which condition best explains this?

  1. The bench set was performed with the valve removed from the line
  2. The actuator spring is too stiff for the specified bench set
  3. The positioner supply pressure is set too low to stroke the actuator
  4. The valve packing friction is excessive or the plug is binding in the cage Correct answer.

Explanation

A bench set of 6–18 psig means the actuator should begin stroking at 6 psig and reach full travel at 18 psig. If it starts correctly but stalls before full stroke, additional force is being consumed by friction or binding in the packing, stem, or plug/cage, so the actuator needs service rather than a spring change.

Question 46Calibration, Maintenance, Repair, and Troubleshooting

A type K thermocouple is installed in a furnace, and the transmitter is 30 m away in a control room. The technician must run wiring from the thermocouple head to the transmitter. Which wiring practice is correct?

  1. Use shielded copper wire and connect the shield to the thermocouple sheath at both ends
  2. Use standard copper instrument wire because the transmitter provides cold-junction compensation
  3. Use type J extension wire because it has a higher temperature rating than type K
  4. Use type K thermocouple extension wire with the correct polarity throughout the run Correct answer.

Explanation

Thermocouple extension wire must match the thermocouple type and be connected with correct polarity so that the new junctions formed at the terminals do not introduce an error. Copper wire would create unintended thermocouple junctions at the head and at the transmitter.

Question 47Calibration, Maintenance, Repair, and Troubleshooting

A current-to-pneumatic (I/P) transducer supplies a 3–15 psig signal to a valve positioner. The technician applies 12 mA and measures 9 psig at the I/P output, but the positioner does not move the valve. The I/P output gauge is stable. What is the most likely next step?

  1. Check the positioner for a plugged supply port or stuck spool Correct answer.
  2. Recalibrate the I/P transducer zero and span
  3. Increase the instrument air supply to 30 psig to overcome the positioner
  4. Replace the I/P transducer because 12 mA should produce 12 psig

Explanation

A 12 mA input is 50% of the 4–20 mA range, so the correct I/P output is 9 psig (50% of 3–15 psig). Since the I/P output is correct and stable, the fault is downstream in the positioner or actuator, such as a plugged supply port or stuck spool.

Question 48Calibration, Maintenance, Repair, and Troubleshooting

Several HART transmitters are wired in a multidrop configuration on a single pair. A technician connects a HART communicator but cannot communicate with any transmitter. The loop has 24 VDC and a 250 Ω resistor. Which condition is the most likely cause?

  1. Each transmitter must be assigned a unique polling address from 1 to 15 Correct answer.
  2. The transmitters must be configured for burst mode before polling is possible
  3. Multidrop mode requires a minimum loop resistance of 500 Ω
  4. The communicator must be connected across the power supply, not across the resistor

Explanation

In HART multidrop mode, each transmitter must have a unique polling address from 1 to 15, and the analog 4–20 mA output is typically disabled. If all devices share the default address 0, the communicator cannot isolate a single device and communication fails.

Question 49Calibration, Maintenance, Repair, and Troubleshooting

A two-wire transmitter has a minimum operating voltage of 12 VDC and draws 4–20 mA. It is powered by a 24 VDC supply through a 250 Ω sense resistor and a 250 Ω barrier resistor. The cable loop resistance is 200 Ω. At 20 mA, what is the voltage available at the transmitter terminals, and is the loop compliant?

  1. 14.0 VDC; the loop is compliant because it exceeds the 12 VDC minimum
  2. 10.0 VDC; the loop is not compliant and the transmitter may drop out Correct answer.
  3. 12.0 VDC; the loop is exactly at the minimum and therefore compliant
  4. 16.0 VDC; the loop is compliant with margin to spare

Explanation

The total loop resistance is 250 Ω + 250 Ω + 200 Ω = 700 Ω, and at 20 mA the drop is 0.020 A × 700 Ω = 14 V. With a 24 VDC supply, only 10 V remains at the transmitter, below its 12 VDC minimum, so the loop is not compliant at maximum current.

Question 50Calibration, Maintenance, Repair, and Troubleshooting

A control valve responds sluggishly and the process oscillates in a sustained cycle. A technician performs a valve signature test and finds that the stem does not move until the controller output changes by 4%, after which it jumps. Which diagnosis and corrective action are most appropriate?

  1. Loop tuning that is too aggressive; increase the controller gain and reset time
  2. Insufficient actuator thrust from an undersized diaphragm; replace the actuator with a larger one
  3. Incorrect bench set causing the valve to seat too tightly; reduce the spring preload
  4. Excessive deadband from packing friction or stem stiction; service the packing and stroke the valve Correct answer.

Explanation

A valve that does not move until the controller output changes by 4% and then jumps is exhibiting stick-slip or stiction, often caused by tight packing or a binding stem. The signature points to mechanical friction; servicing the packing and stroking the valve is the appropriate corrective action.