CCST Instrumentation & Process Control 2 — Questions and Answers
Question 1: A thermocouple produces a millivolt output that drifts with ambient temperature changes. Which correction method is most commonly used to compensate for this error?
- Cold junction compensation (Correct answer)
- Zero suppression
- Span adjustment
- Noise filtering
Correct answer: Cold junction compensation
Cold junction compensation corrects for the reference junction temperature, which fluctuates with ambient conditions and introduces measurement error.
Question 2: In a pneumatic control system, what does an I/P transducer convert?
- Impedance to pressure
- Current (4-20 mA) to pneumatic pressure signal (3-15 psi) (Correct answer)
- Instrument air pressure to voltage
- Pulse signals to analog pressure
Correct answer: Current (4-20 mA) to pneumatic pressure signal (3-15 psi)
An I/P (current-to-pressure) transducer converts a 4-20 mA electrical signal into a 3-15 psi pneumatic output to drive control valves.
Question 3: Which type of control valve trim is specifically designed to provide equal percentage flow characteristics?
- Quick-opening trim
- Linear plug trim
- Parabolic or characterized cam trim (Correct answer)
- Flat disk trim
Correct answer: Parabolic or characterized cam trim
Parabolic or characterized cam trim shapes the relationship between valve travel and flow so that each equal increment of travel produces an equal percentage increase in flow.
Question 4: A differential pressure transmitter installed on an orifice plate reads higher than actual flow. What is the most likely cause?
- The orifice plate is installed backwards (Correct answer)
- The impulse lines are partially blocked
- The square root extraction is disabled
- The transmitter span is set too low
Correct answer: The orifice plate is installed backwards
An orifice plate installed backwards reverses the high/low pressure taps, causing the transmitter to sense incorrect differential pressure and over-read.
Question 5: In ISA S5.1 symbology, a circle on a process flow diagram with the letter 'FIC' indicates which instrument function?
- Flow indicating controller (Correct answer)
- Frequency indicating circuit
- Fluid injection controller
- Fieldbus interface card
Correct answer: Flow indicating controller
Per ISA S5.1, 'F' = flow, 'I' = indicating, 'C' = controller, so FIC denotes a flow indicating controller.
Question 6: What is the primary purpose of a 'positioner' on a pneumatic control valve?
- To act as a safety shutoff
- To ensure the valve stem position accurately follows the controller output signal (Correct answer)
- To reduce supply air consumption
- To convert the valve from air-to-open to air-to-close
Correct answer: To ensure the valve stem position accurately follows the controller output signal
A positioner provides closed-loop feedback on stem position, overcoming friction and hysteresis to make the valve precisely track the controller's demand signal.
Question 7: When calibrating a pressure transmitter with a range of 0-100 psi, what input pressure should produce a 12 mA output?
- 25 psi
- 50 psi (Correct answer)
- 75 psi
- 60 psi
Correct answer: 50 psi
12 mA is the midpoint of the 4-20 mA range, which corresponds to 50% of span, or 50 psi on a 0-100 psi transmitter.
CCST Instrumentation & Process Control 2
A thermocouple produces a millivolt output that drifts with ambient temperature changes.
Which correction method is most commonly used to compensate for this error?