Instrumentation & Process Control Flashcards
7 cards from real CCST practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Instrumentation & Process Control flashcards as text
A thermocouple produces a millivolt output that drifts with ambient temperature changes. Which correction method is most commonly used to compensate for this error?
Answer: Cold junction compensation
Cold junction compensation corrects for the reference junction temperature, which fluctuates with ambient conditions and introduces measurement error.
In a pneumatic control system, what does an I/P transducer convert?
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.
Which type of control valve trim is specifically designed to provide equal percentage flow characteristics?
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.
A differential pressure transmitter installed on an orifice plate reads higher than actual flow. What is the most likely cause?
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.
In ISA S5.1 symbology, a circle on a process flow diagram with the letter 'FIC' indicates which instrument function?
Answer: Flow indicating controller
Per ISA S5.1, 'F' = flow, 'I' = indicating, 'C' = controller, so FIC denotes a flow indicating controller.
What is the primary purpose of a 'positioner' on a pneumatic control valve?
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.
When calibrating a pressure transmitter with a range of 0-100 psi, what input pressure should produce a 12 mA output?
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.