SACA Process Control and Instrumentation 5 — Questions and Answers
Question 1: A thermocouple installation requires a reference junction compensation because:
- Thermocouples are non-linear at high temperatures
- The EMF output depends on the temperature difference between measuring and reference junctions (Correct answer)
- Thermocouples require excitation current to produce a signal
- Cold junctions generate noise in the millivolt range
Correct answer: The EMF output depends on the temperature difference between measuring and reference junctions
A thermocouple generates a voltage proportional to the temperature difference between its hot (measuring) and cold (reference) junctions, so the reference temperature must be known and compensated.
Question 2: Which process variable is typically controlled using a split-range valve configuration?
- Flow in a bypass loop
- Pressure that requires both heating and cooling (Correct answer)
- Level in a gravity-fed tank
- Speed of a variable frequency drive
Correct answer: Pressure that requires both heating and cooling
Split-range control divides the controller output signal between two valves (e.g., one for heating, one for cooling) so a single controller manages both process directions.
Question 3: What does the term 'rangeability' mean when describing a control valve?
- Maximum allowable pressure drop across the valve
- Ratio of maximum to minimum controllable flow (Correct answer)
- Travel distance from fully closed to fully open
- The number of trim sizes available for the valve body
Correct answer: Ratio of maximum to minimum controllable flow
Rangeability is the ratio of the maximum to minimum flow rates over which the valve maintains its specified flow characteristic, indicating how wide a flow range it can control.
Question 4: An RTD with alpha (α) = 0.00385 Ω/Ω/°C is measured. What does this alpha value represent?
- Resistance at 0°C in ohms
- Temperature coefficient of resistance per degree Celsius (Correct answer)
- Self-heating error in watts per degree
- Insulation resistance rating of the sensor
Correct answer: Temperature coefficient of resistance per degree Celsius
Alpha (α) is the temperature coefficient of resistance, indicating how much the RTD resistance changes per ohm per degree Celsius; 0.00385 is the IEC standard for platinum RTDs.
Question 5: In a Fieldbus (Foundation Fieldbus H1) network, what is the role of the Link Active Scheduler (LAS)?
- Converts 4-20 mA signals to digital packets
- Manages deterministic communication scheduling on the bus segment (Correct answer)
- Provides power to all bus devices
- Translates between FF-H1 and FF-HSE protocols
Correct answer: Manages deterministic communication scheduling on the bus segment
The LAS is the bus master that controls the deterministic token-passing schedule, ensuring all devices communicate in an orderly, time-synchronized manner on an H1 segment.
Question 6: A process has a first-order transfer function with a time constant of 60 seconds. Approximately how long will it take for the process output to reach 63.2% of its final value after a step change?
- 6 seconds
- 30 seconds
- 60 seconds (Correct answer)
- 120 seconds
Correct answer: 60 seconds
For a first-order process, the output reaches 63.2% of its total change in exactly one time constant (τ), which is 60 seconds in this case.
Question 7: Which type of pressure measurement is referenced to absolute zero pressure (perfect vacuum)?
- Gauge pressure
- Absolute pressure (Correct answer)
- Differential pressure
- Sealed gauge pressure
Correct answer: Absolute pressure
Absolute pressure is measured relative to a perfect vacuum (zero pressure reference), so it equals gauge pressure plus local atmospheric pressure.
SACA Process Control and Instrumentation 5
A thermocouple installation requires a reference junction compensation because: