ISA CAP Control Systems & Instrumentation 2 — Questions and Answers
Question 1: In a cascade control system, the output of the primary (master) controller becomes the:
- Measured variable of the secondary loop
- Setpoint of the secondary (slave) controller (Correct answer)
- Manipulated variable of the primary loop
- Disturbance input to the process
Correct answer: Setpoint of the secondary (slave) controller
In cascade control, the primary controller's output sets the setpoint for the secondary controller, allowing faster disturbance rejection.
Question 2: A thermocouple produces an EMF based on the Seebeck effect. What is the purpose of cold junction compensation?
- To amplify the thermocouple output signal
- To correct for the reference junction temperature not being at 0°C (Correct answer)
- To linearize the thermocouple characteristic curve
- To protect the thermocouple from electrical noise
Correct answer: To correct for the reference junction temperature not being at 0°C
Cold junction compensation accounts for the actual reference junction temperature so the measured EMF accurately represents the process temperature.
Question 3: What does the term 'rangeability' refer to in a control valve?
- The maximum allowable pressure drop across the valve
- The ratio of maximum to minimum controllable flow (Correct answer)
- The travel distance between fully open and fully closed positions
- The operating temperature range of the valve body
Correct answer: The ratio of maximum to minimum controllable flow
Rangeability is the ratio of the maximum to minimum flow that the valve can control while maintaining its specified flow characteristic.
Question 4: In process control, what is 'offset' in the context of proportional-only control?
- The time delay before the controller begins responding
- A sustained steady-state error between setpoint and process variable (Correct answer)
- The derivative kick experienced on setpoint changes
- The integral windup accumulated during saturation
Correct answer: A sustained steady-state error between setpoint and process variable
Proportional-only controllers have inherent offset because a nonzero error is required to produce a nonzero control output.
Question 5: A 4-20 mA transmitter is configured for a 0-100 psi range. What process pressure corresponds to a signal of 12 mA?
- 50 psi (Correct answer)
- 60 psi
- 40 psi
- 75 psi
Correct answer: 50 psi
12 mA is the midpoint of the 4-20 mA span (16 mA range), representing 50% of the 0-100 psi range, which equals 50 psi.
Question 6: Which control valve characteristic provides approximately equal percentage changes in flow for equal increments of valve travel?
- Linear characteristic
- Quick-opening characteristic
- Equal-percentage characteristic (Correct answer)
- Parabolic characteristic
Correct answer: Equal-percentage characteristic
An equal-percentage characteristic means each increment of valve travel produces the same percentage change in flow relative to current flow.
Question 7: What is the primary advantage of using a Coriolis mass flow meter over a volumetric flow meter in custody transfer applications?
- Lower installation cost and simpler maintenance
- Direct mass measurement unaffected by fluid density changes (Correct answer)
- Higher accuracy only at very low flow rates
- Compatibility with all corrosive fluid types
Correct answer: Direct mass measurement unaffected by fluid density changes
Coriolis meters measure mass flow directly, eliminating errors caused by density variations due to temperature, pressure, or composition changes.
ISA CAP Control Systems & Instrumentation 2
In a cascade control system, the output of the primary (master) controller becomes the: