Control Systems & Instrumentation Flashcards

In a cascade control system, the output of the primary (master) controller becomes the:

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.

A thermocouple produces an EMF based on the Seebeck effect. What is the purpose of cold junction compensation?

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.

What does the term 'rangeability' refer to in a control valve?

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.

In process control, what is 'offset' in the context of proportional-only control?

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.

A 4-20 mA transmitter is configured for a 0-100 psi range. What process pressure corresponds to a signal of 12 mA?

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.

Which control valve characteristic provides approximately equal percentage changes in flow for equal increments of valve travel?

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.

What is the primary advantage of using a Coriolis mass flow meter over a volumetric flow meter in custody transfer applications?

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.