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AutoQuiz: What is the Cause of a High Process Variability?

AutoQuiz is edited by Joel Don, ISA's social media community manager.

Today's automation industry quiz question comes from the ISA Certified Automation Professional certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for information about the CAP program. The following question comes from the CAP study guide, Performance Domain VI, Operations and Maintenance. Long-term support of the system.

If diagnostic software flags a controller as having high process variability when its output is hitting a low limit, the MOST probable cause is a(n):

a) zero shift in the measurement
b) noisy measurement
c) nonlinear control valve
d) oversized control valve
e) none of the above


Controller output successively hits its low limit and manifests a saw tooth oscillation.

An oversized control valve will operate next to the seat where the friction and stick slip is greatest.

A noisy measurement increases high-frequency variability but does not necessarily cause a controller output to hit its limit.

A nonlinear valve may require detuning but does not necessarily cause a controller output to hit its limit.

Zero shift, or bias errors, exists to some degree in all measurements and results in running at a shifted set point. However, is does not cause a controller output to hit its limit unless it is extremely large, which is not as probable as an oversized valve. Such large errors are not common, and during calibration, the technicians would have certainly discovered the problem.

The best answer is D, oversized control valve.


Joel Don
Joel Don
Joel Don is the community manager for ISA and is an independent content marketing, social media and public relations consultant. Prior to his work in marketing and PR, Joel served as an editor for regional newspapers and national magazines throughout the U.S. He earned a master's degree from the Medill School at Northwestern University with a focus on science, engineering and biomedical marketing communications, and a bachelor of science degree from UC San Diego.

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