# ISA Interchange

Welcome to the official blog of the International Society of Automation (ISA).

This blog covers numerous topics on industrial automation such as operations & management, continuous & batch processing, connectivity, manufacturing & machine control, and Industry 4.0.

The material and information contained on this website is for general information purposes only. ISA blog posts may be authored by ISA staff and guest authors from the automation community. Views and opinions expressed by a guest author are solely their own, and do not necessarily represent those of ISA. Posts made by guest authors have been subject to peer review.

# How Do You Determine a Measurement Is Good Enough for Control?

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The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

As part of the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. I will be sharing questions from program participants and the answers. This question is from Danaca Jordan

Danaca Jordan is a manufacturing staff engineer working for a specialty chemical company in the U.S. She began her journey to automation in high school as team captain for FIRST Robotics Team #624 and earned a B.S. in chemical engineering from the University of Houston. Jordan is an active member of AIChE and ISA, and is a successful acolyte of the ISA Mentor Program.

### Danaca Jordan's Question

How do you determine a measurement is good enough for control?

Great question.

First, the measurement needs to be representative of what you are trying to control and significantly affected by the disturbances you are trying to reject. This is best seen by monitoring changes in the measurement for changes in the manipulated and disturbance streams.

Second, the measurement should have a deadtime less than 1/20 of the cyclic disturbance period; otherwise, the disturbance is too fast and is effectively noise with a risk of amplification by resonance with the natural frequency of the loop.

Third, the measurement threshold sensitivity and resolution and repeatability should be less than one quarter of the allowable control error at setpoint for the entire operating measurement range. The measurement must have enough rangeability to handle low operating points. Often measurement errors are expressed as a percent of full scale and are therefore larger in terms of percent of operating setpoint.

Fourth, the measurement drift should be less than one quarter of the allowable control error between scheduled maintenance.

Fifth, the noise must be small and fast enough to be filtered without appreciably slowing the loop’s ability to see disturbances. The filter time to keep the oscillations in the controller output from noise less than the valve deadband should be less than 1/10 the reset time and not significantly affect the tuning of the controller. For more information on measurement location and selection, see my process control post, How to Succeed – Part 4.

See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant) and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).