Flow / Distributed Control

Reader feedback: April 2018

Readers respond to February's Control Talk column and PID controller basics article

Thank you for taking the time to address some of the main areas around some of the more traditional metering technologies (“The art of flowmeter selection”). It was interesting to read someone else's opinion on these common metering technologies, and I liked the “good, bad and ugly” simplification.

I would add something to the differential pressure (DP) discussion in the bad/ugly category, and point out the possible errors in measurement accuracy caused by mechanical damage to the plates/wedge. Prejudiced as I may be, I felt that the area of ultrasonics could have done with a little more attention, especially as there are several varieties around the theme: high accuracy multi-beam inline meters for liquids and gases, clamp-on transit-time meters for reliable measurement of liquids and gases, and Doppler ultrasonic for viscous sludges.

One area that I've written about when looking at all meters is the importance of managing the expectation that the customer has for the technology. Frequently, the customer expects the manufacturer to solve all their problems, perfectly, with one device, and this simply isn't possible. For more information, see https://tinyurl.com/y7u5pkxw.

Andrew Sutton

We appreciate your comments and are glad you enjoyed the article. You're correct that DP meters can be impacted by mechanical damage to the plates/wedge, and ultrasonic meters are just one of a multitude of alternative flow measuring technologies. Unfortunately, we have time and space limitations in doing the "Control Talk" column each month, so we try to cover the most common technologies and the most common strengths and weaknesses. We could write a book on this particular topic, and in fact we're writing 800 pages for the Process/Industrial Instruments and Controls Handbook, Sixth Edition, as we speak. You’ll be happy to know there's a section on ultrasonic flowmeters as well as the rest.

Greg McMillan and Hunter Vegas

I'm a process application engineer, mainly dealing with closed loop/drive logic development and control loop tuning, working with ABB India. I read R. Russell Rhinehart’s article on the basics of the PID controller (“Understanding P, I and D”), and I found it to be clear and comprehensible. Thanks to your work published on ControlGlobal.com, I believe that I would now be able to retain the mathematical concept, which I seldom manage to recall when necessary.

Thank you once again for the article. I look forward to seeing more of your works in process instrumentation and control.

K. Raghu Anand

I greatly appreciate your feedback. I enjoy trying to explain mysteries to the practice community as I did when teaching college students. I've done earlier articles in Control on FOPDT modeling, tuning controllers, and filters, that you may also find useful.

R. Russell Rhinehart