Have you ever watched a circular chart recorder connected to a process pressure transmitter... as the pen hit the peg so hard that ink splattered out the top? If so, you probably wondered what happened to that textbook world where pressure loops were so easy they weren't worth discussing. You are not alone if you have encountered tough pressure control problems. I have run into a few and am still here to talk about them. And what's a few years lopped off a life expectancy, anyway?
I survived my pressure control problems thanks to two words of wisdom I got when I was 4 years old and sitting on my daddy's knee. The secret passed from father to son on what turned out to be that fateful day was deadtime. Of course, it took me 34 years to understand what he was getting at. The intervening period has been filled with math, whose principal function seems to have been to keep me from realizing that deadtime determines loop performance.
Things aren't all bad, of course. If there were no deadtime in a loop, I might be out of a job.
Think of it this way. Without deadtime, any plant engineer who didn't know about instrumentation could buy a proportional
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