Are you grounded in reality?

Stan: We conclude with a real life story from Hunter Vegas, who we herby elect as an honorary columnist, because of his significant contributions to Control Talk over the years.

Hunter: I was working as an instrument engineer at a chemical plant outside of New Orleans. One of the better techs called me to help troubleshoot a problem that bedeviled him. He had a two wire DP transmitter wired to our Texas Instruments (TI) D3 DCS, which I believe was originally EMC, then Rexnord, then TI, then GSE, etc. Anyway, the plant was complaining that the transmitter was reading the correct value out in the field, but the DCS reading was about 20% higher. The tech placed his test meter downstream of the transmitter (in series), and read exactly the same as the transmitter. This suggested there was something wrong with the DCS. However when the tech replaced the transmitter with a two-wire simulator, the DCS readings exactly mirrored the simulator readings. Not knowing what else to do, the tech took the transmitter into the shop for a bench test. On the bench, the transmitter worked flawlessly. However, when the transmitter was re-installed, it still read 20% below the DCS.

At this point the tech gave up and called me. After explaining the situation to me, I asked him to do one thing: "Hook your test meter upstream of the transmitter." The tech thought I was crazy, and said the reading would be the same. I told him to humor me and try it. He called back in disbelief. "Now the meter reading matches the DCS reading, but both are 20% above the transmitter reading!" What happened?

I knew something our tech did not; the D3 system measures the 4-20 mA signal going out to the field. The return leg is simply grounded and is not measured. In this particular case, the transmitter had developed a weak internal ground. When it was installed in the field, the transmitter was returning the correct 4-20 signal, but a few milliamps were shorting through the meter to ground on the incoming side. The DCS was measuring the combination of the actual transmitter reading and the ground current, and thus read about 20% high. When you placed a test meter downstream of the transmitter, you read the correct signal leaving the transmitter. When you placed the test meter upstream of the transmitter, you saw the leakage current.

So why did the transmitter perform flawlessly on the bench? The transmitter was placed on a nice soft rubber non-conducting mat to keep it from getting damaged. Without a path to ground, the transmitter worked flawlessly! 

Greg: In the following trend (below), the process lag was increased from the 5 seconds for the November trend to 50 seconds, and the controller reset time was decreased from 20 to the 5 seconds needed to eliminate the falter in the response of November trend. What is now wrong with tuning?


NEW TUNING TREND

The process lage trend in the November Puzzler was increased from 5 seconds to 50 seconds, and the controller reset time was decreased from 20 seconds to 5 seconds.

This Month's Puzzler: Valve needs all the trimmings? Why did Stan include four sets of different-sized trim when ordering a control valve with a Cv less than 0.1? Send an e-mail with your answer to The Puzzler, CONTROL questions, or comments to controltalk@putman.net.

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About the Authors

Greg McMillan and Stan Weiner, PE bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. They’re accompanied in this edition by honorary columnist Hunter Vegas.