Reader writes: Real semi-wireless installation
Scott A. Somers, process control engineer for Katahdin Paper Co. in East Millinocket, ME sent an email recounting one of his experiences trying to solve a nagging problem. He writes, "Thank you for your article‘ Using Semi-Wireless Instrumentation .’ I had recently completed the design of a semi-wireless installation and my research had revealed exactly what you discussed in your column. I was hoping to find a total wireless solution for a radar level transmitter mounted on a remote fuel oil storage tank. The engineering request included displaying the tank level on a DCS operator display in our steam plant control room. The control room is located roughly 600 feet from the storage tank across a roadway with no existing tray systems to connect the two locations.
"Because of the power requirements of radar level transmitters, a complete wireless solution was not economically feasible, considering the cost of a solar panel and rechargeable battery setup. Fortunately, an existing 120 Vac circuit ran underground past the tank to power receptacles at the unloading platform. This circuit was accessed via a manhole located in close proximity to the tank. As for the wireless part of the installation a third party single-point wireless system was purchased which allowed for up to 1,000 foot reception through obstructions. The wireless signal was converted to 4-20 mA within the steam plant, with a short cable run to the DCS termination rack. The cost savings came in not having to run cable and cable tray, complete with supporting structures to bridge the roadway, the full 600′ between the tank and the control room."
The third-party transmitter in this case is an integrated radio and I/O module for one 4-20 mA current loop device from Phoenix Contact . This type of approach is appropriate if you plan to use wireless one device at a time, rather than a larger network with more nodes.
Given that the article cautioned about using plain lighting and power circuits for instrumentation, I had to ask Scott if he had thought about someone tripping or turning off that branch. He replied, "I’m not really concerned about the reliability of the power source, which originates from an unmanned pump house located about 200 feet from the tank. This is an indication-only loop, not a closed loop. This project was prompted by a safety concern for the steam plant operators who had to trudge out to the tank once a shift, often through knee-deep or higher snow, climb the stairs to the top and‘stick’ it. We would have hours between measurement checks to reset the circuit if it should trip."
-Peter Welander, process industries editor, PWelander@cfemedia.com
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