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MAR-APR 2019

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44 INTECH MARCH/APRIL 2019 WWW.ISA.ORG AUTOMATION BASICS of several individual instruments: a DP-producing element such as an orifice plate, a DP transmitter, a gauge or absolute transmitter, a tem- perature transmitter, and a flow com- puter. Using a single, multivariable DP flowmeter with its sophisticated transmitter and ancillary measure- ments eliminates the need to install all these additional devices, at least in most situations. Some process engineers may be re- luctant to use a secondary variable for a critical measurement. For example, the DP flowmeter can provide its own temperature reading, but perhaps not with an update rate as fast as a stand-alone temperature sensor and transmitter. Of course, it is probably a small number of situations where temperature changes occur so rap - idly, and if a critical loop is based on temperature, it will certainly require its own instrument. Practicality of additional measurements The practicality of using the extra mea- surements as part of a larger process automation strategy will depend on how they are extracted. The DP flow- meter has access to the measurements we have already mentioned—along with any additional information, such as fluid density characteristics and line size, embedded in the configura- tion. The transmitter uses this infor- mation constantly for its own internal calculations. If the DP flowmeter or any other type of multivariable instrument is used in a Foundation Fieldbus or HART- enabled I/O environment, capturing the additional data is very simple. The distributed control system (DCS) sim- ply needs to be programmed to access the data and on how to use it in larger control efforts. In a conventional analog I/O envi- ronment, accessing the extra functions and variables is more complicated. HART multiplexers tend to take a long time to cycle through all the transmit- ters they service, so the additional readings will not have a fast update rate. A HART interface (figure 3) can work with a single multivariable instru - ment, breaking out the additional read - ings and turning them into separate 4–20 mA signals. This works well, but the DCS has to treat them as separate tags just like individual instruments, adding to wiring costs. WirelessHART may be the best ap- proach for retrofits in a simple wired I/O environment, or for new instal - lations. Many plants now have Wire- lessHART networks operating for a variety of purposes and adding an adapter (figure 4) to a DP flowmeter is a simple matter. It can then send all its data through the network to any point in larger systems where it needs to be used. No additional wired I/O slots are necessary. Advantages of multivariable instruments Today's multivariable instruments are possible thanks to advances in trans- mitter electronics. The little circuit board inside the housing is truly a pow- erful computer able to perform calcu - lations with remarkable speed. When applied to DP flowmeters, these capa- bilities provide exceptional accuracy across a huge range of fluid types and characteristics. Where useful, secondary variables can deliver process information with- out additional instruments or process penetrations. This double benefit of performance and cost advantages can help optimize the process while reduc- ing the cost of gathering the data nec- essary for effective decision making. n ABOUT THE AUTHOR Connor Oberle (www.linkedin.com/in/ connoroberle) is a global pressure product manager for Emerson Automation Solu- tions in Shakopee, Minn., responsible for Rosemount™ MultiVariable™ transmit- ters. He has a BS in mechanical engineer- ing from the University of North Dakota. Figure 3. A HART interface ex- tracts the extra variables and presents them as if each were coming from a discrete point. Figure 4. A WirelessHART adapter can send multiple variables via the wireless network without affecting the primary wired I/O connection.

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