Multivariable vs. Single-Variable DP Transmitters
Dispelling Common Myths About Instrument Accuracy
David W. Spitzer, P.E.
Paradigms abound in this world. For example, we spent centuries thinking the earth was flat and the sun rotated around it. Doctors also used to bleed patients to promote healing. One instrumentation paradigm that is occasionally bandied about is that multivariable differential-pressure transmitters that measure pressure, temperature and differential-pressure are more accurate than three individual transmitters measuring the same parameters.
It would seem logical that this was once a true statement. Early multivariable designs used technology that was significantly better than that which was incorporated into the then-available, single-variable transmitters. However, improvements have been made to single-variable transmitters over the years to the extent that it is now questionable as to whether multivariable transmitters offer better performance than single-variable transmitters.
Part of this issue arises from the flexibility of single-variable transmitters. For example, a multivariable transmitter may be capable of measuring pressure up to 1000 PSIa. However, measuring the flow of 25# steam with this transmitter will likely introduce more measurement error than a similar pressure transmitter with an upper-range limit of (say) 100 PSIa. This is because some performance specifications are related to the upper-range limit of the transmitter. Typically, the pressure error associated with a transmitter with a high upper-range limit is typically higher than the pressure error associated with a transmitter with a lower upper-range limit.
An additional aspect of this issue leads one to wonder whether existing multivariable transmitters have better performance specifications than the best pressure, temperature and differential-pressure transmitters currently available. A preliminary (and cursory) examination of some offerings indicates that the best single-variable transmitters can offer better performance than multivariable transmitters in some cases.
This discussion is not intended to imply that single-variable transmitters are superior to multivariable transmitters or that you should prefer one transmitter type over another. Rather, this discussion is intended to alert you that the validity of this paradigm should be examined in the context of the application at hand.
Best-in-class performance may not be the prime objective of your measurement, so a multivariable transmitter may be the proper selection because it offers fewer parts with fewer taps at a lower cost, while meeting the requirements of the application. However, the paradigm that multivariable transmitters are superior to single-measurement transmitters should be questioned when best-in-class performance is the objective of your flow measurement system. You may be surprised by the results of your investigation.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
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