Fine tuning the multivariable way

CONTROL Contributing Editor Wayne Labs takes a look at multivariable pressure transmitters that provide critical process information and meet regulatory specifications.

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MV transmitter accuracy is already at a level where the test equipment used to measure it is at its limits. In “New DPharp EJX Pressure and Differential Pressure Transmitters,” Yokogawa engineers demonstrated the ease of calculating differential and static pressures in a DP instrument by using differential and summational computations of two resonators to produce an accuracy range of ±0.2% for a static pressure of 1.0 MPa. In a more sophisticated mass-flow transmitter, an accuracy range of ±0.1% has been realized for a static pressure of 1.0 MPa. Since these are calculated values, they are “free” process variables that extend the usefulness of the instrument.

Critical improvements in nonlinearity, repeatability and long-term drift are being made possible by fine-tuning error correction algorithms. Repeatability numbers today are as small as ±0.05 to ±0.75% for different pressure variables and ±1° F. Long-term drifts or stability figures hover in the range of ±0.02% (or better) for anywhere from 5 to 15 years depending on the manufacturer and how the drift is calculated. Is 15 years long enough? That depends on how long your process line will run in its current form.

Future is Plug-and-Play
Over the years HART has been very popular because it doesn’t ask users to change out old analog wiring. For new process lines in the U.S., customers are beginning to ask vendors for Foundation fieldbus devices, primarily because of the enhanced communications architecture that can provide data to local controllers, DCSs, and enterprise systems. In Europe, Profibus DP tends to be a common communication medium. Interestingly enough, Joy’s customers have also been asking for CANopen and DeviceNet communications. CANopen tends to be popular in Europe as well as the U.S., and some customers using CANopen in Europe are specifying it in their U.S facilities.

Indeed, as more users request some of these “open” fieldbuses, vendors will respond by offering them as readily as their own proprietary fieldbuses. The long-range benefit, of course, will be plug-and-play devices that can be put in place without regard to vendor. The benefits? By keeping smaller inventories of transmitters on hand, that will lower parts replacement costs, and finding replacements will be quicker and easier.

Better Promotion Needed
With the small market segment that comprises MV transmitters and the slow growth rate of pressure transmitters in general, Cushing suggests that the industry needs to better promote the benefits of the improved performance of MV transmitters. Without such promotion, vendors will have a rough time in financing future R&D efforts. In general, the transmitter market is fast becoming a commodity market where the emphasis is on cost, making it difficult for vendors to put research dollars into their instruments.

Today, there is no technical reason why DP transmitters can’t be shipped as MV devices because most vendors correct their instruments’ DP measurement for static pressure and ambient temperature to get the performance and accuracy their customers demand.

Most instruments already include a calculated static pressure measurement variable, although it’s not always accurate or visible. It should be commonplace in the near future that whenever a user replaces a failed DP transmitter, a MV device will be used in its place to improve accuracy and turndown. With available digital fieldbuses, static pressure and process temperature outputs—as well as level and flow—are practically “free.” And this valuable data will provide information for manufacturers to track products as they’re being made—information that can help make a better product and satisfy regulatory bodies as well.

A Small But Steady Market

MV transmitters are a small part of a fairly slow-growing, but steady market. According to Flow Research’s 2004 study, The World Market for Pressure Transmitters, revenues for pressure transmitters in 2002 totaled $1160 million. Compound annual growth rate (CAGR) through 2007 is projected at 0.8%. Of the total market, MV transmitters account for about 4.3%. MV pressure transmitter manufacturers are trying to find ways to improve their market share—including integrating them with primary elements to form MV flowmeters, building in better accuracy and stability, offering a wider choice of communication protocols, and providing enhanced diagnostics.
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