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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 By Wayne Labs, Contributing Editor

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hether meeting regulatory requirements or improving the bottom line, multivariable (MV) transmitters can provide measurements of multiple process variables (typically pressure, temperature, and often level or flow) with a single point of entry into your process. Combine this feature with digital fieldbus output and onboard control outputs, and you’re on your way to tighter control with lower installation costs. While a digital (smart) MV transmitter may cost more than the devices it replaces, the information it provides will help you fine-tune your control loop to its peak efficiency.

In the early 1990s Dynisco developed a smart MV transmitter that used a temperature measurement to compensate its pressure measurement output. These early MV transmitters were used in monitoring pressure and temperature of melted polymer, allowing users to improve the efficiency of pressure control, increase safety, and meet environmental regulations. Today, the company’s IPX II Series is available in pressure ranges from 0–750 psi to 0–10,000 psi with process temperature measurements to 660° F (350° C) and HART output. Built-in temperature compensation allows an accuracy of up to ±0.15% overall system accuracy, but also provides temperature output when needed.

Meanwhile, newer MV devices are often rated for incendive applications as opposed to many older transmitters did not meet these specs.

Cost-Justifying MVs
Installing a MV transmitter in a new line usually requires just one physical process connection, thus reducing labor and associated installation costs. But this shouldn’t be the primary reason for choosing a MV transmitter.

         

DENSITY COMPENSATION

           
         

Invensys Foxboro’s IMV31 density-compensated multivariable transmitter measures dp, tank pressure, and fluid temperature to provide level measurement in either open or closed tanks.

According to Mike Cushing, ABB pressure product manager, there are two justifications for the cost of MV transmitters. If the user is already making multiple measurements as is done for a steam mass flow application, then it’s easy to demonstrate that a single device is much less expensive than installing two or three separate devices. If, however, only a single measurement is now being made, such as a differential pressure (DP) transmitter used for flow, the benefit may not be as obvious. In this case, a MV transmitter can provide better turndown and sharper accuracy—in many cases decreasing accuracy error from as much as 6–8% to less than 1%.

In the case of a DP transmitter being used for tank level measurement, compensating the output for fluid density changes was a problem for control engineers. Calculations had to be done externally to the transmitter(s)—often in a control system, which led to inaccuracy. Ken Brown, vice president and general manager of Foxboro Measurements and Instruments, said this problem could be solved by adding on-board level calculations to the MV transmitter. For example, by continuously calculating density based on measurements of pressure and temperature, and using this information along with the transmitter’s differential pressure measurement, it’s easy to calculate tank level accurately within the same device.

According to Douglas Joy, Dynisco’s vice president of product marketing, building in a temperature sensor and compensating pressure measurements based on temperature changes can provide pressure measurement system accuracy to 0.15% in a MV transmitter. While the MV transmitter will not always be cheaper to purchase than a separate pressure and temperature sensor, the improvement in accuracy can be equated with reduced operating costs and improved product quality, making the all-in-one approach a sound investment.

Future Sensors, Extended Ranges
Sensor technology has been on a slower evolutionary path than supporting technologies such as fieldbuses and software. Changing strain-gage technology is improving pressure ranges at both ends, says Joy. Moving to a bonded-foil strain gage from a thin-film element allows Dynisco to design a device with a high-end range of 30,000 psi and a low-end range of 25 psi.

SAFETY TAG INCLUDED           
         
Dynisco’s IPX II Series multivariable transmitter measures pressures and temperatures of chemicals, plastics, and polymers.
 
         

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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