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

W

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.

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