Something Old, Something New

Users of the Two Most Measured and Most Important Variables in Process Control—Temperature and Pressure—Are Benefiting from Added Intelligence, Software and Other New Technologies, but Everything Still Depends on Good Old-Fashioned Sensors and Well-Maintained Thermocouples and Resistance Temperature Detectors

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By Jim Montague, Executive Editor

Since about five minutes after the invention of fire, people have tried to measure temperature and pressure. Whoever burned themselves first or tossed the first dried corn or coconut onto a fire no doubt paid closer attention after those first explosions. After all, heat creates pressure, and pressure generates heat. And all the slowly accelerating technology advances and industrial revolutions since then have only made it more important to check temperature and pressure more precisely and more often.

However, temperature and pressure have been measured for so long, and the tools used to measure them have become so ubiquitous that they and their users have grown pretty set in their ways. Perhaps due to their widespread success and long history, thermocouples, many resistance temperature detectors (RTDs) and pressure sensors have grown slightly invisible and perhaps a bit taken for granted. Still, because they play crucial roles in countless applications, many users are reluctant to seek or accept adaptations or innovations in how they’re applied.

Carbon for Cars

For instance, every vehicle with a catalytic converter needs a canister full of activated carbon. That’s a lot of vehicles and a lot of carbon. This is why MeadWestvaco in Covington, Ga., makes about 17,000 tons of it annually, using sawdust and phosphoric acid in an activating kiln process. The only catch is that this application requires precise control and measurement of its char temperatures.

Consequently, MeadWestvaco uses numerous different thermocouples to measure and sometimes recheck 150 different temperatures, mostly on individual devices, but also on some redundant ones too. The process also requires the company to monitor and control gas valves, exhaust temperatures and temperatures of liquids in tanks, including byproducts and slurries. The firm uses ISO-certified thermocouples from JMS Southeast in Statesville, N.C., and some infrared devices, which are calibrated and checked annually.

“I’ve been a field instrumentation guy for six years, and even in that short time we’ve seen better technology for temperature sensing elements,” says Mark Brackenridge, electrical and instrumentation (E&I) supervisor at MeadWestvaco. “For example, all thermocouples still have the same bi-metal junctions, but now we have better metallurgy in the sheaths protecting the sensors, and so they last longer. These sheaths previously were stainless steel, but now we have Stabolloy and Hastelloy, and they last about five years. We store some ¼-in. and 3/8-in. sheathes with different temperature conducting capabilities, but we also tell JMS the lengths and specifications we need, and they make them to order for us.”

Frank Johnson, JMS’ general manager, reports that, not only are many more measurements being made, but the sensors themselves are more capable and have better performance, so the end products they help produce are more consistent. “For example, a plastics manufacturer that used to have one thermocouple or sensor at one point of his process may now have six or seven multi-loop controllers, and be able to profile and scan along the entire length of his application,” says Johnson.    

Temperature, Pressure Evolution

Though basic temperature and pressure sensing methods have remained much the same for decades, RTDs, transmitters and thermocouples also have become more standardized in the past 10 to 15 years, and these sensors have grown more linear, repeatable and accurate, according to Allen Erwin, Yokogawa Corp. of America’s (www.us.yokogawa.com) product manager for transmitters. “There’s a lot less drift and a lot more long-term stability,” says Erwin. “Also, while field-mounted transmitters usually send 4-20 mA or digital signals back to the DCS, the pace of digital is increasing as Foundation fieldbus really began to take hold about five years ago.”

Erwin reports that developers have added data-processing intelligence and more sophisticated communications to their transmitters as they’ve been called on to handle higher-grade products. “Users increasingly need transmitters that don’t drift so they can implement smarter diagnostics and SIL-rated safety devices.”  

Likewise, on the pressure side, transmitters have gone from 0.25% reference accuracy in the 1960s and 70s to achieve 0.05% total performance accuracy in the past 10 to 15 years, reports Scott Nelson, Emerson Process Management’s vice president of worldwide pressure products. “Just as 3-psi to15-psi pneumatics gave way to analog electronic instruments, like the Rosemont 1151, these 4-20 mA devices had to make room for faster hybrid communications like HART and all-digital Foundation fieldbus and Profibus. Of course, all the buzz now is about Wireless HART, which will create chances for more measuring points because the costs are acceptable, and because they can fit into places where wire can’t physically go. Some users tell us they could increase their device count by 50% by going wireless.”

In fact, at a recent IEC meeting in Tokyo, Johnson says he saw a demonstration of a new Type A thermocouple from Russia that uses tungsten and rhenium, so it doesn’t drift as much as former tungsten thermocouples. “Different combinations of materials allow control at a little higher temperatures,” says Johnson. “RTD developers are pursuing extended higher ranges, too. We’ve also seen a lot of evolution in non-contact infrared sensors, which are getting more sophisticated, able to do scan and total-image sensing, and are getting more accurate at single-spot sensing. So, they’re being used a lot more, but users need more basic understanding to apply them, too.”

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