Something Old, Something New

Dec. 22, 2008
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

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 ( 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.”

The Sensor Base

While most temperature and pressure technologies have added data processing, software and intelligence at various stages, the performance of all of these bells and whistles still is founded on the humble sensor and the experience of its operator. 

“The main rule is—you need to devote time to your sensors. They must be high-quality, rugged and accurate,” says Brackenridge. “Measuring temperature means using a thermocouple and sending its signal to the distributed control system (DCS) direct via thermocouple wire, or using an RTD and a transmitter to reach the DCS. Because of the temperatures we deal with, we use Type K or Type J thermocouples, and their wires hook up with the input card on the DCS, which is programmed to handle that type of thermocouple.”

To maintain its sensors correctly, Brackenridge explains that MeadWestvaco examines its thermocouples directly with handheld checkers that hook up across each thermocouple’s terminals, which shows its positive or negative polarity. “If the thermocouple is reading right, it will show a temperature. If not, it will just read as open,” says Brackenridge. “A more scientific method is to remove the old thermocouple and check it against a new one. Likewise, if a reading is fading in and out, then its thermocouple may have to removed and tested on the job. This is because the bi-metallic junction may be damaged inside the thermocouple, even though it looks fine on the outside.”

He adds that MeadWestvaco also can check its thermocouple wiring, troubleshoot poor readings at the DCS, seek out bad input cards by loading temperature data, and fix reading problems by first inputting known values and comparing performance.

Multivariability Multiplies

Perhaps the biggest change in pressure and temperature sensing in recent years is the emergence of multivariable transmitters that can measure static and differential pressure, temperature and flow. This freed users from having to install and maintain separate devices for measuring each variable. 

“Just as we tried to design out all the likely failure points before introducing the Rosemont 3151 pressure transmitter in 2001, these improvements also allowed it to scale up from doing basic pressure measurements to be able to do advanced process diagnostics,” says Nelson. “So transmitters that used to do one variable, such as pressure, could now implement electronics and also measure differential pressure, process temperature, device temperature, static pressure, mass flow and energy use. As a result, the pressure transmitter market is still growing because plants need to be more efficient, safer and more complaint with environmental rules.”  

Listen to Voice of Experience

Besides thoroughly checking and properly maintaining temperature and pressure sensors, Brackenridge adds that it’s also crucial to listen to older engineers, technicians and operators. “We always try to pay attention to the older guys because they’ve seen and done so many things and dealt with so many exceptional situations that you can always learn from them,” explains Brackenridge. Also, he adds, MeadWestvaco previously had separate electrical and instrumentation supervisors, and that he was the firm’s first combined E&I supervisor, though the company still maintains separate electrical and instrumentation shops.  

In fact, practical experience is doubly valuable, not just because so many veterans are retiring, but also because control and automation’s increasing success and pervasiveness is making many exceptional events increasingly rare. As a result, many of today’s engineers don’t know what to do when some really unusual problem occurs because they’ve never seen it happen before, and the guy who did see it has long since moved on.

“Testing in the shop is one thing, but it’s always a different ball game in the field because anything can happen there,” says Brackenridge. “That’s why users have to stay in contact with each other, and why we need technologies that make it easier to look up and troubleshoot devices.”

Despite the need for experienced sensor technicians, however, JMS’ Johnson reports that many users can’t find enough technicians trained in implementing and maintaining traditional temperature and pressure sensors. “So users spend gobs of money on fancy transmitters that they can calibrate and modify, but they don’t pay enough attention to sensors that may have drifted or are about to go bad. That’s why we created a package that can predict drift in thermocouples and RTDs.” 

Closing the Knowledge Gap

Johnson says users need to be aware of what temperature and pressure sensors can do and how these older technologies can benefit their operations. “Thermocouples and RTDs are exposed to a lot of heat, and the steel tubes they’re in can deteriorate quickly,” explains Johnson. “However, we now have special materials to avoid this and help sensors last four times longer, but most users never specify using them. We try to educate and do lunch-and-learns, but a lot of this doesn’t seem to get across.”

Similarly, Johnson adds that DuPont’s Kapton insulation for wires is now more impermeable than the traditional Teflon or fiberglass, but it’s rarely specified either. “We recommend using this better insulation, but PID policies are set ahead of time by many user companies or ISA, and they haven’t included better insulation in their requirements,” he says.

Finally, Johnson reports that Type N thermocouples can handle temperatures as high as Type K, but Type N doesn’t drift as much and will last three times as long. Now, he says, Type N has been available for 30 years, but it was just added to the American Society for Testing and Measurement’s (ASTM) 2760D heat-treating standard.

“Interest in the basic fundamentals of sensors has decreased in the last 30 years as people have retired,” says Johnson. “This is why some users ask us for transmitters with 0.10% accuracy, and then they go buy a sensor that can be off up to 10 °F at different temperature levels. People know about transmitters, but they don’t know about sensors. In fact, the president of JMS Southeast recently met with the head of the instrumentation department at a university in Charlotte about RTDs at 0 °C, and it was the first time they’d heard of sensors.

“For example, I recently built a cedar house, but unfortunately I used galvanized nails that chipped and bled. I remembered too late that I should have used aluminum or stainless-steel nails. The same rules apply to industrial process control. You can’t forget the fundamental things that are naturally part of process control. However, we’re not teaching enough about commodity technologies and how to apply them correctly, and so price pressure wins because users think the components are all the same. If a thermocouple costs $100 and the rest of the system control panel costs $10,000, then people think why should they care a about the lower cost item? They don’t realize how the thermocouple may affect their whole system. Sometimes you just have to pay attention to the basics, but we often see them being overlooked.”

To help users learn about thermocouple and RTD basics, JMS produced and offers a 3-hour DVD of its training course. Johnson also recommends Industrial Temperature Measurement by Tom Kerlin and Bob Shepard, published by ISA. He also suggests that user consult the appendices to ASTMs standards for thermocouples, RTDs and testing devices.

Smartening Up, Securing Acceptance

Though temperature measurement devices remain essentially unchanged in their basic operations, the data processing, emerging software and intelligence around and above them continue to evolve rapidly. For example, all of Meadwest Vaco’s devices usually shoot their signals and loop data back to its DCS, but it and other DCSs also are evolving. In fact, the company recently replaced its DOS-based DCS from Yokogawa Corp. of America with Yokogawa’s CS 300 Microsoft Windows-based DCS.

“It’s all point-and-click, so it’s much easier to program and maintain,” says Brackenridge. “Our networking is still all point-to-point 4-20 mA, but I don’t know if we might try a fieldbus in the future.” 

Despite this ease of use, Brackenridge adds there was some initial resistance to Meadwest’s updating its DCS from some of its mostly older operators. So, not only should less-experienced engineers learn from the veterans, but those older engineers also could stand to learn a few new tricks too. “We did a lot of talking, coaxing and training about the new DCS, but sometimes we also had to say that the new way is just the way it is,” says Brackenridge. “We even set up a simulated process, so our operators could see how the application would react when they used the new DCS to manipulate valves in manual and automatic.” 

Sense of the Future

Whether wired or increasingly wireless, the sheer number of temperature and pressure points and other variables being measured are all but certain to increase in the future.   

“The role of pressure transmitters is changing to include even more process diagnostics and advanced process control, which we call statistical process monitoring. Basically, pressure transmitters can now do stuff that control room couldn’t do in the past,” explains Nelson. “And having transmitters all over a plant gives users a more physical connection to their processes, as well as very fast processing and update rates. This means users can employ pressure transmitters to help see if a distillation column is flooding, or if an application’s fluid composition has changed due to entrapped air or another kind of instability. For example, fluid catalytic crackers (FCCs) need constant flow-rate data, and new pressure transmitters can more closely monitor the overall health of these processes by monitoring for statistical variations.”