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.
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.