by Matt Migliore
While temperature measurement is, like many fluid handling segments, undergoing a period of technological change, the two primary solutions in this space, thermocouples and RTDs (resistance temperature detectors), maintain solid footing in specific application areas. RTDs, for instance, have emerged as a competitive solution for measurements in the -100 C to 600 C range, whereas thermocouples figure to hold on to measurements above 600 C for the foreseeable future.
RTDs vs. Thermocouples
In recent years, RTDs have replaced thermocouples in many -100 C to 600 C applications due to accuracy and performance advantages, but this trend does not seem as if it will threaten thermocouple market share for higher-temperature applications. According to Ron Desmarais, project engineer for Omega Engineering (www.omega.com), though RTD manufacturers continue to work to expand the range of their product offerings, there is a practical ceiling of 800 C to 1000 C for RTD temperature measurements. "Thermocouples really are a better fit for those applications," says Desmarais. As such, he says Omega does not, at this time, recommend RTDs for measurements above 600 C.
One key advance in the RTD segment over the past few years is the evolution of thin-film technology. Thin-film elements are typically smaller than traditional wire-wound elements, so they can be incorporated into sensors where wire-wound elements simply would not fit. This helps to make them more competitive with thermocouples in those applications where small size and fast response really matter. However, Thin-film elements also have a lower operating current than wire-wound devices, which is a bit of a double-edged sword, as it enables the elements to respond more quickly to changes in temperature, but at the same time makes them more susceptible to self-heating effects.
While wire-wound elements remain a good fit for temperature measurements on the upper and lower edges of the RTD range, temperature technology manufacturers are working to expand the range of thin-film elements. "We are getting more and more requirements for sensors incorporating thin-film RTDs due to their faster response times, lower prices and improved accuracies, especially over thermocouples. We have responded to this market by significantly increasing our product offering while making significant investments in technology and facilities to produce thin-film elements in the United States," says Desmarais.
Lara Kauchak, director for Rosemount Temperature, a business unit of Emerson Process Management (www.emersonprocess.com), believes there is a bit of a learning process that is needed among the end-user community in order to push the thin-film market forward. She says, while thin-film elements may better serve the needs of certain applications, end-users often remain unaware of whether they are purchasing thin-film or wire-wound technology for their RTDs and could be unpleasantly surprised with their performance. As a result, she says end-users should specifically request thin-film or high vibration RTDs if they require them for a given application.
Direct Wiring vs. Transmitters
Looking back 10-15 years, temperature measurement installations were performed primarily via direct wire. Now, however, transmitters are gaining significant momentum for new temperature measurement installations.
"It all comes down to process noise in the plant or the application," says Desmarais, noting that transmitters are a good way to overcome the problems associated with RTDs and thermocouples operating in noisy environments. He says, thermocouples are a particularly well suited for transmitters, as they are susceptible to process noise due to their relatively low signal strength and because transmitters enable thermocouples to be converted to standard cable. By eliminating the need to match thermocouple cable for long direct-wire runs, Desmarais says end-users can realize a significant savings in the cost of extension-grade thermocouple wire.
Kauchak cites the ease of installation, cost efficiency, and increased accuracy and diagnostic capability of transmitters as the primary demand drivers over direct wiring installations among end-users. She sees transmitters as a particularly good fit for high-density applications, where there are a number temperature measurement points in close proximity, such as motors, heat exchanges, and distillation columns, for example. In such environments, Kauchak says a high-density transmitter enables the end-user to leverage a single device with multiple inputs to provide diagnostics and temperature measurements for multiple monitoring points. Thus, she says end-users can limit the amount of wiring runs in environments saturated with temperature measurements.
In addition, Kauchak sees transmitters replacing temperature switches in certain safety applications. For example, she says end-users are increasingly employing SIS (safety instrumented system)-certified transmitters over safety switches because of the improved performance and diagnostic capability they provide in certain critical temperature measurement environments.
When applying temperature measurement solutions in a live environment, Desmarais says users must take special care to ensure the sensing element is exposed to the same temperatures as the process being monitored. Often, he says, stem conduction — a common process whereby heat travels up and down the sensing probe — allows ambient temperature to escape from or introduce itself to the sensing element, thereby diminishing the accuracy of the process temperature measurement. Therefore, he says end-users must carefully design their temperature sensing installation to suit the needs of their accuracy requirements.
One key application issue Kauchak says she often encounters with end-users involves the wake frequency calculation for ASME PTC19.3, a guidance document designed to help engineers determine the performance of a thermowell for given applications. She says that many end-users fail to recognize that there is wide variability in the results depending on how the calculation is run and the standard is interpreted. She says that while an updated version of ASME PTC19.3 is expected in the fall with updated guidance on thermowell designs, end-users would be wise to work closely with their temperature measurement supplier to ensure the calculation and results are consistent with their application needs. Further, she says end-user must be careful in how they apply the standard ASME PTC19.3. "The standard is written to provide engineering guidance and is not meant to be used as a pass-fail system," she says. "The user must still work with the supplier to determine whether the solution is providing the level of safety and accuracy they require."
Kauchak also recommends end-users pay special attention to grounding and transient protection when applying temperature measurement solutions. She says many end-users fail to consider grounding and transient protection on startup, only to find it become an issue later on. For example, she says grounding and transient protection may not be an issue in the winter when the temperature measurement solution is installed, but when the summer months come and thunderstorms abound, these issues become a significant concern. "It’s just something that people that have always done direct wiring don’t usually think of when they start moving to transmitters," says Kauchak.
Going forward, both Desmarais and Kauchak see wireless as the big push in the temperature measurement space. According to Kauchak, approximately 80 percent of the temperature points in an average plant are used for monitoring applications, making them a natural fit for wireless communication. Also, she says wireless is a good solution for temperature points, as wiring can often be cumbersome for such applications. For example, she says wireless is an excellent solution for rotating kilns, where wiring is technically very difficult. She says the fact that temperature applications typically require slower update rates than other process measurements makes them advantageous for wireless as well, since battery power is a key concern in wireless application environments.
In an effort to capitalize on the advantages it believes wireless offers in a temperature measurement environment, Kauchak says Emerson Process Management recently released its Rosemount 648 Wireless Temperature Transmitter. The transmitter supports a variety of sensor inputs, including RTD, thermocouple, millivolt, and ohm, and is designed to operate in the same fashion as a wired transmitter. Emerson is also scheduled to release its Rosemount High-Density Wireless 848T transmitter in the summer months. This Wireless HART offering will support multiple temperature points via a single wireless transmitter device.
Meanwhile, Omega Engineering recently released a number of wireless sensor products, including a Wireless Thermocouple Connector System, Wireless RTD Connector System, and the Z-Series Wireless Sensing System, which includes temperature, humidity and barometric pressure sensing capabilities. These products feature stand-alone, compact, battery-powered wireless sensors that are capable of transmitting readings back to a host receiver up to 90 m (300”) away. "There’s been a lot of discussion and anticipation over the last 10 years when it comes to wireless," says Desmarais. "When you can eliminate the wiring in a plant, that’s a good thing. It makes it that much simpler to expand your capability, and I expect you will see a number of new cost-effective products introduced into the marketplace in the near future."
Matt Migliore is the editor of Flow Control magazine. He can be reached at [email protected].