When looking for resources on various aspects of process automation, searches will typically turn up treatments tending more toward either the theoretical or practical side of a topic. Theoretical information is important and necessary in many cases, but often solving a given problem calls for something more practical. Fortunately, Emerson has produced both kinds of information over the years, but a recent example moves more toward the practical side.
In the March 2018 issue of Process Heating, Todd Olin and Neil Larson take a practical approach in Selecting Process Temperature Sensors. This is billed as the first of two articles on the topic.
Temperature sensors come in many shapes and sizes, but for industrial use, two technologies cover the majority of applications: thermocouples (TCs) and resistance temperature detectors (RTDs). It is not difficult to find resources discussing the differences between the two technologies. But, for purposes of this article, which is part one of a two-part series, we will focus on some of the more nuts-and-bolts aspects of the selection process. In addition, this article will describe ways to improve temperature-measurement performance regardless of the selected sensor technology.
Naturally, comparing and contrasting the two technologies requires at least a bit of theory since the two approaches really are quite different. TCs are a thermoelectric circuit able to turn a thermal gradient into a corresponding voltage, while an RTD changes resistance with temperature. Good to know, but what does it mean in the real world, and which is better for my application? As in most cases, the universal engineering answer applies: it depends. TCs cover a wider temperature range, so if the temperature is above roughly 750 ºC, RTDs are out. But there are more subtle considerations.
One of the biggest advantages of a thermocouple over an RTD is its ruggedness. Because the sensor consists of two wires, typically welded together, it is not delicate. So long as the junction is sound, the insulation is intact and the wire itself is not corroded, a thermocouple will work. The wire used is usually of a relatively heavy gauge, so a thermocouple can withstand a lot of punishment and vibration.
An RTD uses a characteristic found in many metals whereby the electrical resistance increases as temperature increases. This is a phenomenon known as thermal resistivity. A given metal will have a given resistance at a given temperature. If this is properly quantified, an RTD can provide an accurate temperature reading.
So there is a ruggedness versus accuracy argument, but there are many more considerations the article examines. The whole picture is far more nuanced, and the list of qualifications it addresses goes into a lot of detail. Forgive a spoiler, but if circumstances permit, Todd and Neil generally side with RTDs when there is a valid choice. They point to the fact that RTDs are still improving as a technology and their characteristics are becoming more consistent. They also have specific accuracy classes per IEC spelling out measurement tolerances.
For ultimate accuracy, RTDs can be fine-tuned when used in conjunction with a specific transmitter. In a calibration lab, a sensor and transmitter can be paired and adjusted to characterize and correct readings in a specific range. This approach isn’t for everybody, but it is an option. Of course, those driven to the utmost in accuracy need to pay attention to more than just the sensor.
There is a chain of components between the sensor and the final input point, which can be on anything from a local transmitter display to a plant-wide control or monitoring system. Every termination, and even the cable itself, can cause reading drift by interfering with the voltage or resistance measurement. The transmitter or input card also will have its own error factor, so it is important to understand how these all add up. Suffice it to say, an expensive Class A RTD supported by lower-grade components will be no better than the worst link in the chain.
The article covers a wide variety of points supporting the selection process, and it tends to stay with nuts-and-bolts issues and practical considerations. We’re looking forward to what comes out in Part 2 in April. You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Temperature Group and other specialty areas for suggestions and answers.