How to Select Temperature Measurement Components

We tend to think of instruments as single things, such as a pressure transmitter or flow meter, but when it comes to measuring temperature, there is an enormous range of sensors, thermowells, transmitters and many other parts available as individual components. Fortunately, there are complete assemblies for those who don’t want to design from scratch. But where something more specialized is required, sorting through the whole range of pieces and parts can be daunting.

 This sorting process is the topic of a new article by Todd Olin and Neil Larson in the April 2019 issue of Process Heating magazine titled How to Select Temperature Measurement Components. They look at the whole process chain of sensing a temperature, converting it to engineering units and getting it to a user and/or automation host system. Naturally, they begin with the sensor itself as the first link.

 At some point, the raw analog signal from an RTD or a thermocouple has to be turned into a meaningful temperature value. This can be as simple as using a multi-tester to take an ohm reading. When using a PT100 RTD, a reading of 113.61 ohms corresponds to 95°F. This figure can be found on an RTD Chart showing the range of resistance readings and their temperatures. Similar charts are available for the different thermocouple types. This might be useful for a science fair project or perhaps in a troubleshooting situation, but a real-world application calls for something more practical.

Practical is the key word here for anything in a real-world manufacturing situation, but it can have any number of meanings. A given sensor may be used to control a critical temperature loop, or it may require nothing more than an individual visual display so an operator can read it. It’s all about the necessary functionality for the application. One of the key links is a transmitter which takes the weak voltage produced by a thermocouple or resistance value of an RTD and makes it more robust so it can be sent to a host system more reliably.

A transmitter takes the raw signal from a sensor and converts it to a 4 to 20 mA signal with some other type. Depending on the type of transmitters used, the signal might be converted to HART; or to a fieldbus signal such as Foundation Fieldbus or Profibus PA; or to a wireless signal such as WirelessHART. The default capabilities available from virtually any transmitter will include basic functions such as providing a reference for thermocouples, converting to appropriate engineering units, damping and ranging. Smart transmitters can add diagnostic functions to monitor the sensor itself, look for problems that might be developing and support redundant sensor arrangements.

 The flexibility of these systems is driven by the range of form factors for these devices. Transmitters alone come in several configurations for mounting at the sensor, in a control cabinet, with or without a local display and various other options. For example, the RosemountTm 248 Temperature Transmitter can meet the requirements of a single point measurement application in a wide range of process environments.

 Regardless of how the system is configured, it’s important to ensure every link in the chain is capable of delivering the kind of precision and reliability needed to satisfy the needs of the process.

 The effectiveness of a self-designed temperature setup depends on understanding the function of each component and its position in the chain from sensor to host system. Accuracy depends on the sensor itself, but it also on the transmitter, lead wires and even the skill of the individuals making the terminations. The range of components available from manufacturers can cover virtually any imaginable application, delivering whatever performance is required.

 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.