To Thermocouple or RTD When Using Temperature Sensor Transmitters, that is the Question
Or perhaps, thermistor?
By Mike Moore, Inside Technical Sales Representative, Process Automation Products | March 19, 2018
Wixom, MI: In the process manufacturing and even R&D / test world, temperature measurement and monitoring is one of the more common and widely applied applications. As a manufacturer of Temperature Transmitters, Acromag covers a wide range of sensor input transmitters and alarms, and with it questions about the best sensor for the job.
What at first blush may seem a simple straight-forward proposition, temperature range, units (commonly C or F), limit set points, temperature sensor type, output signal, etc. may well run into unexpected issues at time of installation and start-up.
In the Acromag white paper Criteria for Temperature Sensor Selection - A Comparison of Thermocouple and RTD Temperature Sensors, a comparison table for many measurement characteristics is given between Thermocouple (TC) and RTD sensors on page three. Accuracy, stability/drift, sensitivity and measurement range are some of the more obvious factors considered, while un-thought of characteristics like EMI resistance to signal strength, need for reference junction, vibration/shock resistance and even response time may be conditional choices that if left out could affect the measurements.
When advising customers on configuring temperature sensor transmitters, one question that comes up often is, "Do I need thermocouple CJC or Cold Junction Compensation turned on or off?" It would seem a good rule-of-thumb to configure the TC transmitter with CJC On for better measurement accuracy. However, it may, in fact, lead to measurement problems as stated in the above mentioned white paper Part 3: "The thermal lag of cold junction compensation might make a thermocouple a poor choice where its mating amplifier or cold junction is subject to rapid or quickly changing ambient temepratures. Further, the presence of cold junction compensation in the mating amplifier often makes it subject to the self-heating or warm-up of the measuring circuit itself, driving longer warm-up periods for the measuring system to reach thermal equilibrium."
Further thermocouple sensor considerations are covered in the Part 1 white paper The Basics of Temperature Measurement Using Thermocouples, after all a thermocouple operates by creating a mV signal from the junction of two dissimilar metals, right? Me too, but not so fast, as stated in this white paper: "You may have been told something like 'a thermocouple produces a small voltage created by the junction of two dissimilar metals'. This simplification of the thermocouple is at best only half true, and very misleading. The reality is that it is the temperature difference between one end of a conductor and the other end that produces the small electromotive force (emf), or change imbalance, that leads us to the temperature difference across the conductor."
In applying the proper transmitter to RTD sensors, the complexity increases beyond the RTD type (Pt, Cu, Ni, Platinum 100 ohm being most common), two, three or four wire RTD and temperature range to linearization, Alpha values and other considerations as discussed in white paper Part 2, The Basics of Temperature Measurement Using RTDs. Along with some handy wiring diagrams, this white paper covers potential errors one may run into when picking out RTD's. "RTD's are susceptible to three dominant groups of errors: there are errors that result due to the inherent tolerances builti into the element, errors due to the thermal gradients that develop between the element and the material being sensed, and electrical errors encountered in the wiring between the sensor element and the measuring instrument. While many of these errors are electrical, others simply occur as a result of the mechanical construction of the RTD." Reference tables are included with RTD Materials, Ohms, and Alpha data and their primary benefits.
And what of our friend Thermistor? Well, that is a rare measurement bird indeed. However, I am seeing more thermistor applications arise and so will prod engineering for another white paper featuring thermistors and their application considerations. We could call it something like -- oh I don't know -- maybe "The Basics of Temperature Measurement Using Thermistors".
Ok, so you have a temperature sensor requirement, be it in a process application, manufacturing plant or test and measurement system that seems like a no-brainer, and while there is something to be said for the tried and true, there is also the old adage of not being able to make an old dog do new tricks. Each application with its own unique variables of location, environment, materials, sensor distance and I/O present signal quality challenges that may not have been experienced before. As a sensor/signal I/O transmitter manufacturer, Acromag is here to assist with not only choosing the proper transmitter for the sensor, but also in support material for the best sensor for the job, so you don't get yourself overheated on your next temperature measurement application.