Which kind of temperature sensor is the most accurate?
Dear Control Engineering: Having just read the article Temperature sensors: Make the right choice, RTD vs. TC, I would like to offer some clarification to the subject. As a rookie EE in 1967 my assignment was as an instrumentation engineer for the support labs and test stands that NASA had installed to test the LM and CSM engines at a simulated altitude of 100,000 feet. The site location was the NASA White Sands Test Facility near Las Cruces, NM. There was some minimal utilization of RTDs (resistance temperature detectors) because we regarded them to yield only an approximate temperature result. Our temperature sensor of choice was the TC (thermocouple) because properly applied the temperature result was far more accurate than with RTDs. By properly applied I mean that the same material as was utilized by the TC junction was used throughout the TC circuit and the all important ice reference junction was also properly applied. We checked the TC per its published curve using a Wheatstone bridge. For most of the non-flight related applications we fabricated our own TCs and tested them using a reference junction, identical to the TC being tested, immersed in a beaker of ice water. Our results were very accurate, and unless there has been a major shift in RTD technology in the recent past, they won’t even come within the fabled "order of magnitude" yielded by the TCs. Our system was periodically double checked utilizing/calibrated using a temperature standard one level away from the Washington National Bureau of Standards.
Eugene Wilkins, PE, CEO of Complere Engineering Group, Inc., Modesto, Ca.
The article’s author, Ashleigh Hayes, marketing engineer for Emerson Process Management responds:
While the reader brings up a valid argument for TC accuracy, it is important to remember the applications and industry we are addressing. Both RTD’s and TC’s have advantages and limitations in what they can do; the key is to select the right solution for the specific application and industry the measurement is being taken in.
To his specific point, TC’s are able to maintain high accuracy if they are operated under specific, controlled conditions and monitored regularly. In a lab environment this can be done as the conditions are controlled and someone can check, and potentially replace, the TC frequently. Along with that, in a lab environment ice baths can be used for cold junction compensation which helps to provide a highly accurate measurement.
When speaking about process industries and a typical environment such as an oil refinery, this kind of monitoring and testing is not feasible. Often temperature sensors are installed into the process and left for very long periods of time. At the time of installation, a TC may be of a similar accuracy as an RTD, however over time the degradation of the TC will lead to the RTD being more accurate.
Further, many transmitters will not use the same material throughout the measurement circuit as what is used to create the TC junction. Alternative or compensation materials are more common in process plant applications as they are less expensive; even though using dissimilar materials can lead to a less accurate thermocouple measurement.
As for RTDs, their technology has developed over the decades. The platinum used in RTDs is more pure and manufacturing processes have become more reliable with better repeatability. Standards such as the IEC 60751 and ASTM E1137 have been periodically updated due to advances in RTD manufacturing and capabilities which help make today’s RTDs more accurate and precise than ever.
When choosing between an RTD and a TC, it is important to keep in mind what type of application and environment the temperature sensor is being used in. With the right equipment and maintenance TCs can be used in higher accuracy applications, however without controlled conditions and regular attention being paid to each measurement point, an RTD is the better choice when it comes to accuracy.
A good resource for information on RTDs and TCs is the The Engineers Guide to Industrial Temperature Measurement from Emerson available free at Rosemount.com/TempGuide.