Resistance Temperature Detectors

Resistance Temperature Detectors (RTDs) operate within a narrow range of temperatures, but have much better accuracy than thermocouples (below 600 degrees Celsius). These are usually constructed with very fine platinum wire tightly wrapped around ceramic or glass. This produces a resistance-to-temperature relationship. Because it is a resistance-based measurement, an excitation current is needed to operate an RTD (1 mA).  

The resistance of an RTD follows a predefined slope. RTDs are specified with a base resistance. A 200 PT100 RTD has a slope of 0.00200 Ohms/degrees Celsius from 0 to 100 degrees Celsius. Within that range (0 to 100 degrees Celsius), the slope will be linear. RTDs come in two, three, and four-wire packages, with four-wire models used for high-accuracy calibration systems. RTDs are often used with bridge circuits to increase resolution, with software linearizing the results:

Wire-wound RTD

RTDs are rarely used above 600 degrees Celsius, which limits their application in industry. At high temperatures, the platinum can become contaminated, leading to incorrect results; however, when measuring within their specified range, RTDs are fairly stable and accurate.