What are the differences between a thermocouple and an RTD?

The most notable difference between a thermocouple and an RTD (Resistance Temperature Detector) is the principle of operation. A thermocouple operates on the principle that two dissimilar metals joined together will produce a voltage related to a temperature difference. An RTD, operates on the principle that electrical resistance of certain metals changes in a predictable way depending on the rise or fall in temperature.

Advantages of the thermocouple include a wide temperature measuring range (depending on the thermocouple type the range can be as much as from -300°F to 2300°F), fast response time (under a second in some cases), low initial cost, and durability. Overall, thermocouples are able to withstand rugged applications. 

Advantages for RTDs include stable output over a long period of time, ease of recalibration, and accurate readings over narrow temperature spans. Disadvantages, when compared to the thermocouples, are: smaller overall temperature range ( -330°F to 930°F), higher initial cost and they are more fragile in rugged, industrial environments.