Unfortunately, the final factor in selection of an RTD or thermocouple is usually cost. For decades, thermocouples have been the most widely used form of electrical temperature sensor because they are less expensive. While this is still true, it must be pointed out that the cost for a platinum RTD has come down to a competitive range due to the increased usage of thin-film sensors.

Thin-film RTD sensors certainly are not new technology anymore. They were developed in Germany in the 1970s. Currently, they are being used in ever-increasing numbers in applications from -58 to 1,112°F (-50 to 600°C) in industrial environments, the food and beverage industry, laboratory equipment and some automotive uses.

Whether or not an RTD can actually be used in place of a thermocouple will depend on the specific design and application. But if it's feasible, the actual price difference from thermocouple to RTD may be less than $10. If this is part of a large assembly, particularly one involving a thermowell and transmitter, that will amount to a small percentage of the total unit cost.

It should be pointed out that there will be hidden savings on the installation of the RTD because standard instrument wire can be used, as no specially compensated cables are required. Add to that the long-term savings of reduced calibration intervals, less drift and longer life expectancy, and RTDs should certainly be considered for any industrial application where a thermocouple might be used.