It is not practical to compare RTDs and thermocouples generally. However, if we compare their performance in terms of specific criteria, we can see which is best suited for specific applications.
RTDs vs. Thermocouples
- Temperature range: Thermocouples are best for working at high temperatures. New manufacturing techniques have improved the measurement range of RTD probes, but more than 90% of RTDs are designed for temperatures below 400°C. In contrast, some thermocouples can be used at up to 2500°C.
- Cost: Thermocouples are generally cheaper than RTDs. An RTD will often cost two or three times more than a thermocouple with the same temperature and style.
Savings can be made on RTD installation, which is cheaper as inexpensive copper wire is used. However, this saving is not enough to compensate for the higher device price.
- Sensitivity: While both sensor types respond quickly to temperature changes, thermocouples are faster. A grounded thermocouple will respond nearly three times faster than a PT100 RTD.
The fastest-possible temperature sensor is an exposed tip thermocouple. However, manufacturing improvements have also greatly improved the response times of thin-film PT100 probes.
- Accuracy: RTDs are generally more accurate than thermocouples. RTDs have typically an accuracy of 0.1°C, compared to 1°C for most. However, some thermocouple models can match RTD accuracy. The many factors that can affect sensor accuracy include linearity, repeatability or stability.
- Linearity: The temperature-resistance relation in RTD is almost lineal in the sensor range, while a thermocouple has an ‘S’-type plot.
- Stability: RTD probe readings stay stable and repeatable for a long time. Thermocouple readings tend to drift because of chemical changes in the sensor (such as oxidation). RTDs’ linearity and lack of drift make them more stable in the long term.