RTDs vs Thermocouples
RTDs and thermocouples are the two most common temperature sensors. Learn the key differences, when to use each, and common mistakes.
The Key Difference
RTDs measure temperature by the change in electrical resistance of a metal (usually platinum). Thermocouples generate a small voltage at the junction of two dissimilar metals. This fundamental difference determines their respective strengths and weaknesses.
RTDs are more accurate and stable but limited to moderate temperatures. Thermocouples are less accurate but can measure very high temperatures that would destroy an RTD.
When to Use RTDs
- Process temperature measurement: Where accuracy of 0.5 degrees C or better is needed.
- Temperatures below 600 degrees C: RTDs are the standard for process temperatures in the -200 to 600 degrees C range.
- Long-term stability matters: RTDs drift less than 0.1 degrees C per year. Thermocouples drift 1 to 5 degrees C per year.
- Low vibration: RTDs are more fragile than thermocouples. Avoid in high-vibration environments unless specially designed.
When to Use Thermocouples
- High temperatures (above 600 degrees C): Furnaces, kilns, exhaust gas, flue gas. RTDs cannot survive these temperatures.
- Harsh environments: High vibration, shock, or pressure. Thermocouples are more robust.
- Fast response needed: Exposed-junction thermocouples respond faster than RTDs in thermowells.
- Cost-sensitive applications: Thermocouples are cheaper than RTDs, especially for multiple points.
Thermocouple Types
| Type | Materials | Range | Typical Use |
|---|---|---|---|
| K | Chromel/Alumel | -200 to 1,260 degrees C | General purpose, most common |
| J | Iron/Constantan | -40 to 750 degrees C | Vacuum, reducing atmospheres |
| T | Copper/Constantan | -200 to 350 degrees C | Low temperature, cryogenic |
| N | Nicrosil/Nisil | -200 to 1,300 degrees C | High temperature, more stable than K |
| R/S | Platinum/Rhodium | 0 to 1,450 degrees C | Very high temperature, furnaces |
Matching the transmitter
Each thermocouple type has a different voltage-temperature relationship. The transmitter or controller must be configured for the correct thermocouple type. Connecting a Type K thermocouple to a transmitter configured for Type J gives completely wrong readings. Always verify the configuration matches the sensor.
Tip: If temperature readings are way off, check three things first: (1) Is the transmitter configured for the correct sensor type? (2) Is the correct extension cable being used? (3) Is the wiring polarity correct? These three causes account for most temperature measurement problems.