Temperature Sensors
Temperature is one of the most measured variables in process plants. Learn about RTDs, thermocouples, thermistors, and how to select the right sensor.
Main Types of Temperature Sensors
- RTD (Resistance Temperature Detector): A precision wire winding (usually platinum) whose resistance changes with temperature. Very accurate and stable. Most common type: Pt100 (100 ohms at 0 degrees C). Range: -200 to 600 degrees C.
- Thermocouple: Two dissimilar metal wires joined at a junction. The junction generates a small voltage proportional to temperature. Less accurate than RTD but can handle much higher temperatures. Range: -200 to 1,800 degrees C depending on type.
- Thermistor: A semiconductor whose resistance changes dramatically with temperature. Very sensitive but non-linear and limited temperature range. Used for low-cost, high-sensitivity applications.
- Infrared (pyrometer): Measures temperature by detecting infrared radiation. Non-contact. Used for moving surfaces, high temperatures, or where contact is impractical.
RTD vs Thermocouple
| Feature | RTD (Pt100) | Thermocouple |
|---|---|---|
| Accuracy | +/- 0.1 to 0.5 degrees C | +/- 1 to 5 degrees C |
| Stability | Excellent (drifts less than 0.1 degrees C/year) | Good (drifts 1 to 5 degrees C/year) |
| Temperature range | -200 to 600 degrees C | -200 to 1,800 degrees C |
| Response time | Slower (1 to 10 seconds) | Faster (0.1 to 5 seconds) |
| Cost | Higher | Lower |
| Lead wire effect | 3-wire or 4-wire required | Compensation cable required |
| Best for | Process temperature, accuracy-critical | High temperature, harsh environments |
Installation Considerations
- Thermowell: A protective sleeve inserted into the process. The sensor slides into the thermowell. Allows sensor replacement without shutting down the process. But adds thermal lag and reduces accuracy.
- Immersion depth: The sensor should be immersed at least 5 to 10 pipe diameters or sufficiently past the pipe wall to avoid measuring the wall temperature rather than the fluid temperature.
- Lead wire routing: Keep sensor leads away from power cables to avoid electromagnetic interference. Use shielded cable for RTDs.
- Junction type: RTDs can be 2-wire, 3-wire, or 4-wire. 3-wire compensates for lead resistance. 4-wire is most accurate. Always match the transmitter to the sensor wiring type.
Use the correct extension cable for thermocouples
Thermocouples generate a voltage at the junction. If you connect with standard copper wire, you create a second junction at the connection point, adding an unwanted voltage. Always use thermocouple extension cable made of the same material as the thermocouple. Using the wrong cable is a common source of temperature measurement errors.
Tip: Calibrate temperature sensors annually using a dry block calibrator. Compare the sensor reading to the calibrator at three points (low, mid, high). RTDs should be within +/- 0.5 degrees C. Thermocouples within +/- 2 degrees C. Replace sensors that drift outside specification.