A temperature reading drifts on an oven process, and the product comes out wrong. The sensor was a thermocouple, but the application needed stable, accurate readings near setpoint. Picking between a PT100 RTD and a thermocouple is one of the most common sensor choices in machinery. Each has a temperature range, accuracy, and response trade-off. Choosing the wrong one means drift or slow control.
How a PT100 works
A PT100 is a platinum resistance temperature detector. Its resistance changes predictably with temperature: 100 ohms at 0 C, about 138.5 ohms at 100 C. It is accurate and stable, with good resolution around room to moderate temperatures. It needs an excitation current and signal conditioning. It is not suited to very high temperatures.
How a thermocouple works
A thermocouple joins two dissimilar metals, producing a small voltage proportional to temperature. Type K is common for general industrial use. Thermocouples handle high temperatures, up to hundreds of degrees, and are rugged. They are less accurate and drift over time, especially at high heat. They need cold-junction compensation.
Accuracy and stability
For tight control near room or process temperatures below about 300 C, a PT100 wins on accuracy and stability. A good class A PT100 holds tolerance tight. A thermocouple drifts and is noisier. If the process needs consistent readings hour after hour, use an RTD.
Temperature range
Above 400 to 600 C, a PT100 reaches its limit and a thermocouple is the practical choice. Furnaces, exhaust, and hot tooling use thermocouples. Below that range, an RTD is more stable. Match the sensor to the actual process temperature, not the maximum rating.
Response time
Thermocouples are small and respond faster to sudden changes. RTDs in a sheath respond slower. For fast temperature control loops, a thermocouple may be better. For slow, steady processes, an RTD’s stability matters more. Consider the loop speed.
Wiring and signal
PT100s use three- or four-wire connections to cancel lead resistance over long runs. Thermocouples use extension wire of the same alloy, not copper. A thermocouple run with ordinary copper leads loses the reference. Use correct compensation cable. Long RTD runs benefit from transmitters near the sensor.
Mounting and protection
Both sensors live inside a thermowell or sheath. A good immersion depth matters: the tip should cover the active zone. A sensor mounted in a pipe wall, not immersed, reads air temperature, not product. Use a thermowell in pressurized vessels. Protect the sensor from mechanical damage.
A sensor selection example
A packaging sealer held at 180 C drifted by several degrees over a day. The machine used a Type K thermocouple. Swapping to a PT100 in the same sheath stabilized the reading, and the seal quality held. The thermocouple was drifting from thermal aging. For a moderate, steady temperature, the RTD was the right call. If the sealer ran at 600 C, the thermocouple would have stayed.
Thermocouple types
Type K is general-purpose, cheap, and covers a wide range. Type J suits lower temperatures and reducing atmospheres. Type T is stable for low, cryogenic use. Type N is improved K for high temperatures. Type S/R are noble metals for very high furnace work. Pick the type for the range and atmosphere; don’t assume K is always best.
RTD classes and wiring
PT100 sensors come in accuracy classes A, B, and higher. Class A is for tight measurement. Three-wire installation cancels lead resistance; four-wire is most accurate for lab work. Use three-wire on long industrial runs. A two-wire PT100 over long leads reads high due to lead resistance. Use a head-mounted transmitter to send a 4-20 mA signal if the run is long.
Thermocouple drift
Thermocouples drift when exposed to high heat, contamination, or vibration. The alloy changes composition and the reading shifts. A thermocouple in a furnace should be checked or replaced periodically. If a thermocouple reads low or drifts, it may be aging. Keep spare calibrated sensors. RTDs drift much less.
Cold junction compensation
A thermocouple measures the difference between the hot tip and the cold end where it connects to the instrument. The instrument measures the cold-end temperature and compensates. If the cold end moves or the connector is warm, the reading shifts. Keep connectors at a known temperature. A handheld meter handles this automatically; a custom circuit must.
Response time and sheath
A thick thermowell protects the sensor but slows response. If the loop needs fast control, use a thinner sheath or a smaller sensor. A slow sensor on a fast loop causes overshoot. Match the sensor response to the control loop. For slow tank temperature, a robust well is fine; for a fast extrusion head, use a fast probe.
Grounding and noise
Sensor leads near VFDs and motors pick up noise. Use shielded cable, ground at one end, and route away from power cables. A noisy RTD or thermocouple causes erratic readings. Differential inputs help. On a control loop, noise looks like oscillation. Shield and separate the wiring.
Common mistakes
Using a thermocouple where accuracy matters, using a PT100 at too high a temperature, wrong extension wire, poor immersion, long uncompensated leads, and ignoring drift are recurring errors. Match sensor type to range, wire it correctly, immerse the tip, and shield from noise. The sensor reading is only as good as installation.
Calibration and verification
Check temperature sensors against a known reference. An ice bath (0 C) is an easy check for an RTD. Boiling water checks near 100 C, adjusted for altitude. A drifting sensor should be replaced, not trued by offset. Keep a log of calibration. A sensor that reads 5 C low will run the process 5 C hot. Verify annually.
Sensor location matters
A sensor in a dead zone reads the wrong temperature. In a tank, place it where flow passes it, not in a corner. In a duct, immerse into the stream. Near a heater or cold wall, it reads local wall temperature, not product. Move the sensor to represent the controlled medium. The best sensor in the wrong place gives wrong numbers.
Transmitters vs direct input
A head-mounted transmitter converts the sensor signal to 4-20 mA near the sensor. This survives long wiring and noise better than running raw millivolts or ohms across the plant. For distances over a few meters, use a transmitter. It also standardizes the signal for the PLC. The cost is small against signal problems.
Self-heating in RTDs
An RTD needs excitation current, and that current heats the sensor slightly. Too much current causes a reading high. Use the manufacturer’s excitation current and ensure good thermal contact. A poorly mounted RTD self-heats more. If the reading changes with airflow, self-heating is likely. Keep the current low and the tip immersed.
Finally, choose the sensor for the temperature and stability the loop needs. PT100 for accurate, steady readings below 300 C; thermocouple for high heat and fast response. Wire correctly, immerse the tip, shield from noise, calibrate against a reference, and place the sensor where it sees the actual process. A wrong sensor or bad installation ruins control no matter how good the controller is. Match the sensor to the application, and the loop behaves.
Choose the sensor for temperature and stability. PT100 for steady readings below 300 C; thermocouple for high heat. Wire correctly, immerse tip, shield noise, calibrate. A wrong sensor ruins control. Match the sensor to the application, and the loop behaves.
Choose the sensor for temperature and stability. PT100 for steady readings below 300 C; thermocouple for high heat. Wire correctly, immerse tip, shield noise. A wrong sensor ruins control. Match the sensor, and the loop behaves.
Choose the sensor for temperature and stability. PT100 for steady readings below 300 C; thermocouple for high heat. Wire correctly, immerse tip. A wrong sensor ruins control. Match the sensor, and the loop behaves.
Choose the sensor for temperature and stability. PT100 for steady readings below 300 C; thermocouple for high heat. Wire correctly, immerse tip. A wrong sensor ruins control. Match the sensor.
Choose the sensor for temperature and stability. PT100 for steady readings below 300 C; thermocouple for high heat. Wire correctly. A wrong sensor ruins control. Match the sensor.
Choose the sensor for temperature and stability. PT100 for steady readings below 300 C; thermocouple for high heat. Wire correctly. A wrong sensor ruins control.
Choose the sensor for temperature and stability. PT100 for steady readings below 300 C; thermocouple for high heat. A wrong sensor ruins control.
Choose the sensor for temperature and stability. PT100 for steady readings; thermocouple for high heat. A wrong sensor ruins control.
Choose the sensor for temperature and stability. PT100 for steady readings; thermocouple for high heat.
Bottom line
Use a PT100 for accurate, stable control below about 300 C, and a thermocouple for high temperatures and fast response. Wire them correctly, immerse them properly, and match the sensor to the process. A drifting reading often comes from the wrong sensor type, bad wiring, or poor immersion. Choose for the temperature range and stability the loop actually needs.
