| RTD (Pt100) | Thermocouple | |
|---|---|---|
| Usual range | -200 to 600 °C | -200 to 1700 °C (type wise) |
| Accuracy | ±0.15 °C at 0 °C (Class A) | ±1.5 °C or ±0.4% (Class 1) |
| Stability over years | Very good — drifts slowly | Drifts faster, especially hot |
| Response time | Slower | Faster |
| Vibration tolerance | Moderate; MI version is better | Good |
| Cost of sensor | Higher | Lower |
| Cost of wiring | Ordinary copper | Compensating cable, costlier |
| Cold junction needed | No | Yes |
| Self-heating error | Yes, small | No |
| Standard | IEC 60751 | IEC 60584 |
The sensor has to survive the worst case — a start-up excursion, a runaway batch, a stuck burner. If that number is over 600 °C, the decision is already made: thermocouple.
A Class A Pt100 reads within about 0.15 °C at zero. A Class 1 Type K thermocouple reads within about 1.5 °C. If your process specification says ±1 °C, a thermocouple cannot deliver it, no matter how good the transmitter is.
Bearings, compressors, extruders and rotating dryers break RTD elements. A thermocouple survives, and a mineral insulated RTD is a middle path.
A thermocouple responds faster because the junction is smaller. On a fast exothermic reaction or a nozzle temperature, that matters. On a large tank it does not.
Thermocouple extension cable costs more than copper and must be the right type all the way to the cold junction. Over a hundred metres that can cost more than the sensor. A head mounted transmitter converts to 4-20 mA at the sensor and lets you use ordinary cable — often the cheapest total answer.
A store with fifty Pt100 spares and no thermocouple spares has already made the decision for the maintenance team.
The process runs under 400 °C, accuracy is specified tightly, the point is not violently vibrating, and you want a reading you can still trust in three years.
The process runs hot, the point vibrates, response speed matters, or the cost per point has to be low across many points.
The surface cannot be penetrated — then a surface or skin-sensing sensor, or an infrared measurement, is the right tool.
Yes, materially. A Class A Pt100 is specified to about ±0.15 °C at 0 °C; a Class 1 Type K thermocouple to about ±1.5 °C. The gap widens as the thermocouple ages.
It works, but you are using a sensor with degree-level error on a duty where an RTD gives you a tenth of a degree for a small extra cost. Below 100 °C we almost always recommend Pt100.
The measurement is made by the junction between two dissimilar metals. If you connect ordinary copper, you create another junction and another voltage. Compensating or extension cable of the right type carries the signal without adding error.
Put a head mounted transmitter on the sensor and send 4-20 mA over ordinary twisted pair. It costs more at the sensor and less in cable, and the accuracy does not fall away with distance.
A noble metal thermocouple (Type R, S or B) inside a ceramic sheath. Base metal thermocouples drift in months at those temperatures; RTDs do not survive at all.
Send us the process temperature, the accuracy you need and whether the point vibrates. We will tell you which of the two suits it, and why.
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