Almost all industrial temperature measurement comes down to one of two physical effects: a resistance that changes predictably with temperature, or a voltage generated at the junction of two dissimilar metals. RTDs use the first; thermocouples use the second. Both turn up constantly in wellsite and surface equipment, and the choice between them is rarely arbitrary.
RTDs: resistance that tracks temperature
A Resistance Temperature Detector (RTD) is built around a length of very pure metal — almost always platinum, in the common “Pt100” and “Pt1000” types — whose electrical resistance increases in a known, repeatable way as it gets hotter. A Pt100 element measures exactly 100 Ω at 0 °C, and its resistance at any other temperature is described by a standard curve (the Callendar–Van Dusen equation) that instrumentation is built to interpret automatically. Because the relationship is so well characterised, RTDs are the standard choice wherever accuracy and stability matter most.
Measuring resistance accurately over a run of cable is its own small problem, since the cable itself has resistance that would otherwise be read as part of the sensor. That’s why RTDs are wired as 3-wire or 4-wire circuits rather than 2-wire: the extra wires let the instrument measure and subtract the lead resistance, rather than mistake it for a temperature reading.
Thermocouples: voltage from dissimilar metals
A thermocouple is simpler in construction: two wires of different metal alloys (types K, J, and T are common) are joined at one end to form a measurement junction. Where that junction is hotter or colder than the point where the wires connect to the measuring instrument, a small voltage appears — the Seebeck effect. That voltage is a function of the temperature difference between the measurement junction and the reference (or “cold”) junction, not of absolute temperature directly, so accurate readings depend on knowing or compensating for the reference junction’s own temperature. Modern instruments do this electronically, measuring the temperature at their own terminals and correcting for it automatically.
Comparing the two in practice
| RTD (Pt100) | Thermocouple | |
|---|---|---|
| Typical accuracy | Higher, more stable long-term | Lower, can drift with junction ageing |
| Response time | Slower (larger sensing mass) | Faster |
| Usable range | Roughly −200 °C to 850 °C | Type-dependent, up to 1300+ °C |
| Signal | Resistance — needs excitation current | Self-generated millivolt signal |
| Robustness | More fragile element | Generally more rugged, simpler construction |
| Typical use here | Process and surface equipment monitoring where stability matters | Wider-range or faster-response duty, and where simplicity favours it |
Which one fits your application
Neither type is universally “better” — the operating envelope decides it. Where long-term stability and the best achievable accuracy matter, an RTD is usually the right call. Where the range is wider, the environment is harsher, or a faster response is needed, a thermocouple often wins. Hazardous-area classification, process connection, and sheath material selection matter just as much as the sensing principle itself.
See how this fits into what we supply on the What We Supply page, or get in touch with the application and we’ll tell you honestly what fits.