How do RTD and thermocouple sensors differ in mechanism and typical use?

• A. RTD: resistance changes with temperature for high accuracy; Thermocouple: generates voltage with temperature difference for wide range and fast response.
• B. RTD: generates voltage with temperature; Thermocouple: resistance changes with temperature for high accuracy.
• C. RTD: uses magnetic field; Thermocouple: uses optical signal.
• D. RTD: digital output; Thermocouple: analog output.

Answer: (A)

RTD sensors rely on the metal’s resistance changing with temperature. The metal (often platinum) has a predictable, highly repeatable rise in resistance as it gets hotter, so you measure temperature by sensing that resistance change. This makes RTDs very accurate and stable, with good linearity over a defined range, though they aren’t as fast or as high-temperature capable as some alternatives. They require a small excitation current to measure resistance, and sometimes multiple-wire configurations are used to cancel lead resistance for better accuracy. RTDs are commonly used where precision and long-term stability matter, such as in process control and lab instrumentation, with typical useful ranges from well below freezing up to around 850°C (depending on the metal).

Thermocouples, by contrast, work on the Seebeck effect: when two dissimilar metals are joined, a voltage is produced that is proportional to the temperature difference between the measurement junction and a reference junction. This voltage is very small, so it requires sensitive electronics to read it. Thermocouples can cover an extremely wide temperature range, respond quickly, and are rugged and inexpensive, making them ideal for many high- or extreme-temperature applications. However, they’re generally less accurate and stable over time than RTDs and need reference-junction (cold-junction) compensation to convert the voltage into an accurate temperature reading.

So the key difference is mechanism (resistance change versus voltage generation) and typical use (RTDs for high accuracy and stability; thermocouples for wide range and fast response).