Compare RTD and thermocouple sensors in terms of sensing principle and typical accuracy/temperature range.

• A. RTD changes resistance with temperature and offers higher accuracy and stability over moderate ranges; thermocouples generate a voltage proportional to temperature difference and cover wider ranges but with less long-term stability.
• B. RTD generates voltage proportional to temperature difference; thermocouples changes resistance with temperature.
• C. RTDs have the widest temperature range and best long-term stability; thermocouples are more accurate at low temperatures only.
• D. RTDs generate a current proportional to temperature; thermocouples measure resistance with high stability.

Answer: (B)

Sensing principle first: RTDs work by the metal’s resistance changing with temperature. As temperature rises, the resistivity of the metal increases, and by measuring that resistance with a bridge or precision ohmmeter you infer the temperature. Thermocouples rely on the Seebeck effect: joining two dissimilar metals creates a small voltage that is proportional to the temperature difference between the measurement junction and a reference (cold) junction. That voltage lets you determine temperature after appropriate calibration and junction compensation.

Accuracy and temperature range follow from those principles. RTDs are known for high accuracy and excellent stability over a moderate temperature span. They are very linear, repeatable, and drift very little over time, making them ideal for precision temperature measurements in ordinary process or lab ranges (roughly from -200°C up to about 600–850°C depending on the device). Thermocouples, by contrast, cover a much wider temperature range—from well below freezing to well above 1000°C—because different metal couples can withstand extreme temperatures. However, the voltage signal is small and sensitive to wiring and junction conditions, so thermocouples generally exhibit lower long-term stability and accuracy than RTDs and typically require careful cold-junction compensation.

So the correct idea is that RTDs sense temperature through resistance changes with high accuracy and stability in a moderate range, while thermocouples generate a voltage from the temperature difference and span a broader range but with less precision and stability.