What principle does a resistance temperature detector (RTD) use?

A resistance temperature detector (RTD) operates on the principle that the electrical resistance of certain materials changes in a predictable way as the temperature changes. Specifically, most metals exhibit a positive temperature coefficient, meaning that their resistance increases as the temperature rises. This property allows RTDs to measure temperature with high accuracy and stability. By measuring the resistance of the RTD, the corresponding temperature can be deduced using calibration curves or established relationships for the specific material used in the RTD, typically platinum.

The other options, while they pertain to other physical principles, do not apply to how an RTD functions:

• The change in liquid volume relates to thermometric principles used in liquid-in-glass thermometers, which does not involve electrical resistance.

• The change in thermal conductivity is relevant in other forms of thermal measurement but is not the basis of how an RTD operates.

• The change in pressure is related to pressure sensors and not relevant to the temperature measurement function of an RTD.

Thus, the correct answer accurately reflects the fundamental operational principle of resistance temperature detectors.