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The following chart reveals the main differences between RTDs and Thermocouples.

RTDs

An RTD is a sensor whose resistance changes as its temperature changes. The resistance increases as the temperature of the sensor increases. The resistance vs temperature relationship is well known and is repeatable over time.

An RTD is a passive device. It does not produce an output on its own. External electronic devices are used to measure the resistance of the sensor by passing a small electrical current through the sensor to generate a voltage.

Thermocouples

A thermocouple is a device consisting of two different conductors (usually metal alloys) that produce a voltage proportional to a temperature difference between either ends of the pair of conductors. In contrast to most other methods of temperature measurement, thermocouples are self-powered and require no external form of excitation.