The operating principle of Resistance Temperature Detectors (RTDs) is based on the property that the resistance of metallic conductors varies with temperature. The core principle is as follows: when temperature changes, the amplitude of lattice vibration in metallic conductors such as platinum and copper alters, which impedes electron movement to varying degrees and consequently changes the resistance value. Taking Pt100 platinum RTD as an example, its standard resistance is 100 Ω at 0 °C, and the resistance increases by approximately 0.385 Ω for every 1 °C rise in temperature, showing highly linear characteristics within a certain temperature range. In practical measurement, a constant‑current source or bridge circuit converts resistance variation into electrical signals. Three‑wire or four‑wire connection is applied to eliminate lead‑wire errors. Finally, resistance values are converted into temperature values according to international standard reference tables.
Thermocouples feature a simple structure and rugged construction. Thermocouple sensors measure temperature based on the Seebeck effect. Two dissimilar conductors are connected to form a closed loop. A tiny voltage difference develops in the loop whenever the temperatures at the two junctions differ. This voltage difference is known as thermal voltage or thermoelectromotive force. Its magnitude depends on the materials of the two conductors, as well as the temperature difference between the "measuring junction" (the joint of the two conductors) and the "cold junction" (the open‑circuit ends of the conductors). For this reason, thermocouples are generally only used for temperature‑difference measurement. The absolute temperature at the measuring junction can be obtained if the cold‑junction temperature is known, or after separate temperature measurement and compensation. Standards IEC 60584 and ASTM E230/ANSI MC96.1 list common thermocouple conductor material combinations and their corresponding thermal‑voltage‑to‑temperature characteristics.
