TM131

The working principle of Resistance Temperature Detector (RTD) is temperature measurement based on the characteristic that the resistance of metallic conductors varies with temperature. The core principle: when temperature changes, the lattice‑vibration amplitude of metallic conductors such as platinum and copper alters, which results in varying obstruction to electron movement and a corresponding change in resistance. Taking Pt100 platinum RTD as an example, its nominal resistance is 100 Ω at 0 °C; resistance increases by approximately 0.385 Ω per 1 °C temperature rise, 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 / four‑wire connection is adopted to eliminate lead‑wire errors. Finally, resistance values are converted into temperature values according to international standard reference tables. Thermocouples feature simple structure and high robustness. Thermocouple sensors measure temperature based on the Seebeck effect. Two dissimilar conductors are connected to form a closed loop. A tiny voltage difference will be generated in the loop provided that temperatures at two junctions differ. This voltage difference is known as thermoelectric voltage or thermoelectromotive force. Its magnitude depends on the conductor materials and the temperature difference between the “measuring junction” (junction of the two conductors) and the “cold junction” (open ends of the conductors). Therefore, thermocouples are normally used for differential‑temperature measurement. Absolute temperature at the measuring junction can be obtained if the cold‑junction temperature is known, or measured separately for compensation. IEC 60584 and ASTM E230/ANSI MC96.1 standards list common thermocouple conductor material combinations and corresponding thermoelectric‑voltage‑versus‑temperature characteristics.

High‑end Application

RTD or thermocouple thermometers are available with integral thermowell, or for installation into field‑mounted thermowells. Optionally they can be fitted into modular transmitters supporting universal communication protocols and Bluetooth® technology. Especially with iTHERM QuickNeck for tool‑free recalibration, vibration resistance, fast sensor response time and thermowell technologies (iTHERM StrongSens, QuickSens), along with explosion‑proof approvals, the devices deliver enhanced measurement accuracy and reliability.