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The Resistance Temperature Detector (RTD) or resistance thermometer uses the fact that the resistance of metals increases with temperature. Examples are RTD's are shown schematically below.

Resistance Temperature Detectors

Further Information
The resistance of commercially available RTDs ranges from 10 to 25,000 W. More common ones are 100, 200, and 1000 W strain-free platinum (>99.999%) probes and 10 W copper probes. Generally, the higher the resistance, the less affected the RTD will be due to small resistance/voltage fluctuations in the lead wires and circuit.

Common metals used in RTDs include platinum, copper, nickel, BalcoTM (70% Ni-30% Fe), and tungsten. Their temperature ranges are listed in the following table.

Material Temperature
Platinum (Pt) -260~1000 °C
(-440~1800 °F)
< 550 °C (1022 °F) in most applications
Copper (Cu) -200~260 °C
(-330~500 °F)
Nickel (Ni) -200~430 °C
(-330~800 °F)
Linearity is not good
(70% Ni-30% Fe)
-100~230 °C
(-150~450 °F)
Linearity is not good; cheap to fabricate; high resistance
Tungsten (W) -100~1200 °C
(-150~2200 °F)
Pros and Cons

  • Pros:
  - Stable and accurate.
  - Linearity is better than thermocouples.
  - Higher signal-to-noise ratio.
  • Cons:
  - More expensive.
  - Self heating.
  - Requires a current source.
  - Response time may not be fast enough for some applications.