3 RTD temperature probe
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3.2 Operating principle
RTD temperature probes take advantage of the temperature-dependent change in the resistance of the
metals. For PTC resistors (PTC; positive temperature coefficient) such as platinum, the resistance in-
creases as the temperature rises.
For thermistors (NTC; negative temperature coefficient), however, the resistance decreases as the tem-
perature rises.
For platinum sensors, the characteristic line has been defined between -200 °C and +850 °C according
to DIN EN 60751. (However, the standard stipulates that limit deviations are only defined up to max.
660 °C.)
As a result, resistance measurements taken using the connection lines and the measuring surface of the
temperature sensor allows conclusions to be drawn about the temperature, provided that the exact re-
lationship between temperature and resistance has been established.
For PTCs, the nominal value is taken to be the resistance at 0 °C.
Example:
Resistance/ohm
Temperature/°C
Resistance at 0 °C Nominal value Sensor designation
100 Ω 100 Pt100 temperature sensor
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