Glossary · Sensors and measurement
Resistance temperature detector (RTD)
Also known as: RTD, Pt100, Resistance thermometer
German: Widerstandsthermometer (RTD)
In temperature measurement, a resistance temperature detector (RTD) is a sensor whose electrical resistance changes with temperature in a known way; the most common type is the platinum sensor Pt100 with 100 ohms at 0 °C, specified in IEC 60751.
- Sensors
- Standards
In one sentence
An RTD measures temperature by the change in electrical resistance of a metal element, most commonly a platinum Pt100 per IEC 60751.
Example
A Pt100 in a thermowell measures the jacket temperature of a reactor and is wired in a four-wire circuit to a head-mounted transmitter.
How it applies
- Engineering: RTDs offer good accuracy and stability in a moderate temperature range. IEC 60751 defines tolerance classes. Two-, three- and four-wire connections differ in how well they compensate lead resistance; four-wire is the most accurate.
- Installation: Thermowells protect the sensor but add response time; immersion length affects the measurement error.
- Maintenance: Check sensors against a reference at defined intervals, especially where the value affects product quality or safety.
- Documentation: Specify sensor type, tolerance class and connection method in the instrument data sheet and circuit diagram. A mismatch between the wiring and the transmitter setting produces a constant offset that is easy to overlook.
RTD vs. thermocouple
An RTD is more accurate and stable in the lower and medium temperature range. A Thermocouple covers higher temperatures, responds faster and is more robust, but is less accurate and needs cold-junction compensation.