Glossary · Sensors and measurement
Thermocouple
Also known as: TC
German: Thermoelement
In temperature measurement, a thermocouple is a temperature sensor made of two different metal conductors joined at the measuring point; the temperature difference between the measuring junction and the reference junction produces a small voltage (Seebeck effect). Common types such as K, J, N and S are specified in IEC 60584-1.
- Sensors
- Standards
In one sentence
A thermocouple generates a small voltage from the temperature difference between two joined metals and covers wide ranges up to high temperatures.
Example
Type K thermocouples measure the zone temperatures of a heat treatment furnace, connected with compensating cable to the temperature controller.
How it applies
- Engineering: Thermocouples cover wide temperature ranges and are robust and fast. They require cold-junction compensation and thermocouple or compensating cables of the correct type up to the reference point.
- Installation: Wrong cable types or reversed polarity cause errors that look plausible. Grounded thermocouples may need isolated inputs.
- Maintenance: Thermocouples age at high temperatures and drift. Replacement intervals and checks against a reference help detect this. Many transmitters detect a broken thermocouple.
- Documentation: State the type, the tolerance class, the cable type and the reference junction location in circuit and instrument documentation. Mixing up types is a common fault after replacement.
Thermocouple vs. RTD
A thermocouple is suited to high temperatures and fast response. A Resistance temperature detector (RTD) is more accurate and stable at moderate temperatures and needs no cold-junction compensation.