In brief
A thermocouple is a temperature sensor made of two joined metals that generates a temperature-dependent thermoelectric voltage through the Seebeck effect.
Contents
What is Thermocouple?
A thermocouple consists of two wires of different metals joined at the measuring point. If there is a temperature difference between the measuring junction and the reference junction, a small voltage is produced, from which the evaluation unit calculates the temperature. Thermocouples need no auxiliary power, respond quickly and are suitable for high temperatures, for example in furnaces, hardening shops, glassworks or steelworks. What is measured is always the temperature difference; the temperature of the reference junction is therefore recorded and compensated in the evaluation.
The pairings are standardised as types designated by letters, with reference values and tolerances governed by IEC 60584. Common types are type K (nickel-chromium/nickel), type J (iron/copper-nickel), type N and precious metal types such as S, R or B for very high temperatures. Designs include mineral-insulated thermocouples, screw-in sensors with thermowell and versions with ceramic protection tubes. Suitable thermocouple or compensating cables are used for the connection to the evaluation unit.
Thermocouples age, especially at high temperatures, and drift as a result. Other sources of error are incorrect compensating cables, reversed polarity and damaged protection tubes. In high-temperature processes they are therefore regarded as wear parts. The achievable service life depends heavily on temperature, atmosphere and protection tube.
In practice: maintenance and procurement
For an order, the thermocouple type, tolerance class, design, diameter, insertion length, protection tube material, process connection and type of connection are needed. Where replacements are frequent, keeping a stock of measuring inserts of the same length is worthwhile.
Synonyms and abbreviations: TC · type K thermocouple · mineral-insulated thermocouple · MI thermocouple · thermocouple probe
Frequently asked questions
What types of thermocouple are there?
Several types are standardised and designated by letters. Type K is the most widely used general-purpose type, type J is often used at lower temperatures, and type N is regarded as more stable at higher temperatures. Precious metal thermocouples such as type S, R or B are used at very high temperatures.
What is a compensating cable for a thermocouple?
A compensating cable connects the thermocouple to the evaluation unit and has thermoelectric properties similar to those of the thermocouple itself within the relevant temperature range. A normal copper cable would generate additional voltages at the junctions and distort the measurement. Cable type and thermocouple type must match.
What information is needed to replace a thermocouple?
Needed are type, tolerance class, sheath or protection tube diameter, insertion length, protection tube material, process connection, terminal head or connector type and details of the measuring junction, such as insulated or grounded. The maximum operating temperature is also important, as it determines material and design. If the nameplate is missing, the colour of the connecting cable often helps to identify the type.