How a thermocouple works
Two different metals joined at the measuring point produce a small voltage that depends on the temperature difference between that junction and the reference (cold) junction at the instrument. Thermocouple types are defined in IEC 60584 by their metal pairs.
Common types at a glance
| Type | Metals | Approximate upper range* | Typical uses |
|---|---|---|---|
| K | Nickel-chromium / nickel-aluminium | about 1200 °C | General purpose: furnaces, flue gas, process |
| J | Iron / copper-nickel | about 750 °C | Plastics, older equipment, reducing atmospheres |
| T | Copper / copper-nickel | about 350 °C | Low and sub-zero temperatures |
| N | Nicrosil / nisil | about 1250 °C | More stable alternative to K at high temperature |
| R and S | Platinum-rhodium / platinum | about 1600 °C | Steel, glass, heat treatment, reference sensors |
| B | Platinum-rhodium / platinum-rhodium | about 1700 °C | Very high temperatures: steel melting, kilns |
*Practical limits depend on wire diameter, sheath, atmosphere and required life. Check the specification for your application.
Base metal vs noble metal
Base-metal types (K, J, T, N) are economical and widely used. Noble-metal types (R, S, B) cost more but are stable at high temperatures where base metals drift or fail.
Construction: mineral insulated and thermowells
Mineral-insulated (MI) thermocouples have wires packed in compacted magnesium oxide inside a metal sheath — robust, bendable and fast-responding. A thermowell protects the sensor from pressure, flow and corrosion and lets it be replaced without opening the process. Ceramic protection tubes are used at very high temperatures.
Extension and compensating cables
Connect thermocouples to instruments with cable of the matching type; ordinary copper cable introduces errors at the junctions. Extension cables use the same alloys as the thermocouple; compensating cables use alloys with matching output over a limited temperature range.
Thermocouple or RTD?
RTDs are more accurate and stable at moderate temperatures; thermocouples cover much higher temperatures and harsh conditions. See our RTD vs thermocouple comparison.
Frequently asked questions
Why does my thermocouple read wrong?
Typical causes are the wrong cable type, reversed polarity, drift after long high-temperature use, or a damaged sheath. Calibration against a reference sensor confirms the reading.
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