Temperature Measurement · Glossary

What is a Thermocouple?

two dissimilar metalsTemperature Measurement

A thermocouple is two different metal wires joined at one end; it produces a small voltage that depends on the temperature difference between its ends.

Thermocouple Explained

The voltage comes from the Seebeck effect along the wires. It depends on the temperature of the hot (measuring) junction and the cold (reference) junction, so cold junction compensation is needed.

Thermocouples are rugged, cheap and cover from cryogenic to about 1700 °C. They are less accurate and stable than RTDs.

Thermocouple Formula

E ≈ S × (T_hot − T_cold)

E	output voltage
S	Seebeck coefficient (about 41 µV/°C for type K near room temperature)

Example: Accurate conversion uses the IEC 60584-1 reference tables, since S changes with temperature.

Calculators for Thermocouple

CalculatorThermocouple CalculatorFree online calculator with formula and worked example.CalculatorThermocouple TableFree online calculator with formula and worked example.CalculatorRTD and Thermocouple Tolerance CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadTemperature Measurement Cheat SheetFree printable PDF cheat sheet.Free downloadRTD and Thermocouple Calibration SOPFree step by step procedure, Word and PDF.

Frequently Asked Questions

What is a Thermocouple?

A thermocouple is two different metal wires joined at one end; it produces a small voltage that depends on the temperature difference between its ends.

Is a thermocouple polarity sensitive?

Yes. Reversed wires make temperature changes read in the wrong direction.

Why does a thermocouple read room temperature when open?

Many transmitters detect an open circuit and drive the output to a fault value; without detection the reading can float.

Learn more on the blog

Thermocouple and RTD installation precautions

A practical guide on instrumentationblog.in with more detail on thermocouple and how it is applied on real plants.

Read Full Article

More Temperature Measurement Terms

Temperature termThermocouple Tolerance ClassThermocouple tolerance class is the permitted deviation of a new thermocouple from its reference table,…Temperature termThermocouple Extension CableThermocouple extension cable carries the thermocouple signal to the instrument using wires that match the…Temperature termMineral Insulated CableMineral insulated cable has sensor wires packed in compressed magnesium oxide inside a metal sheath; it…Temperature termThermowellA thermowell is a closed end tube fitted into the process that protects a temperature sensor and allows…Temperature termWake FrequencyWake frequency is the frequency of vortices shed by flow past a thermowell; if it approaches the well…Temperature termInsertion LengthInsertion length is the length of a thermowell or sensor that projects into the process from the mounting…

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