Fundamentals of Thermocouples

Simply put, a thermocouple converts a temperature reading into a voltage reading. Thermocouple types differ by metallic composition and are designated by a single letter such as J, T, or K. With a known thermocouple type and a measured voltage from DAQ hardware, the temperature can be calculated.

A thermocouple is created when two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermo-electric voltage is known as Seebeck voltage, named after Thomas Seebeck, who discovered the phenomenon in 1821.


An ice bath is sometimes used as a known temperature for calibration. The metal-metal junction at the DAQ hardware acts much like the metal-metal junction on the thermocouple and thus produces a voltage. This extraneous voltage is accounted for when some source of compensation is used, such as the ice bath. This is know as cold-junction compensation, which may be provided internally in thermocouple specific National Instruments products.

Below is a chart that relates the voltage output of the thermocouple to the temperature. It is not a linear relationship, however may be approximated as one. This voltage is amplified by signal conditioning equipment and then input to the computer through the DAQ card, thus turning a real life temperature measurement into a digital value on a computer that can be analyzed and presented.

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