Thermocouple temperature sensor

Thermocouple temperature sensor operating principle

A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals joined at one end. When the junction of these metals is heated or cooled, a current is established in the circuit, generating an electromotive force (EMF) which depends on the nature of the wires used.
The high-quality metals used are homogeneous, allowing the thermocouple to be subjected to different temperature gradations along its length without disturbing the electromotive force.
Measuring devices receive the thermocouple directly and handle the cold junction themselves.
These thermocouple alloys are generally available in wire form.

There are many models and technical specifications. It is extremely important to understand the basic structure, operating mode and temperature ranges to determine the type and material of the thermocouple best suited for your application.

There are different types of thermocouples:

Platinum thermocouple type S
Nickel alloy thermocouple type K
Type T, J and N thermocouples

Thermocouple temperature sensor installation and wiring

Polarity must be respected, otherwise additional parasitic EMFs may occur.
Use cables specially adapted to thermocouples, so-called "extension" or "compensation" cables.

Isolation

Thermocouples are often exposed to "harsh" atmospheres, subject to electrical, electromagnetic and other types of interference.
To compensate for the temperature measurement disturbances created by these parasites, metal-sheathed thermocouples are supplied with an insulation resistance between the wires and the protector. Rigorously tested at the factory, this insulation resistance is of the order of 1000 to 5000 MW minimum at 100 to 500Vdc depending on the thermocouple diameter and at ambient temperature, for a thermocouple less than 1 meter long, in compliance with the standard.
These resistance values decrease when the thermocouple is subjected to high temperatures, until they become almost zero at very high temperatures.
For this reason, we recommend measuring instruments with galvanic separation of input and output, or input and output and power supply.