What is a Thermocouple?

Thermocouples are the most common sensor used for exhaust gas temperature measuring. Consisting of a pair of dissimilar metal wires joined at one end, producing a net thermoelectric voltage when measured between the open pair.

In other words; A thermoelectric device creates a voltage when there is a different temperature on each side. This value changes according to the size of the temperature difference between each end, and materials used of the wire pair and the uniformity of the wire-pair relative to the Seebeck coefficient. Thermocouples measure a relative temperature not an absolute temperature. In other words, a thermocouple requires a reference of known temperature. Commonly commercialized thermocouples utilize and independent sensor, such as a thermister to provide the reading of the reference temperature fro compensation of the cold junction.

The use of these materials has been proven to be the best material suited for Exhaust Gas Temperature sensing, providing durability and accuracies that are acceptable.

The discovery of thermoelectricity is accredited to Thomas Johann Seebeck;(1770-1831) Estonia, In 1821 he discovered that a compass needle deflected when placed near a closed loop formed from two dissimilar metal conductors if the junctions were maintained at different temperatures. He also observed that the deflection of the compass needle was proportional to the temperature difference. Subsequently he described that it depended on the type of conducting materials used, and not on the temperature distributed along the conductors. The Seebeck coefficient is defined as the open circuit voltage produced between two points on a conductor, where a uniform temperature difference of 1K Ohms exists between those points.

This effect is non-linear and depending on the materials used for creating the junction result in different curves. Materials have attributes that make them better suited for various temperature ranges. So the combination of materials over the course of time has produced standards for certain temperature ranges for example type K is made up of Chromel{90 percent nickel and 10 percent chromium} and Alumel (consisting of 95% nickel, 2% manganese, 2% aluminium and 1% silicon)