Thermocouples

Temperature from Voltage

There are two principles or effects that allow us to determine temperature from voltage. The Seebeck effect and the Thomson effect. William Thomson, later Lord Kelvin, discovered that heat can be absorbed or released in a conducting element depending upon the direction of the current. This could also be stated as electrons will diffuse with the differential heating of an element, and this diffusion is specific to the conductor's composition.

Thomas Seebeck discovered that if two dissimilar conductors were coupled and heated at one junction a potential would develop between the ends of the conductor. Seebeck investigated the relationships of different conductors to each other and found an organization much like static charge's triboelectric series.

Conductors further from each other in the Seebeck series evolve larger voltages across them, and current always flows down according to the materials' relative positions in the series.

The proportional change of voltage with temperature across a thermocouple allows us to determine temperature at the joint: V = αΔT where α is the Seebeck coefficient, measured in Volts/Kelvin. A linear increase in voltage with temperature can be assumed for temperature changes within several hundred Kelvin.

The high temperature reading above illustrates an important point- Very slight changes in the composition of conductors can translate to considerable variations in α. This has led to the adoption of eight standard thermocouple combinations that allow temperature measurements to be taken in different ranges at varying sensitivity. It is possible to use the simple Seebeck coefficent equation to determine temperature with these thermocouples. However, the voltage temperature relationships of these alloys have been thoroughly investigated and documented, so it is more common to obtain temperature from a lookup table.