Thermocouples

A thermocouple is a devise which comprises of two conductors made from dissimilar metals these generate a very small voltage known as contact potential. The voltage generated varies with temperature. They are available in a variety of types dependant on conductor material. Each combination of conductor material has a different temperature range and output voltage. Thermocouples are available mounted in various housing to suit specific applications.

Principles of operation

In the early part of the ninetieth century Thomas Seebeck, an Estonian-born German physicist, discovered that in a circuit made with two dissimilar conductor materials, a voltage would be generated whenever there was a difference in temperature between the two junctions.

In the top diagram, left, a voltage will be generated if the right-hand junction is heated. The voltage will be proportionate to the temperature, so the hotter the junction the higher the voltage. It’s not quite a linier relationship, but very close. If the voltage were measured and converted to a temperature, the figure arrived at won’t be the actual temperature, it will be the difference in temperature between the hot and the cold junction. So if the cold junction is at an ambient of 22°C and the hot junction at 100°C then the voltage reading will correspond to 78°C.

To make Seedbeck’s discovery more practical, the cold junction needs to be at a know temperature. Looking at the centre diagram, using 0°C as a reference for the cold junction means that the temperature corresponding to the voltage generated by the hot junction will be an absolute measurement.

Rather than having bowls of ice water slopping around all over the place, modern instruments contain electronics to replicate the cold junction which makes life a whole lot less complicated as shown in the lower diagram. Whilst the built in cold junction compensation makes life easer in some respects, it makes it a little more complicated in others. You will notice that the system using the water bath uses copper wires to connect to the volt meter. This is normally bad practice when connecting thermocouples as again you have a junction of dissimilar metals which will also generate a voltage if their temperature differs.

In the centre diagram the use of copper wires is possible because both cold junctions are at the same temperature in the water bath.  Where the cold junction compensation is located in the instrument it is necessary to use the same, or very similar, conductor materials for the wiring between the thermocouple and the instrument.