Thomas Seebeck discovered in 1821 that when two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. (Seebeck effect). The junctions can be exposed, grounded or ungrounded. The thermocouple is normally directly connected to a standard temperature controller. Thermocouples are among the easiest temperature sensors used in science and industry and very cost effective. (usually less than $ 50.00) Most of the thermocouples are custom made with lead times of 24hrs if needed.
Other elements in a closed loop heating cycle:
A couple important things:
- Thermocouples measure their own temperature.
- Thermocouples can err in reading their own temperature, especially after being used for a while, or if the insulation between the wires loses its resistance due to moisture or thermal conditions
- Beware of electrical hazards using thermocouples, they are electrical conductors. RTD's are less sensitive to electrical noice.
- Thermocouples DO NOT MEASURE AT THE JUNCTIONS! They can't, it is physically impossible to have a temperature gradient at a point.
- The distance between thermocouple and heater element will generate a thermal lag which can be compensated by the temperature controller.