Thermocouple Basics

General description

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:

  1. Thermocouples measure their own temperature.
  2. 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
  3. Beware of electrical hazards using thermocouples, they are electrical conductors. RTD's are less sensitive to electrical noice.
  4. Thermocouples DO NOT MEASURE AT THE JUNCTIONS! They can't, it is physically impossible to have a temperature gradient at a point.
  5. The distance between thermocouple and heater element will generate a thermal lag which can be compensated by the temperature controller.