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Thermocouple

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.

Basic thermocouple circuit

Other elements in a closed loop heating cycle:

Thermocouples are classified with letters and are color coded. The most common types are J (black)-, K (yellow)- and T- (blue) type. The wire color indicates what type of thermocouple it is. (example: white + red wire insulations are used for J-type thermocouples; the colors of jacks and plugs for a J-type thermocouple is black.

Thermocouple color codes:

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.

Application
The advantages of thermocouples like self powered, simple, inexpensive and useful for wide temperature ranges often times overcome the disadvantages like non-linear, least sensitive and least stable.

High temperature thermocouples are defined as sensors used at temperatures of 2000degF and beyond. These sensors are generally made from noble metals or refratory ceramics (Molybdenum, Tantalum, Alumina, Circonia or Quartz).This type of thermocouples can measure temperatures up to 5000 degF and are used in high temperture furnaces, rocket engines and amunitions. Thermocouples made out of noble materials are expensive. It may make sense to use instead non-contact temperature measuring methods like infrared sensors. (also called Pyrometer, Spectralmeter)

Thermocouples must be selected to meet application conditions. Selection considerations are:

  • Temperature (select type J, K, M, T or other)
  • Response time (larger diameter reduce response time)
  • Service life (larger diameter increases service life)

There are hugely different styles of thermocouples available for different industries like plastic, pharmaceutical, food. For example: the plastic industry prefers cost effective thermocouples without MgO, spring adjustable and bayonet cap style for quick install. Other industries do like thermocouple assemblies consisting of thermo-elements swaged in hard packed Magnesium Oxide (MgO) encased in an annealed metal sheath so they can be formed and bent into a radius twice the size of its outer sheath.


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