Thermocouples and Infrared Sensors
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. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve. Nearly all thermocouples are custom made with lead times of one to two days.
Other elements in a closed loop heating cycle:
- temperature controller
- heating element
- electrical power switching
Thermocouples are classified with letters and are color coded. The most common types are J (black)-, K (yellow)-, S & R (green) 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.
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.
- 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.
- 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 2000 degF and beyond. These sensors are generally made from noble metals. These types of thermocouples can measure temperatures up to 4200 degF and are used in high temperature laboratory and industrial furnaces. Thermocouples made out of noble materials are expensive. Thermocouples must be selected to meet application conditions. Selection considerations are:
- Temperature (select type J, K,B, S, R, C or other)
- Response time (larger diameter reduce response time)
- Service life (larger diameter increases service life)