Resistances, which are used in obtaining electrical current by heating contact surfaces of 2 different metals (like iron and constantan etc.) are called as thermocouple.
Electrical current, which occur on thercouple, depends on temeperature of contact point and caused by different thermal and electrical features of metals. In other words, electrons, in high temperatured fields, have high thermic energy and these electrons move towards cold field. In consequence of this process, a voltage in microvolt or milivolt range occur in discharge ends.
They are elements, which are used for measuring of temeperature of hot environments (like, oven, cooker, boiler etc). In measuring of temperature of these kinds of environments, thermocouple based measuring circuit or resistance based (thermosresistance) measuring circuits are used. In termocouple based measuring system, there is a design which convert the heat to voltage. But in resistance based measuring systems, there are elements which resistance changes according to temperature. Resistance based measuring system is used mainly in measuring of lower temperatures (like -200 ile +850 °C).
Thermocouple element types and measuring ranges;
Cu-Const.......T type...-200..300'C Fe-Const.......J type...-200..800'C Cr-Al.............K type..-200..1200'C NiCr-Ni..........K type..-200..1200'C Cr-Const.......E type..-200..1200'C Nikrosil-Nisil.N type......0..1200'C Pt%10Rh-Pt..S type......0..1500'C Pt%13Rh-Pt..R type......0..1600'C Pt%18Rh-Pt..B type......0..1800'C Tn-Tn%26Re.W type......0..2000'C
Thermocouple types according to their shapes;
L - Type Thermocouples
Thermocouples with insert Portable type Thermocouples Mineral insulated Thermocouples Bayonet type Thermocouples