The measurement principle of a thermocouple (TC) is based on the contact of two different metals.
If two dissimilar metals are connected together, a signal in millivolt (mV) will be generated. This thermoelectrical voltage (electromotive force - EMF) is a result of the combined metals under change of temperature. Reference tables explain how to transfer the mV signal into degree Celsius.
Measuring the temperature by thermoelectric voltage is a comparison measurement between the hot end of the thermocouple and the cold junction. Thermocouples are standardized from -274 °C to +1800 °C.
- Thermocouples per EN 60584 or IEC 584
The thermocouples described in these standards are generally divided into two groups. The precious metal thermocouple types S (Pt10%Rh-Pt), R (Pt13%Rh-Pt) and B (Pt30%Rh-Pt6%Rh), and the base metal thermocouple types E (NiCr-CuNi), J (Fe-CuNi), K (NiCr-NiAl), N (NiCrSi-NiSi) and T (Cu-CuNi).
These standardized types are incorporated in many international standards and they are compatible regarding to their basic thermoelectric voltage (EMF values). For example, it is possible to use a type K acc. to EN 60584 as a type K acc. to ANSI-MC 96.1 or even as a type K acc. to JIS C 1602. The allowable tolerances, however, deviate from each other among the foresaid standards. Detailed information of the accuracy classes and their tolerances is described in the corresponding standards.
- Thermocouple Extension and Compensating Cables
It is often necessary to locate the cold junction of the thermocouple in a larger distance from the measuring point due to safety concerns or constructional reasons. For the connection between the two points, thermocouple extension and compensating cables are used.
The application range for these cables is limited in most standards to a temperature of -25 °C (-13 °F) to +200 °C (+400 °F), or depends on the temperature resistance of the insulation material used. The insulation material is to be selected in order to fulfill the requirements at site concerning chemical and heat resistance, moisture protection and other technical requirements.
Thermocouple extension cables:
These cables are made of thermocouple wires, solid or stranded conductors, which have the same nominal composition as the corresponding thermocouple. The TC extension cables are characterized by the letter "X" (extension) as a suffix to the thermocouple type (e.g. type JX).
For cost reasons, especially for precious metal thermocouples, it is economical to use another, less costly material for the connection between the thermocouple and the cold junction. These substitute alloys have the same thermoelectrical properties over a limited temperature range as the corresponding thermocouple.
The compensating cables are characterized by the letter "C" (compensation) which is added to the TC type (e.g. type KC). If there are different substitute alloys allowed as a compensating cable, the wire code is amended with letter "A" and "B" (e.g. type KCA or type KCB).
The measurement principle utilized in a Resistance Temperature Detector is the temperature dependence of the resistance of platinum. You will find additional information on Resistance Temperature Detectors by clicking on the following link: Resistance Temperature Detector (RTD)