Over the years, the development of temperature measurement technology has produced numerous different measurement methods for temperature measurement. Mineral insulated temperature sensors as sheathed thermocouples belong to the group of temperature sensors, which cover a wide range of applications as proven resistance thermometers.
Function Principle Thermocouples
With thermocouples, different metals are joined together at a certain distance, usually by soldering joints. One connection point represents a measuring point, the other serves as a reference point. If different temperatures prevail at these two connection points, an electric thermoelectric voltage is generated between these contact points, which allows a small but measurable current to flow. This phenomenon is also known as the Seebeck effect.
Cause of the thermoelectric emf
Metals as chemical raw materials consist of atoms in whose atomic shells the electrons move on different paths around the atomic nucleus. The outer shell of each atomic shell is occupied by valence electrons. These valence electrons are bound to the atomic nucleus with a very specific binding energy, whereby the size of the binding energy varies for different metals. If this binding energy is overcome by external energy input in the form of heat, valence electrons of the metal with the smaller binding energy at the solder joint can jump over to the metal with the larger binding energy. There an excess of electrons is created, and the metal with the lower binding energy has an electron deficiency. The resulting potential difference is the cause of the thermoelectric emf, on which the functional principle of a thermocouple is also based.
Physical details on the magnitude of the thermoelectric emf
The Seebeck coefficient (S), which is specified in microvolts/Kelvin (µV/K), plays a decisive role for the magnitude of the thermoelectric emf. The product of the Seebeck coefficient multiplied by the temperature difference gives the theoretical thermoelectric emf in Kelvin.
The size of the Seebeck coefficient is given for different metals in relation to platinum (Pt). The base temperature is T = 273 K (0 °C).
Constantan = -35 µV/K
Nickel = -15 µV/K
Aluminium = 3.5 µV/K
Copper = 6.5 µV/K
Silver = 6.5 µV/K
Nickel-chromium = 25 µV/K
However, the physical relationship between temperature and thermoelectric voltage is not linear, but approximates the shape of a parabola.
Uth = a * T + b * T2
The coefficients a and b here are thermoelectric material constants. The temperature difference is indicated here with T, at a reference temperature of T0 = 273 K (0 °C).
Types and applications of thermocouples
Manufacturers of thermocouples differentiate between types K, N, J, E, T, R, S and B. These types are intended for different application and temperature ranges and are sorted according to the following definition:
Type K: NiCr-NiAl thermocouples are suitable for oxidizing or protective gas atmospheres up to 1200 °C.
Type J: Fe-CuNi thermocouples are used in vacuum, in inert gas atmospheres and for oxidation and reduction applications up to 750 °C.
Type N: NiCrSi-NiSi thermocouples are used in oxidizing atmospheres, inert gas atmospheres and dry atmospheres up to 1200°C. The accuracy at high temperatures is very high. However, the atmospheres must not contain sulphur.
Type E: NiCr-CuNi thermocouples can be used in oxidizing or inert gas atmospheres up to temperatures of 900 °C. Type E achieves the highest thermoelectric voltage of all thermocouples.
Type T: Cu-CuNi thermocouples can also be used at temperatures below 0 °C up to 350 °C in oxidizing, reducing and inert gas atmospheres. They are also insensitive to rust in humid atmospheres.
TYPE S: Pt10%Rh-Pt thermocouples are suitable for continuous use in oxidizing or inert gas atmospheres up to 1600 °C. Impurities must be avoided as they cause brittleness.
Type R: Pt13%Rh-Pt thermocouples can be permanently used in oxidizing or inert gas atmospheres up to 1600 °C. Contaminating influences must be avoided to avoid embrittlement.
Type B: Pt30%Rh-Pt6%Rh thermocouples can be permanently used in oxidizing or inert gas atmospheres up to 1700 °C. In the short term, they can also be used in vacuum environments. Contaminating influences must be avoided to avoid embrittlement.
Heating conductors and heating elements must operate in the correct temperature range in practical applications. Thermocouples often serve here as monitoring and control elements and are successfully used for such purposes.