Thermoelectric voltage series

To attend a thermocouple temperatures to measure the need for electric thermal stresses causal thermal forces of the different materials used, usually are the metals or alloys , to be known.

The thermoelectric voltage of a thermocouple that can be achieved at a given temperature difference is greater, the greater the distance between the metals in the thermoelectric series. The thermoelectric voltage of a thermocouple results from the temperature difference and the difference in the thermal forces of the two materials used. The thermal force is a temperature-dependent material constant.

The thermal stresses of the materials are given as a first approximation as a proportionality factor compared to a reference material. As a constant factor, it is assumed that there is only a slight and in practical applications negligible dependence of the proportionality factor on the absolute temperature, which is guaranteed for certain materials and limited temperature ranges. This factor, then called the proportionality constant, with the dimension mV / K, is called thermal sensitivity, k-value or k-factor . In the thermoelectric series, they are specified in the same way as the electrical series, relative to platinum , based on a temperature difference of 100 Kelvin. These k values ​​of the materials apply to the normal temperature of 273  K (0 ° C).

material	α in µV / K at 273 K.
Bismuth	−72
Constantan	−35
nickel	−15
potassium	−9.0
sodium	−2.0
platinum	0
mercury	0.6
carbon	3
aluminum	3.5
lead	4.0
Rhodium	6th
copper	6.5
gold	6.5
silver	6.5
cadmium	7.5
iron	19th
Nichrome	25th
antimony	47
Germanium	300
silicon	440
Tellurium	500
selenium	900

Non-linear thermal voltages at low temperatures

The thermal voltages of specific thermocouples are often almost linearly dependent on the temperature over certain ranges of the absolute temperature, but there are sometimes strong deviations for other ranges. For example, the value of NiCr / Ni thermocouples (type K) in the range from 0 to 800 ° C is 4… 4.26 mV / 100 K, but drops sharply towards lower temperatures and is only 1 at −250 ° C mV / 100 K as shown in the diagram on the right with the pink gradient.

But non-linearities do not only occur at low temperatures. For example, Pt / PtRh10 thermocouples (type S) have a value of around 0.65 mV / 100 K at room temperature, which increases to values ​​of 1.2 mV / 100 K at 1000 ° C.

In the linear range of constant k values, this is possible by restricting the temperature range to a sufficiently narrow range, the thermal voltage U zu results

${\displaystyle U=(k_{\mathrm {a} }-k_{\mathrm {b} })\cdot (T_{1}-T_{2})}$

With

${\displaystyle k_{\mathrm {a} },k_{\mathrm {b} }}$ - k values ​​of the two metals "a" and "b"

and

${\displaystyle T_{1},T_{2}}$ - Temperatures of the two joints of the materials.

Due to the temperature-dependent k-values, thermocouples do not provide a temperature-proportional voltage signal over a larger temperature range. This non-linearity must be taken into account or compensated for for precise measurements. There are tables for common thermocouple combinations in which the thermal voltages for each temperature can be read in 0.1 Kelvin steps. The temperature can be determined using these tables or the non-linearity is taken into account using empirical formulas that only apply to certain areas. Tables (basic values ​​of thermal voltages ) can be found in the standard IEC 60584 Part 1 or from the manufacturers of the thermocouples.

See also

• Thermoelectricity

Web links

Commons : Thermocouple diagrams  - collection of images, videos and audio files

Individual evidence

1. ^ Bernhard Frenzel, Florian Gebhard: Physics formula collection . Springer DE, 2009, ISBN 3-8348-0875-X ( limited preview in the Google book search).
2. ↑ Seebeck Coefficients.