Cambridge University Logo
Department of Materials Science and Metallurgy

Michele Scervini's Research Activity
Thermocouples: Operating Principle
Thermocouple configurations
Thermocouples in Gas Turbine
Drift: a short explanation
Drift in Type K bare wire thermocouples
Drift in type K MIMS thermocouples
HEATTOP project
Link to other websites
Michele Scervini's Contacts


Type K thermocouple is a Ni base thermocouple introduced by Hoskins Manufacturing at the beginning of the 20th century: the positive and negative thermoelements were called Chromel and Alumel respectively. Table3 reports indicative compositions for Chromel and Alumel.

Being Type K thermocouple defined according to the voltage versus temperature relationship, there is no need to use necessarily Chromel and Alumel compositions: other manufacturers produce positive (KP) and negative (KN) thermoelements with different compositions, which comply with the type K voltage versus temperature relationship.

Elements Chromel (wt%) Alumel (wt%)
Ni 90 95
Cr 10 -
Mn - 3
Al - 2
Table3: Approximate composition for Chromel and Alumel thermoelements.


All the manufacturers of KP and KN alloys have selected compositions in order to have single phase alloys from ambient temperature to the solidus line: this prevents phase transformations along the thermoelements. Phase transformations in an alloy can produce changes in the existing phases and changes in their compositions: the Seebeck coefficient is dependent on composition and crystallography and for this reason single phase thermoelements are preferential choice. Figure13 is showing composition of Chromel on the Ni-Cr phase diagram.

  Ni-Cr phase diagram  
Figure13: Ni-Cr phase diagram (adapted from [1]).


The composition of the type K thermoelements has been selected in order to have a thermocouple with high Seebeck coefficient: this may be achieved coupling a positive thermoelement with positive Seebeck coefficient with a negative thermoelement with negative Seebeck coefficient. Pure Ni has a negative Seebeck coefficient, but alloying Ni with other elements can result in an alloy with a positive Seebeck coefficient: this is the case of alloying Ni with Cr. Figure14 shows the effect of different Cr contents on the Seebeck coefficient of Ni: a maximum value for the Seebeck coefficient is obtained for Cr content of about 10at%, which matches the Chromel composition.

The type K negative thermoelement is instead designed to contain about 5% of alloying elements selected to have a negative Seebeck coefficient over the whole temperature range.

  effect of Cr on the Seebeck coefficient of Ni  
Figure14: Effect of Cr on the seebeck coefficient of Ni (Data adapted from [2]).


When type K bare wires thermoelements are exposed in air, oxidation takes place. Let us consider the effect of oxidation on the positive thermoelement: in a Ni-Cr alloy, Cr is oxidized preferentially forming Cr rich oxides. In Chromel the Cr content of the alloy is not enough to form a continuous external chromium oxide: an external Ni oxide is formed with an underlying Cr rich oxide layer. Figure15 shows the oxide layers formed after 110h at 1200°C.

The content of Cr in the Cr oxide is higher than the Cr content of the alloy: a depletion of Cr from the base metal occurs in order to form Cr rich oxides and as a result the Cr content in the base metal and the Seebeck coefficient are lowered (see Figure14). Figure16 shows the Cr depletion in the base metal as a result of oxidation: the depletion affects deeper and deeper layers of base metal as the exposure time increases.

  Oxide layers on Chromel after 110h at 1200C  

Figure15: Scanning Electron Microscope Images of KP thermoelements cross section exposed in air at 1200°C for 110h; the image of the cross section of the thermoelements (top left) is shown along with elemental maps for Oxygen (top right), Nickel (bottom left) and Chromium (bottom right).

  Cr depletion as a result of oxidation  
Figure16: Cr depletion in a Chromel wire as a result of oxidation in air. (Data adapted from [3])


Chromel is affected by a short range order transformation occurring at temperatures between 200°C and 600°C; in this temperature range locally oredered atomic arrangements are developed, which produce an increase in the Seebeck coefficient, as shown in Figure17: the change is time dependent and at each temperature it increases with time till a maximum value. The change is maximum at about 400°C, while above this temperature it decreases. Above 600°C the positive thermoelement is completely disordered.

When the junction of a type K thermocouple is exposed at temperature above 600°C, a portion of the positive thermoelement will undergo the short range order transformation and drift will take place.

  short range order  
Figure17: Short range order transformation in Chromel (Data adapted from [4]).



[1] ASM Handbook Volume 3 "Alloy phase diagrams" (1992) ASM International

[2] M.V. Vedernikov, N.V. Kolomoets, Soviet Physics-Solid State 2 (1961) 2420

[3] N.A. Burley, "Solute depletion and thermo-E.M.F. drift in nickel-base thermocouple alloys". Journal of the
Institute of metals
, 97 252-254 (1969)

[4] A.W. Fenton, "Errors in thermoelectric thermometers". Proceedings of the Institution of the Electrical
116 1277-1285 (1969)


Previous Page Next Page

Last update: August 31st 2009
© Michele Scervini 2009