WIKA TR Série Mode D'emploi page 9

3. Specifications
3.2 Thermocouples
3.2.1 Sensor types
Model Recommended max. operating temperature
K 1,200 °C
J 800 °C
E 800 °C
N 1,200 °C
3.2.2 Potential measurement uncertainties
Important factors which counteract the long-term stability of thermocouples.
Ageing effects/poisoning
Oxidation processes in thermocouples which are not appropriately protected ("bare"
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thermocouple wires) result in falsifications of the characteristic curves.
Foreign atoms (poisoning) that diffuse into the original alloys lead to changes of these original
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alloys and thus falsify the characteristic curve.
The influence of hydrogen leads to the embrittlement of the thermocouples.
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The Ni leg of the type K thermocouple is often damaged by sulphur which is contained in exhaust
gases, for example. Thermocouple types J and T age slightly, as the pure metal leg oxidises first.
In general, rising temperatures cause accelerated ageing effects.
Green rot
If type K thermocouples are used at temperatures from approx. 800 °C to 1,050 °C, considerable
changes of the thermoelectric voltage can occur. The cause of this is a chromium depletion or
the chrome oxidation in the NiCr leg (+ leg). The precondition for this is a low concentration of
oxygen or steam in the immediate environment of the thermocouple. The nickel leg is not affected
by it. The consequence of this effect is a drift of the measured value caused by decreasing
thermoelectric voltage. This effect is accelerated if there is a shortage of oxygen (reducing
atmosphere), since a complete oxide layer, which would protect it from further oxidation of the
chromium, cannot be formed on the surface of the thermocouple.
The thermocouple is permanently destroyed by this process. The name green rot is derived from
the greenish shimmering colouration on the breaking point of the wire.
The thermocouple type N has in this regard an advantage due to its silicium content. Here, a
protective oxide layer forms on its surface under the same conditions.
K effect
The NiCr leg of a type K thermocouple has an ordered alignment with respect to the alignment
in the crystal lattice below approx. 400 °C. If the thermocouple is heated further, a transition to
a disordered state occurs in the temperature range between approx. 400 °C and 600 °C. Above
600 °C, an ordered crystal lattice is restored.
WIKA operating instructions resistance thermometers, thermocouples
EN
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