WIKA TR10-L Manual De Instrucciones página 15

4. Design and function
4.4 Resistance thermometers
For specifications, see WIKA data sheet TE 60.12 (TR10-L) / TE 61.01 (TR10-0) and the
technical information IN 00.17 "Usage limitations and accuracies of platinum resistance
thermometers per IEC 60751".
4.5 Thermocouples
4.5.1 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
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these original 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. Type J and T thermocouples 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 [1,472 °F] to
1,050 °C [1,922 °F], considerable changes of the thermoelectric voltage can occur. The
cause of this is a chromium depletion or the chromium 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 atmos-
phere), 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 silicon content.
Here, a protective oxide layer forms on its surface under the same conditions.
WIKA operating instructions, models TR10-L, TC10-L, TR10-0, TC10-0 (Ex d)
EN
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