WIKA TR10-L Mode D'emploi page 10

3. Specifications
3.2 Thermocouples
3.2.1 Sensor types
Type Recommended max. operating temperature
GB
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"
■
thermocouple wires) result in falsifications of the characteristic curves.
Foreign atoms (poisoning) that diffuse into the original alloys lead to changes of these
■
original alloys and thus falsify the characteristic curve.
The influence of hydrogen leads to the embrittlement of the thermocouples.
■
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 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.
10
WIKA operating instructions models TR10-L, TC10-L (Ex d)