Wika TR12-M Manual do Utilizador descarregar pdf (Página 10)

14064370.01 08/2013 GB/D

WIKA operating instructions models TR12, TC1210

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 eects/poisoning

■

Oxidation processes in thermocouples which are not appropriately protected (“bare”

thermocouple wires) result in falsications of the characteristic curves.

■

Foreign atoms (poisoning) that diuse into the original alloys lead to changes of these

original alloys and thus falsify the characteristic curve.

■

The inuence 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. Thermocouple types J and T age slightly, as the pure metal leg

oxidises rst.

In general, rising temperatures cause accelerated ageing eects.

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 aected by it. The consequence of this eect is a drift of

the measured value caused by decreasing thermoelectric voltage. This eect 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.

3. Specications

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