Wika TR10-J Instrukcja Użytkownika Pobierz pdf (Strona 9)

WIKA operating instructions resistance thermometers, thermocouples

11208287.06 10/2013 GB/D/F/E

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.

K eect

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.

3. Specications

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