WIKA TC12-M - Handbuch (Seite 11)

GB
3. Specifications
14083458.01 09/2013 GB/D
WIKA operating instructions models TR12, TC12 (Ex i) 11
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.
If these thermocouples cool too quickly (quicker than approx. 100 °C per hour), the
undesirable disordered crystal lattice occurs again during cooling in the range from
approx. 600 °C to approx. 400 °C. In the characteristic curve of type K, however, a
consistently ordered alignment state is assumed and provided with values. This results in
a fault of thermoelectric voltage of up to approx. 0.8 mV (approx. 5 °C) in this range. The K
effect is reversible and is largely eliminated again by annealing above 700 °C, followed by
correspondingly slow cooling.
Thin sheathed thermocouples are particularly sensitive in this regard. Cooling in resting air
can already lead to deviations of 1 °C.
In type N thermocouples, it has been possible to reduce this short-range-order effect by
alloying both legs with silicium.
The application range of these thermometers is limited both by the permissible maximum
temperature of the thermocouple and by the max. temperature of the thermowell material.
Listed models are available both as single or dual thermocouples. The thermocouple will be
delivered with an insulated measuring point, unless explicitly specified otherwise.