WIKA TR12-M - Handbuch (Seite 11)

GB
14064370.01 08/2013 GB/D
WIKA operating instructions models TR12, TC12 11
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 respect. Cooling in resting air
can already lead to deviations of 1 K.
In type N thermocouple, 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.
Tolerance value
For the tolerance value of thermocouples, a cold junction temperature of 0 °C has been
taken as the basis. When using a compensating cable or thermocouple cable, an additional
measuring error must be considered.
For limiting deviations and further specifications, see the corresponding WIKA data sheet
and technical information sheet IN 00.23, “Application of thermocouples”.
3. Specifications