WIKA TR10-A - Handbuch (Seite 13)

WIKA operating instructions RTD and TC, intrinsically safe designs (Ex i)
3345267.12 01/2014 GB/D
13
GB
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 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 IN 00.23, “Application of thermocouples”.
3. Specifications