WIKA TR10-L Betriebsanleitung Seite 16

4. Design and function
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 [752 °F]. If the thermocouple
is heated further, a transition to a disordered state occurs in the temperature range
between approx. 400 °C [752 °F] and 600 °C [1,112 °F]. Above 600 °C [1,112 °F], an
EN
ordered crystal lattice is restored.
If these thermocouples cool too quickly (quicker than approx. 100 °C [212 °F] per hour),
the undesirable disordered crystal lattice occurs again during cooling in the range
from approx. 600 °C [1,112 °F] to approx. 400 °C [752 °F]. 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 [41 °F]) in this range. The K effect is reversible and is largely eliminated
again by annealing above 700 °C [1,292 °F], 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 [34 °F].
In type N thermocouples, it has been possible to reduce this short-range-order effect by
alloying both legs with silicon.
The actual operating temperature of the thermometer is limited both by the maximum
permissible working temperature of the thermocouple, as well as by the maximum
permissible working temperature of the thermowell/protection tube material.
Listed types are available both as single or dual thermocouples. The thermocouple will
be delivered with an ungrounded measuring location, unless explicitly specified other-
wise.
Tolerance value
For the tolerance value of thermocouples, a cold junction temperature of 0 °C [32 °F]
has been taken as the basis. When using a compensating cable or thermocouple cable,
an additional measuring deviation must be considered.
For tolerance values and further specifications, see the corresponding WIKA data sheet
and technical information IN 00.23, "Application of thermocouples".
For further specifications, see WIKA data sheet TE 65.12 (TC10-L), TE 66.01 (TC10-0)
and the order documentation.
16 WIKA operating instructions, models TR10-L, TC10-L, TR10-0, TC10-0 (Ex d)