Design And Function; Short Description - WIKA TR Serie Manual De Instrucciones

3. Specifications / 4. Design and function
Potential measuring uncertainties due to aging effects
Thermocouples age and change their temperature/thermoelectric voltage curve. Type J (Fe-CuNi)
thermocouples age slightly due to oxidation of the pure metal leg. As for type K and to a lesser
extend type N (NiCrSi-NiSi) thermocouples, considerable changes in thermo-electric voltage
can occur at high temperatures due to chromium depletion in the NiCr leg, which results in a
decreasing thermoelectric voltage.
This effect is accelerated if there is a shortage of oxygen, since a complete oxide skin is unable
to form on the surface of the thermocouple and protect it from further oxidation. The chromium
in the alloy oxidizes, but not the nickel, giving rise to the "green rot" that eventually destroys
the thermocouple. During the fast cooling of NiCr-Ni thermocouples, that has been operating
at temperatures above 700 °C, certain states occur within the crystal structure (short-range
order), which in type K elements can result in a change in thermoelectric voltage of up to 0.8 mV
(K effect).
It has been possible to reduce the short-range order effect in type N (NiCrSi-NiSi)
thermocouples by alloying both legs with silicon.
The effect is reversible and can be cancelled by annealing above 700 °C with subsequent slow
cooling. Thermocouples with smaler diameters react sensitively in this respect. Even cooling in
still air can cause deviations greater than 1 K.
The application range of these thermometers is limited by the permissible max. temperature of the
thermocouple as well as the max. temperature of the thermowell material.
Listed thermocouples are available both as single or duplex thermocouples. The thermocouple
will be delivered with an insulated measuring point, unless explicitly specified otherwise.
Sensor tolerance value
A cold junction temperature of 0 °C is taken as basis with the definition of the sensor limiting
error of thermocouples. When using a compensating cable or thermocouple cable, an additional
measuring error must be considered.
For tolerance values and further specifications see current WIKA data sheet or order documentation.

4. Design and function

4.1 Short description

These thermometers (resistance thermometers and thermocouples) detect temperatures in
processes.
These thermometers are suitable, depending on the version, for low, medium and high process
requirements.
They can be assembled as cable probes or as thermometers with a connection head. Optionally,
a temperature transmitter can be built into the connection head. For this temperature transmitter,
separate operating instructions will be enclosed.
WIKA operating instructions resistance thermometers, thermocouples
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