WIKA TR10-A Manual De Instrucciones página 11

5. Commissioning, operation
5.1.5 Mounting instructions for electrical thermometers with ceramic protection tube
Ceramic thermowell materials withstand changes in temperature only to a limited extent. A
temperature shock can therefore easily result in stress cracks and consequently in damage
to the protection tube.
EN
For this reason, preheat thermocouples with ceramic or sapphire protection tubes before
installation, and then slowly immerse them into the hot process.
In accordance with DIN 43724, an insertion speed of 1 cm/min is recommended for protec-
tion tubes with a diameter of 24/26 mm. For smaller diameters of 10/15 mm, the speed can
be increased to 50 cm/min. As a basic principle, higher process temperatures require a
lower insertion speed.
In addition to the protection from thermal stress, ceramic protection tubes must also be
protected from mechanical loads. The reason for these harmful stresses are bending forces
in case of a horizontal mounting position. As a consequence, an additional support must
be provided in case of a horizontal mounting position depending on the diameter, greater
nominal lengths and the design.
In principle, the deflection problem also occurs for metal protection tubes, particularly
for insertion lengths > 500 mm. For process temperatures > 1,200 °C, vertical mounting
should be preferred.
Due to the high thermal, chemical, and mechanical stresses to which ceramic and sapphire
protection tubes are subjected during operation, a general indication regarding the
service life can only be given to a limited extent. This is particularly valid for applications in
high-load processes, such as gasification reactors. According to this, the process-related
parts of the thermocouples are wear parts which are not covered by the warranty.
Ceramic protection tubes with purge connection
For ceramic protection tubes with purge connection the following basic settings are
recommended:
Pressure of purge gas: 0.25 ... 0.35 bar [3.6 ... 5.1 psi] over maximum process pressure
Flow rate of purge gas: approx. 10 ... 12 LPH
Purge gas: nitrogen
Depending on the process an adjustment of given values can be required. The sole
responsibility for this rests with the end user.
WIKA operating instructions, models TRxx and TCxx 11