3345267.08 06/2011 GB/D
32 WIKA operating instructions RTD and TC, intrinsically safe designs
GB
8.5 TC93 coaxial multipoint thermocouple
WARNING!
The circuits of the coaxial multi-point thermocouple are operationally connected to
one another. When applied in practice, a separate assessment must be made, and/or
special conditions may have to be observed when connecting the coaxial multipoint
thermocouple. In addition, an assessment of an intrinsically safe system (e.g. when
connecting several sensor circuits of dierent transmitters to one another) must be
made.
9. Calculation examples for self-heating at the sensor/thermowell tip
The self-heating at the sensor tip or thermowell tip depends upon the sensor type (TC/RTD),
the probe diameter, the thermowell design and the power supplied to the sensor in the event of
a failure. The table below shows the possible combinations. The table shows that when a failure
occurs, thermocouples produce much less self-heating than resistance thermometers.
Thermal resistance [R
th
in K/W]
Sensor Probe Ø in mm
2.0-
<3.0
3.0-
<6.0
6 - 8 3.0 -
6.0
1)
0.5-
<1.5
1.5-
<3.0
3.0-
<6.0
6.0-
12.0
Sensor type RTD RTD RTD RTD TC TC TC TC
without thermowell 245 110 75 225 105 60 20 5
with multi-part thermowell
(straight and tapered)
(e.g. TW22, TW35, TW40, TW45, etc.)
135 60 37 - - - 11 2.5
with thermowell - machined from solid
material (straight and tapered)
(e.g. TW10, TW15, TW20, TW25, TW30,
TW50, TW55, TW60, etc.)
50 22 16 - - - 4 1
Special thermowell – EN 14597 - - 33 - - - - 2.5
Tx55 (tubular holder) - 110 75 225 - - 20 5
Built into a blind bore
(minimum wall thickness 5 mm)
50 22 16 45 22 13 4 1
1) surface-sensitive
When using multiple sensors and simultaneous operation, the sum of the individual powers must
not exceed the value of the maximum permissible power. The maximum permissible power must
be limited to 1.5 W maximum. This must be guaranteed by the plant operator.
8. Electrical connection values / 9. Calculation examples for ...
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