WIKA TR10-A - Handbuch (Seite 32)

3345267.12 01/2014 GB/D
32 WIKA operating instructions RTD and TC, intrinsically safe designs (Ex i)
GB
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.
9.1 Calculation example for RTD measuring point with thermowell
Use at the partition wall to Zone 0: Calculate the maximum permissible temperature T
max
at the
thermowell tip for the following combination:
RTD measuring insert Ø 6 mm with built-in model T32.1S head-mounted transmitter, fitted into
a Design 3F multi-part thermowell . Power supply is, for example, via a Model KFD2-STC4-EX1
transducer power supply (WIKA article No. 2341268).
T max is obtained by adding the temperature of the medium and the self-heating. The self-heating
of the thermowell tip depends on the supplied power P o of the transmitter and the thermal
resistance R th .
The following formula is used for the calculation: T
max
= P
o
x R
th
+ T
M
T
max
= Surface temperature (max. temperature at the thermowell tip)
P
o
= from transmitter data sheet
R
th
= Thermal resistance [K/W]
T
M
= Temperature of the medium
Prerequisite is an ambient temperature T
amb
of -20 ... +40 °C.
9. Calculation examples for self-heating at the sensor/thermowell tip