Calculation Examples For Self-Heating At The Sensor/ Thermowell Tip; Sensor Type - WIKA RTD Zusatzinformation

5. Calculation examples for self-heating at the sensor ...
5. 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
EN
(TC/RTD), the sensor diameter, the thermowell design and the power supplied to the
temperature transmitter 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

Sensor type

Measuring insert diameter
Without thermowell
With fabricated thermowell
(straight and tapered), e.g. TW22,
TW35, TW40, TW45 etc.
With thermowell - solid-body
material
(straight and tapered), e.g. TW10,
TW15, TW20, TW25, TW30,
TW50, TW55, TW60, etc.
Special thermowell in
accordance with EN 14597
Tx55 (retaining tube)
Built into a blind bore
(minimum wall thickness 5 mm)
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.
18
in K/W]
th
RTD
2.0 ... 3.0 ...
< 3.0 < 6.0
245 110
135 60
50 22
- -
- 110
50 22
WIKA additional information RTD and TC, intrinsically safe designs (Ex i)
TC
6.0 ... 3.0 ... 0.5 ...
8.0 6.0 < 1.5
1)
75 225 105
37 - -
16 - -
33 - -
75 225 -
16 45 22
1.5 ... 3.0 ... 6.0 ...
< 3.0 < 6.0 12.0
60 20 5
- 11 2.5
- 4 1
- - 2.5
- 20 5
13 4 1