Calculation Examples For Self-Heating At The Thermowell Tip; Sensor Type - WIKA TR12 Serie Manual De Instrucciones

5. Calculation examples for self-heating at the thermowell tip

5. Calculation examples for self-heating at the thermowell tip
The self-heating at the thermowell tip depends upon the sensor type (TC/RTD), the
measuring insert diameter and the thermowell design. The table below shows the possible
EN
combinations. The heating at the sensor tip of the bare measuring insert is clearly higher;
the representation of these values was omitted on the grounds of the required assembly
with a thermowell.
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
With fabricated thermowell
(straight and tapered), e.g. TW35, TW40 etc.
With thermowell - solid-body material
(straight and tapered), e.g. TW10, TW15, TW20, TW25,
TW30 etc.
Built into a blind bore
(minimum wall thickness 5 mm)
5.1 Example calculation for variant 2 with TC sensor
Under the same conditions it gives a lower value for the self-heating, since the supplied
power is not only converted at the probe tip, but rather over the entire length of the
measuring insert.
Thermal resistance [R
Self-heating: 0.8 W * 3 K/W = 2.4 K
T = T + self-heating: 150 °C + 2.4 °C = 152.4 °C
max M
As a safety margin for type-tested instruments (for T6 to T3), an additional 5 °C must be
subtracted from the 200 °C; hence 195 °C would be permissible. This means that in this
case temperature class T3 is not exceeded.
In this example it is clear that the self-heating here is almost negligible.
20
in K/W]
th
in K/W] from table = 3 K/W
th
WIKA additional information TR12/TC12, intrinsically safe designs (Ex i)
RTD TC
3.0 - 6.0 - 3.0 -
< 6.0 ≤ 8.0 < 6.0
60 37 11
22 16 4
22 16 4
6.0 -
≤ 8.0
2.5
1
1