Calculation Examples For Self-Heating At The Thermowell Tip - WIKA TR10-W Betriebsanleitung

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

9. Calculation examples for self-heating at the thermowell tip
The self-heating at the thermowell tip depends upon the sensor type (measuring resistor/
thermocouple), the measuring insert diameter and the thermowell design. The table below
shows the possible 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
Measuring insert diameter
Sensor type
With fabricated thermowell
(straight and tapered)
(e.g. TW30, TW35, TW40)
With solid material thermowell (straight and tapered)
(e.g. TW10, TW15, TW20, TW25, TW30)
Installed in a blind bore
(minimum wall thickness 5 mm)
9.1 Example calculation for variant 2, resistance thermometer
Use at the partition to zone 0, marking II 1/2G Ex d IIC T1-T6 power-limited circuit using a
fuse with 32 mA.
Calculate the maximum permissible temperature T
combination:
Ø 6 mm RTD measuring insert with built-in head-mounted transmitter, assembled with solid
material thermowell.
T
is obtained by adding the temperature of the medium and the self-heating. The
max
self-heating depends on the supplied power P
calculated supplied power, P
only realised at the sensor tip.
It thus gives the following formula: T
T = Surface temperature (max. temperature at the thermowell tip)
max
P = 0.8 W (fuse with 32 mA, a complete short-circuit of the transmitter is assumed)
o
R = Thermal resistance [K/W]
th
T = Temperature of the medium
M
WIKA operating instructions models TR10-W, TC10-W (Ex d)
in K/W]
th
, comes from the chosen standard value for the fuse and is
o
= P
max o
3 mm 6 mm
RTD RTD
60 37
22 16
22 16
at the thermowell tip for the following
max
and the thermal resistance R
o
x R + T
th M
GB
3 mm 6 mm
TC TC
15 5
10 3
10 3
. The
th
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