Examples Of Calculations, Self-Heating Of The Sensor Tip - WIKA TR10-W Manual De Instrucciones

8. Examples of calculations, self-heating of the sensor tip

8. Examples of calculations, self-heating of the sensor tip
The self-heating at the thermowell tip depends on the sensor type (measuring
resistor/thermocouple), the measuring insert diameter and the design of the
thermowell. The following table indicates the possible combinations. The heating
at the sensor tip of the bare measuring insert is considerably higher; as it must
be combined with a thermowell, these values are not shown. The table shows
that the self-heating of thermocouples is considerably lower than the self-heating
of resistance thermometers.
Thermal resistance [Rth in °K/W]
Measuring insert diameter
Sensor type
With multipart thermowell
(straight and tapered)
(e.g. TW30, TW35, TW40)
With solid thermowell (straight and tapered)
(e.g. TW10, TW15, TW20, TW25, TW30)
Mounted in a pocket hole
(minimum wall thickness 5 mm)
8.1 Calculation example for version 2, resistance thermometer
Use at the transition to zone 0, certification II 1/2G Ex d IIC T1-T6, power
limitation circuit by means of a 32 mA pre-fuse.
The maximum possible temperature T
combination must be determined:
RTD measuring insert Ø 6 mm with integral head-mounted transmitter, mounted
in a solid thermowell.
T results from the addition of the medium temperature and the self-heating.
max
The self-heating depends on the supplied power P
R . The calculated supplied power P
th
for the pre-fuse and is only active at the sensor tip.
Thus, the following formula applies: T
T = Surface temperature (max. temperature at the thermowell tip)
max
P = 0.8 W (pre-fuse with 32 mA, a complete short circuit of the transmitter is
o
assumed)
R = Thermal resistance [K/W]
th
T = Medium temperature
M
WIKA operating instructions models TR10-W, TC10-W
3 mm 6 mm
RTD RTD
60 37
22 16
22 16
at the thermowell tip for the following
max
and the thermal resistance
o
results from the selected standard value
o
= P x R + T
max o th
3 mm 6 mm
TC TC
15 5
10 3
10 3
M
GB
15