WIKA TC12-M - Handbuch (Seite 31)

GB
9. Calculation examples for self-heating at the thermowell tip
14083458.01 09/2013 GB/D
WIKA operating instructions models TR12, TC12 (Ex i) 31
The table shows that when a failure occurs, thermocouples produce much less self-heating
than resistance thermometers.
Thermal resistance [Rth in K/W]
Sensor type RTD TC
Measuring insert diameter 3.0 -
< 6.0
6.0 -
≤ 8.0
3.0 -
< 6.0
6.0 -
≤ 8.0
With fabricated thermowell (straight and tapered)
(e.g. TW30, TW35, TW40)
60 37 15 5
With solid material thermowell (straight and tapered)
(e.g. TW10, TW15, TW20, TW25, TW30)
22 16 10 3
Built into a blind bore
(minimum wall thickness 5 mm)
22 16 10 3
9.1 Example calculation for variant 2 with RTD sensor
Use with a thermowell at the partition to zone 0, marking II 1/2 G Ex ia IIC T1-T6 Ga/Gb.
Calculate the maximum permissible temperature Tmax at the thermowell tip for the following
combination: Ø 6 mm RTD measuring insert with built-in head-mounted transmitter,
assembled with solid material thermowell.
T
max
is obtained by adding the temperature of the medium and the self-heating. The
self-heating depends on the supplied power Po and the thermal resistance R
th
. The
calculated supplied power, Po, comes from the chosen standard value for the fuse and is
only realised at the sensor tip.
It thus gives the following formula: T
max
= P
o
x R
th
+ T
M
T
max
= Surface temperature (max. temperature at the thermowell tip)
P
o
= 0.8 W (fuse with 32 mA, a complete short-circuit of the transmitter is assumed)
R
th
= Thermal resistance [K/W]
T
M
= Medium temperature
Example: Resistance thermometer RTD
Diameter: 6 mm
Medium temperature T
M
= 150 °C
Supplied power: P
o
= 0.8 W
Temperature class T3 (200 °C) must not be exceeded
Thermal resistance [R
th
in K/W] from table = 16 K/W
Self-heating: 0.8 W x 16 K/W = 12.8 K
T
max
= T
M
+ self-heating: 150 °C + 12.8 °C = 162.8 °C
As safety margin for type-examined 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.