WIKA TR10-W Betriebsanleitung Seite 22

9. Calculation examples for self-heating at the thermowell tip
Example: Resistance thermometer
Diameter: 6 mm
Temperature of the medium: T
Supplied power: P
Temperature class T3 (200 °C) must not be exceeded
GB
Thermal resistance [R
Self-heating: 0.8 W x 16 K/W = 12.8 K
T = T + self-heating: 150 °C + 12.8 °C = 162.8 °C
max M
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.
Additional information:
Temperature class for T3 = 200 °C
Safety factor for type-tested instruments (for T6 to T3)
Safety factor for type-tested instruments (for T1 to T2)
Safety factor for applications for instrument category 1 (zone 0)
application here.
1) EN 60079-0 / EN 60079-1
2) EN 60079-26
9.2 Example calculation for variant 2, thermocouple
Under the same conditions it gives a lower value for the self-warming, since the supplied
power is not only converted at the sensor tip, but rather over the entire length of the
measuring insert.
Thermal resistance [R
Self-heating: 0.8 W x 3 K/W = 2.4 K
T = T + self-heating: 150 °C + 2.4 °C = 152.4 °C
max M
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.
In this example it is clear that the self-heating here is almost negligible.
22
= 150 °C
M
= 0.8 W
o
in K/W] from table = 16 K/W
th
in K/W] from table = 3 K/W
th
WIKA operating instructions models TR10-W, TC10-W (Ex d)
= 5 K
1)
= 10 K
1)
= 80 % finds no
2)