Additional Information - WIKA TR12 Serie Manual De Instrucciones

5. Calculation examples for self-heating at the thermowell tip
The following formula is used for the calculation: 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 = Medium temperature
M
Example
Resistance thermometer RTD
Diameter: 6 mm
Medium temperature: T
Supplied power: P = 0.8 W
o
Temperature class T3 (200 °C) must not be exceeded
Thermal resistance [R
Self-heating: 0.8 W * 16 K/W = 12.8 K
T = T + self-heating: 150 °C + 12.8 °C = 162.8 °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.

Additional information:

Temperature class for T3 = 200 °C
Safety margin for type-tested instruments (for T3 to T6)
Safety margin for type-tested instruments (for T1 to T2)
Safety margin for applications for instrument category 1 (zone 0)
application here
1) EN 60079-0 section 26.5.1.3
2) EN 1127-1: 2011 section 6.4.2
5.2 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 * 1 K/W = 0.8 K
T = T + self-heating: 150 °C + 0.8 K = 150.8 °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.
WIKA additional information TR12/TC12, flameproof enclosure (Ex d)
= 150 °C
M
in K/W] from table = 16 K/W
th
in K/W] from table = 3 K/W
th
= P * R + T
max o th M
= 5 K
1)
= 10 K
1)
= 80 % finds no
2)
EN
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