Additional Information - WIKA RTD Zusatzinformation

5. Calculation examples for self-heating at the sensor ...
5.1 Calculation for RTD measuring point with thermowell
▶ Use at the partition to zone 0
Calculate the maximum possible temperature, T
combination:
▶
RTD measuring insert Ø 6 mm with built-in model T32.1S head-mounted transmitter,
fitted into a design 3F fabricated thermowell
▶
Power supply is, for example, via a model KFD2-STC4-EX1 isolated barrier (WIKA
article no. 2341268)
T is obtained by adding the temperature of the medium and the self-heating. The
max
self-heating of the thermowell tip depends on the supplied power P
the thermal resistance R
The following formula is used for the calculation: T
T = Surface temperature (max. temperature at the thermowell tip)
max
P = from transmitter data sheet
o
R = Thermal resistance [K/W]
th
T = Medium temperature
M
Example
Resistance thermometer RTD
Diameter: 6 mm
Medium temperature: T
Supplied power: P = 15.2 mW
o
Temperature class T3 (200 °C) must not be exceeded
Thermal resistance [R
Self-heating: 0.0152 W * 37 K/W = 0.56 K
T = T + self-heating: 150 °C + 0.56 °C = 150.56 °C
max M
The result shows that in this case self-heating at the thermowell tip is negligible. 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 margin for type-tested instruments (for T3 to T6)
Safety margin for type-tested instruments (for T1 to T2)
2) IEC/EN 60079-0: 2009 section 26.5.1
WIKA additional information RTD and TC, intrinsically safe designs (Ex i)
.
th
= 150 °C
M
in K/W] from table = 37 K/W
th
, at the thermowell tip for the following
max
o
= P * R + T
max o th M
= 5 K
2)
= 10 K
2)
of the transmitter and
19
EN