WIKA RTD Zusatzinformation Seite 21

5. Calculation examples for self-heating at the sensor ...
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 = 1,150 mW
o
Temperature class T3 (200 °C) must not be exceeded
Thermal resistance [R
Self-heating: 1.15 W * 75 K/W = 86.25 K
T = T + self-heating: 150 °C + 86.25 °C = 236.25 °C
max M
The result shows, in this case, substantial self-heating at the sensor tip. As safety margin
for type-examined instruments (for T3 to T6), 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 exceeded significantly and therefore not permissible. An additional thermowell
or transmitter could be used as a remedy.
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)
1) IEC/EN 60079-0: 2009 Ch. 26.5.1
5.3 Calculation for the above-mentioned RTD with thermowell
▶ RTD measuring insert Ø 6 mm without transmitter, built into a 3F design fabricated
thermowell
Thermal resistance [R
Self-heating: 1.15 W * 37 K/W = 42.55 K
T = T + self-heating: 150 °C + 42.55 °C = 192.55 °C
max M
The result shows, in this case, substantial self-heating at the sensor tip. As safety margin
for type-examined instruments (for T3 to T6), 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.
WIKA additional information RTD and TC, intrinsically safe designs (Ex i)
= 150 °C
M
in K/W] from table = 75 K/W
th
in K/W] from table = 37 K/W
th
= P * R + T
max o th M
= 5 K
1)
= 10 K
1)
EN
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