WIKA TR10-A - Handbuch (Seite 34)

3345267.12 01/2014 GB/D
34 WIKA operating instructions RTD and TC, intrinsically safe designs (Ex i)
GB
The following formula is used for the calculation: T
max
= P
o
x R
th
+ T
M
T
max
= Surface temperature (max. temperature at the probe tip)
P
o
= from the Zener barrier data sheet
R th = Thermal resistance [K/W]
T
M
= Temperature of the medium
Prerequisite is an ambient temperature T
amb
of -20 ... +40 °C.
Example
Resistance thermometer RTD
Diameter: 6 mm
Temperature of the medium T M = 150 °C
Supplied power: P o = 1150 mW
Temperature class T3 (200 °C) must not be exceeded
Thermal resistance [Rth in K/W] from table = 75 K/W
Self-heating: 1.15 W x 75 K/W = 86.25 K
T max = T M + self-heating: 150 °C + 86.25 °C = 236.25 °C
The result shows, in this case, substantial self-heating at the probe tip.
As safety margin for type-examined instrument (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 exceeded significantly and therefore not permissible. An additional
thermowell could be used as a remedy.
Additional information
Temperature class for T3 = 200 °C
Safety margin for type-examined instruments (T6 to T3) 1) = 5 K
Safety clearance for type-examined instruments (T1 to T2) 1) = 10 K
1) EN/IEC 60 079-0: 2009 Section 26.5.1
9.3 Calculation example for the above-mentioned resistance thermometer with thermowell
RTD measuring insert Ø 6 mm without transmitter, built into a 3F design multi-part thermowell.
Thermal resistance [R th in K/W] from table = 37 K/W
Self-heating: 1.15 W x 37 K/W = 42.55 K
T max = T M + self-heating: 150 °C + 42.55 °C = 192.55 °C
The result shows, in this case, substantial self-heating at the probe tip.
As safety margin for type-examined instrument (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.
9. Calculation examples for self-heating at the sensor/thermowell tip