Calculation Examples For Self-Heating Of The Pipe Body At The Sensor Mounting Point - WIKA TR25 Manual De Instrucciones

8. Calculation examples for self-heating of the pipe body at ...
8. Calculation examples for self-heating of the pipe body at
the sensor mounting point
In-line resistance thermometer model TR25 with built-in head-mounted transmitter
EN
model T32.1S.
Power supply is, for example, via a model KFD2-STC4-EX1 repeater power supply
(WIKA article no. 2341268).
T is obtained by adding the temperature of the medium and the self-heating. The
max
self-heating depends on the supplied power P
resistance R .
th
The following formula is used for the calculation: T
T = Surface temperature (max. temperature of the pipe body at the sensor mounting point)
max
P = from transmitter data sheet
o
R = thermal resistance [K/W]
th
T = medium temperature
M
The precondition is an ambient temperature T
Thermal resistance for TR25 (R
Example
Medium temperature: T
Supplied power: P
Temperature class T3 (200 °C) must not be exceeded
Thermal resistance [R
Self-heating: 0.0152 W x 60 K/W = 0.91 K
T = T + self-heating: 150 °C + 0.91 °C = 150.91 °C
max M
The result shows that, in this case, self-heating of the pipe body at the sensor mounting
point 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 (T6 to T3)
Safety factor for type-tested instruments (T1 to T2)
1) IEC/EN 60079-0: 2009 section 26.5.1
26
) 60 K/W
th
= 150 °C
M
= 15.2 mW
o
in K/W] = 60 K/W
th
of the transmitter and the thermal
o
= P x R + T
max o th
of -20 ... +40 °C.
amb
= 5 K
1)
= 10 K
1)
WIKA operating instructions model TR25 (Ex i)
M