EN
14073045.04 03/2019 EN/DE
WIKA operating instructions model TR25 (Ex i)
26
10. Calculation examples for self-heating of the tubular body
at the sensor installation point
In-line resistance thermometer model TR25 with built-in head-mounted transmitter
model T32.1S.
Power supply is, for example, via a model KFD2-STC4-EX1 transducer power supply
(WIKA Article No. 2341268).
T
max
is obtained by adding the temperature of the medium and the self-heating. The
self-heating depends on the supplied power P
o
of the transmitter and the thermal
resistance R
th
.
The following formula is used for the calculation: T
max
= P
o
x R
th
+ T
M
T
max
= Surface temperature (max. temperature of the tubular body at the sensor installation
point)
P
o
= From transmitter 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.
Thermal resistance for the TR25 (R
th
) 60 K/W
Example
Temperature of the medium: T
M
= 150 °C
Supplied power: P
o
= 15.2 mW
Temperature class T3 (200 °C) must not be exceeded
Thermal resistance [R
th
in K/W] = 60 K/W
Self-heating: 0.0152 W x 60 K/W = 0.91 K
T
max
= T
M
+ self-heating: 150 °C + 0.91 °C = 150.91 °C
The result shows that in this case self-heating of the tubular body at the sensor installation
point is negligible.
As safety clearance for type-examined instruments (for T6 to T3), another 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 forT3 = 200 °C
Safety clearance for type-examined instruments (T6 to T3)
1)
= 5 K
Safety clearance for type-examined instruments (T1 to T2)
1)
= 10 K
1) IEC/EN 60079-0: 2009 Section 26.5.1
9. Calculation examples for self-heating of the tubular ...
