WIKA TR10-A Benutzerhandbuch
Seite 33
WIKA operating instructions RTD and TC, intrinsically safe designs (Ex i)
3345267.12 01/2014 GB/D
33
GB
Example
Resistance thermometer RTD
Diameter: 6 mm
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] from table = 37 K/W
Self-heating: 0.0152 W x 37 K/W = 0.56 K
T
max
= T
M
+ self-heating: 150 °C + 0.56 °C = 150.56 °C
The result shows that in this case self-heating at the thermowell tip 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 for T3 = 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) EN/IEC 60 079-0: 2009 Section 26.5.1
Simplified verification of intrinsic safety for the above-mentioned combination
Measuring insert
Head transmitter
Power supply
U
i
: DC 30 V
U
o
: DC 6.5 V
U
i
: DC 30 V
U
o
: DC 25.4 V
I
i
: 550 mA
I
o
: 9.3 mA
I
i
: 130 mA
I
o
: 88.2 mA
P
i
(max) at the sensor: 1.5 W
P
o
: 15.2 mW
P
i
: 800 mW
P
o
: 560 mW
C
i
: negligible
C
o
: 24 µF
C
i
: 7.8 nF
C
o
: 93 nF
L
i
: negligible
L
o
: 365 mH
L
i
: 100 µH
L
o
: 2.7 mH
Upon comparing the values, it is obvious that it is permissible to connect these units to one
another. However, the operator must also take into account the values for inductance and
capacitance of the electrical connection leads.
9.2 Calculation example for a sheathed cable with RTD sensor
Use at the partition wall to Zone 0: Calculate the maximum permissible temperature T
max
at the
probe tip for the following combination:
Resistance thermometer without thermowell (TR10-H) Ø 6 mm without transmitter, mounted by
means of a compression fitting with stainless steel sealing ring. Power supply is, for example, via
a model Z954 Zener barrier (WIKA Article No. 3247938), for example.
T
max
is obtained by adding the temperature of the medium and the self-heating. The self-heating
of the thermowell tip depends on the supplied power P
o
of the Zener barrier and the thermal
resistance R
th
.
9. Calculation examples for self-heating at the sensor/thermowell tip