WIKA RTD Zusatzinformation Seite 20

5. Calculation examples for self-heating at the sensor ...
Sensor with transmitter and barrier
EN
ϑ
C / L = capacitance and inductance of the electrical connecting cable
c c
Simplified verification of intrinsic safety for the above-mentioned combination
Measuring insert
U : DC 30 V
i
I : 550 mA
i
P (max) at the sensor = 1.5 W
i
C : negligible
i
L : negligible
i
Upon comparing the values, it is obvious that it is permissible to connect these instruments
to one another. However, the operator must also take into account the values for
inductance and capacitance of the electrical connection leads.
5.2 Calculation for a sheathed element with RTD sensor
▶
Use at the partition to zone 0
Calculate the maximum possible temperature, T
combination:
▶ RTD without thermowell (TR10-H) Ø 6 mm without transmitter, mounted by means of a
compression fitting with stainless steel ferrule.
▶ The power supply is, for example, via a Zener barrier, for example model Z954 (WIKA
article no. 3247938)
T is obtained by adding the temperature of the medium and the self-heating. The
max
self-heating of the thermowell tip depends on the supplied power P
and the thermal resistance R
20
U ≥ U
i o
l ≥ l
i o
P ≥ P
i o
C + C ≤ C
i c o
L + L ≤ L
i c o
Head-mounted transmitter
≥ U
o
≥ I : 9.3 mA
o
≥ P
o
≤ C
o
≤ L
o
.
th
WIKA additional information RTD and TC, intrinsically safe designs (Ex i)
Hazardous area
U ≥ U
i o
l ≥ l
i o
P ≥ P
i o
C + C ≤ C
i c o
L + L ≤ L
i c o
: DC 6.5 V U : DC 30 V
i
I : 130 mA
i
: 15.2 mW P : 800 mW
i
: 24 µF C : 7.8 nF
i
: 365 mH L : 100 µH
i
, at the sensor tip for the following
max
Non-hazardous area
Isolated barrier
≥ U : DC 25.4 V
o
≥ I : 88.2 mA
o
≥ P : 560 mW
o
≤ C : 93 nF
o
≤ L : 2.7 mH
o
of the Zener barrier
o