WIKA TR12-M - Handbuch (Seite 22)

GB
8. Safety-related instructions for the different variants
11419904.02 09/2013 GB/D
WIKA operating instructions models TR12, TC12 (Ex d) 22
A heating in the connection head does not occur with variant 1. However, an impermissible
heat reflux from the process which can exceed the operating temperature of the housing or
the temperature class, must be prevented through suitable heat insulation or a suitably long
neck tube.
8.2 Variant 2:
ATEX/IECEX Ex d housing or connection head with built-in head-mounted transmitter.
The evaluation is made via a current (4 … 20 mA), voltage (2 … 10 V) or field bus signal,
which is generated from a head-mounted transmitter.
Use in zone 1, marking II 2G Ex d IIC T1-T6 Gb
The flameproof housing or the connection head is in zone 1 (or zone 2). The sensor is in
zone 1. In the case of a separation of Ex-zones, a thermowell (from corrosion-resistant
steel, min. wall thickness 1 mm) must be used.
Use with a thermowell at the partition to zone 0, marking II 1/2G Ex d IIC T1-T6 Ga/Gb
The flameproof housing or the connection head is in zone 1 (or zone 2). The sensor is
within a thermowell (min. wall thickness 1 mm) which extends into zone 0 via a process
connection.
The thermometer should be operated with a power-limiting circuit.
P
max
: 2 W
U
max:
30 V
A power supply with Ex ia circuitry fulfills these conditions, but is not required if the limits
can be achieved through other measures. The responsibility rests with the operator.
WIKA recommends realising the power limitation through a suitable fuse in the 4 ... 20 mA
current loop of the head-mounted transmitter. In the event of a failure, the head-mounted
transmitter will circuit will be interrupted through the fuse tripping.
Example for calculating the fuse for a maximum power at the sensor of 0.8 Watt:
The internal resistance of thermocouples is significantly lower than the thermal resistance
of a Pt100 sensor, so the much less favourable case for a resistance thermometer has been
calculated.
P
max
= (1.7 x I
s
)² x Rw
I
s
= Fuse rating
P
max
= maximum power at sensor = 0.8 W
R
w
= Resistance of the sensors (temperature dependant)
at 450 °C = 264.18 Ω in accordance with DIN EN 60751 for Pt100.
The following defines the fuse rating:
I
s
= sqrt (P
max
/ R
w
) / 1.7
I
s
= sqrt (0.8 W / 265 Ω) / 1.7
I
s
= 32.32 mA
This results in a rated current for a fuse link = 32 mA